Secure Coding Practices for Middleware

1
Secure Coding Practices
for Middleware
EGI Technical Forum
Lyon September 19, 2011
This research funded in part by Department of Homeland Security grant FA8750-10-2-0030 (funded through AFRL).
Past funding has been provided by NATO grant CLG 983049, National Science Foundation grant OCI-0844219, the
National Science Foundation under contract with San Diego Supercomputing Center, and National Science
Foundation grants CNS-0627501 and CNS-0716460.
Barton P. Miller
James A. Kupsch
Computer Sciences Department
University of Wisconsin
bart@cs.wisc.edu
Elisa Heymann
Computer Architecture and
Operating Systems Department
Universitat Autònoma de Barcelona
Elisa.Heymann@uab.es

2
Who we are
Elisa Heymann
Eduardo Cesar
Jairo Serrano
Guifré Ruiz
Manuel Brugnoli
Bart Miller
Jim Kupsch
Karl Mazurak
Daniel Crowell
Wenbin Fang
Henry Abbey

3
What do we do
• Assess Middleware: Make cloud/grid
software more secure
• Train: We teach tutorials for users,
developers, sys admins, and managers
• Research: Make in-depth assessments
more automated and improve quality of
automated code analysis

4
Studied Systems
Condor, University of Wisconsin
Batch queuing workload management system
15 vulnerabilities 600 KLOC of C and C++
SRB, SDSC
Storage Resource Broker - data grid
5 vulnerabilities 280 KLOC of C
MyProxy, NCSA
Credential Management System
glExec, Nikhef
Identity mapping service
Gratia Condor Probe, FNAL and Open Science Grid
Feeds Condor Usage into Gratia Accounting System
3 vulnerabilities 1.7 KLOC of Perl and Bash
Condor Quill, University of Wisconsin
DBMS Storage of Condor Operational and Historical Data
6 vulnerabilities 7.9 KLOC of C and C++

5
Studied Systems
Wireshark, wireshark.org
Network Protocol Analyzer
in progress 2400 KLOC of C
Condor Privilege Separation, Univ. of Wisconsin
Restricted Identity Switching Module
21 KLOC of C and C++
VOMS Admin, INFN
Web management interface to VOMS data
35 KLOC of Java and PHP
CrossBroker, Universitat Autònoma de Barcelona
Resource Mgr for Parallel & Interactive Applications
97 KLOC of C++
ARGUS 1.2, HIP, INFN, NIKHEF, SWITCH
gLite Authorization Service
42 KLOC of Java and C

6
In Progress
VOMS Core INFN
Network Protocol Analyzer
in progress 161 KLOC of Bourne Shell,
C++ and C
Google Chrome, Google
Web browser
in progress 2396 KLOC of C and C++

7
Who are we
http://www.cs.wisc.edu/mist/
http://www.cs.wisc.edu/mist/papers/VAshort.pdf
What do we do

8
Who funds us
• European Commission
– EGI
– EMI
• Spanish Government
• United States
– DHS
– NSF
• NATO

9
Roadmap
– Introduction
– Handling errors
– Pointers and Strings
– Numeric Errors
– Race Conditions
– Exceptions
– Privilege, Sandboxing and Environment
– Injection Attacks
– Web Attacks
– Bad things

10
Discussion of the Practices
• Description of vulnerability
• Signs of presence in the code
• Mitigations
• Safer alternatives

11
Handling Errors
• If a call can fail, always check for errors
optimistic error handling (i.e. none) is bad
• Error handling strategies:
– Handle locally and continue
– Cleanup and propagate the error
– Exit the application
• All APIs you use or develop, that can fail,
must be able to report errors to the caller
• Using exceptions forces error handling

13
Buffer Overflows
http://cwe.mitre.org/top25/archive/2011/2011_cwe_sans_top25.html#Listing
1. Improper Neutralization of Special Elements used in an SQL
Command ('SQL Injection')
2. Improper Neutralization of Special Elements used in an OS
Command ('OS Command Injection')
3. Buffer Copy without Checking Size of Input ('Classic Buffer
Overflow')
4. Improper Neutralization of Input During Web Page Generation
('Cross-site Scripting')
5. Missing Authentication for Critical Function
6. Missing Authorization
7. Use of Hard-coded Credentials
8. Missing Encryption of Sensitive Data
9. Unrestricted Upload of File with Dangerous Type
10. Reliance on Untrusted Inputs in a Security Decision

14
Buffer Overflows
• Description
– Accessing locations of a buffer outside the boundaries
of the buffer
• Common causes
– C-style strings
– Array access and pointer arithmetic in languages
without bounds checking
– Off by one errors
– Fixed large buffer sizes (make it big and hope)
– Decoupled buffer pointer and its size
• If size unknown overflows are impossible to detect
• Require synchronization between the two
• Ok if size is implicitly known and every use knows it (hard)

15
Why Buffer Overflows
are Dangerous
• An overflow overwrites memory adjacent
to a buffer
• This memory could be
– Unused
– Code
– Program data that can affect operations
– Internal data used by the runtime system
• Common result is a crash
• Specially crafted values can be used for an
attack

16
Buffer Overflow of User Data
Affecting Flow of Control
char id[8];
int validId = 0; /* not valid */
gets(id); /* reads "evillogin"*/
/* validId is now 110 decimal */
if (IsValid(id)) validId = 1; /* not true */
if (validId) /* is true */
{DoPrivilegedOp();} /* gets executed */
e v i l l o g i 110
‘n’
0 0 0
id validId
0 0 0 0
id validId

17
Buffer Overflow Danger Signs:
Missing Buffer Size
• gets, getpass, getwd, and scanf family
(with %s or %[…] specifiers without width)
– Impossible to use correctly: size comes solely
from user input
– Source of the first (1987) stack smash attack.
– Alternatives:
Unsafe Safer
gets(s) fgets(s, sLen, stdin)
getcwd(s) getwd(s, sLen)
scanf("%s", s) scanf("%100s", s)

18
strcat, strcpy, sprintf,
vsprintf
– Impossible for function to detect overflow
• Destination buffer size not passed
– Difficult to use safely w/o pre-checks
• Checks require destination buffer size
• Length of data formatted by printf
• Difficult & error prone
• Best incorporated in a safe replacement function
Proper usage: concat s1, s2 into dst
If (dstSize < strlen(s1) + strlen(s2) + 1)
{ERROR("buffer overflow");}
strcpy(dst, s1);
strcat(dst, s2);

19
Buffer Overflow Danger Signs:
Difficult to Use and Truncation
• strncat(dst, src, n)
– n is the maximum number of chars of src to append
(trailing null also appended)
– can overflow if n >=(dstSize-strlen(dst))
• strncpy(dst, src, n)
– Writes n chars into dst, if strlen(src)<n, it fills the
other n-strlen(src) chars with 0’s
– If strlen(src)>=n, dst is not null terminated
• Truncation detection not provided
• Deceptively insecure
– Feels safer but requires same careful use as strcat

20
Safer String Handling:
C-library functions
• snprintf(buf, bufSize, fmt, …) and
vsnprintf
– Returns number of bytes, not including 0 that
would’ve been written.
– Truncation detection possible
(result >= bufSize implies truncation)
– Use as safer version of strcpy and strcat
Proper usage: concat s1, s2 into dst
r = snprintf(dst, dstSize, "%s%s",s1, s2);
If (r >= dstSize)
{ERROR("truncation");}

21
ISO/IEC 24731
Extensions for the C library:
Part 1, Bounds Checking Interface
• Functions to make the C library safer
• Meant to easily replace existing library
calls with little or no other changes
• Aborts on error or optionally reports error
• Very few unspecified behaviors
• All updated buffers require a size param
• http://www.open-std.org/jtc1/sc22/wg14

22
Stack Smashing
• This is a buffer overflow of a variable local
to a function that corrupts the internal
state of the run-time system
• Target of the attack is the value on the
stack to jump to when the function
completes
• Can result in arbitrary code being
executed
• Not trivial, but not impossible either

23
Pointer Attacks
• First, overwrite a pointer
– In the code
– In the run-time environment
• Heap attacks use the pointers usually at the
beginning and end of blocks of memory
• Second, the pointer is used
– Read user controlled data that causes a
security violation
– Write user controlled data that later causes a
security violation

24
Attacks on Code Pointers
• Stack Smashing is an example
• There are many more pointers to functions or
addresses in code
– Dispatch tables for libraries
– Return addresses
– Function pointers in code
– C++ vtables
– jmp_buf
– atexit
– Exception handling run-time
– Internal heap run-time data structures

25
Buffer Overflow of a
User Pointer
{
char id[8];
int (*logFunc)(char*) = MyLogger;
gets(id); /* reads "evilguyx “ */
/* equivalent to system(userMsg) */
logFunc(userMsg);
e v i l g u y x
id logFunc
id logFunc
Ptr to MyLogger
Ptr to system
Ptr to system

27
Integer Vulnerabilities
• Description
– Many programming languages allow silent loss of
integer data without warning due to
• Overflow
• Truncation
• Signed vs. unsigned representations
– Code may be secure on one platform, but silently
vulnerable on another, due to different underlying
integer types.
• General causes
– Not checking for overflow
– Mixing integer types of different ranges
– Mixing unsigned and signed integers

28
Integer Danger Signs
• Mixing signed and unsigned integers
• Converting to a smaller integer
• Using a built-in type instead of the API’s
typedef type
• However built-ins can be problematic too:
size_t is unsigned, ptrdiff_t is signed
• Assigning values to a variable of the
correct type before data validation
(range/size check)

29
Numeric Parsing
Unreported Errors
• atoi, atol, atof, scanf family (with %u,
%i, %d, %x and %o specifiers)
– Out of range values results in unspecified
behavior
– Non-numeric input returns 0
– Use strtol, strtoul, strtoll, strtoull,
strtof, strtod, strtold which allow error
detection

31
Race Conditions
• Description
– A race condition occurs when multiple threads of
control try to perform a non-atomic operation on a
shared object, such as
• Multithreaded applications accessing shared data
• Accessing external shared resources such as the file system
• General causes
– Threads or signal handlers without proper
synchronization
– Non-reentrant functions (may have shared variables)
– Performing non-atomic sequences of operations on
shared resources (file system, shared memory) and
assuming they are atomic

32
File System Race Conditions
• A file system maps a path name of a file or other
object in the file system, to the internal identifier
(device and inode)
• If an attacker can control any component of the
path, multiple uses of a path can result in
different file system objects
• Safe use of path
– eliminate race condition
• use only once
• use file descriptor for all other uses
– verify multiple uses are consistent

File System Race Examples
• Check properties of a file then open
Bad: access or stat  open
Safe: open  fstat
• Create file if it doesn’t exist
Bad: if stat fails  creat(fn, mode)
Safe: open(fn, O_CREAT|O_EXCL, mode)
– Never use O_CREAT without O_EXCL
– Better still use safefile library
• http://www.cs.wisc.edu/mist/safefile
James A. Kupsch and Barton P. Miller, “How to Open a File and Not Get
Hacked,” 2008 Third International Conference on Availability, Reliability and
Security (ARES), Barcelona, Spain, March 2008.
33

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Race Condition Temporary Files
• Temporary directory (/tmp) is a dangerous area
of the file system
– Any process can create a directory entry there
– Usually has the sticky bit set, so only the owner can
delete their files
• Ok to create true temporary files in /tmp
– Create using mkstemp, unlink, access through
returned file descriptor
– Storage vanishes when file descriptor is closed
• Safe use of /tmp directory
– create a secure directory in /tmp
– use it to store files

35
Race Condition Examples
• Your Actions Attackers Action
s=strdup("/tmp/zXXXXXX")
tempnam(s)
// s now "/tmp/zRANDOM" link = "/etc/passwd"
file = "/tmp/zRANDOM"
symlink(link, file)
f = fopen(s, "w+")
// writes now update
// /etc/passwd
time
Safe Version
fd = mkstemp(s)
f = fdopen(fd, "w+")

Successful Race Condition Attack
void TransFunds(srcAcct, dstAcct, xfrAmt) {
if (xfrAmt < 0)
FatalError();
int srcAmt = srcAcct.GetBal();
if (srcAmt - xfrAmt < 0)
FatalError();
srcAcct.SetBal(srcAmt - xfrAmt);
dstAcct.SetBal(dstAcct.getBal() + xfrAmt);
}
36
Balances
Thread 1 Thread 2 Bob Ian
XfrFunds(Bob, Ian, 100) XfrFunds(Bob, Ian, 100) 100 0
srcAmt = 100
srcAmt = 100
srcAmt – 100 < 0 ?
srcAmt – 100 < 0 ?
srcAcct.SetBal(100 – 100) 0
dst.SetBal(0 + 100) 100
dst.SetBal(0 + 100) 200
time

Mitigated Race Condition Attack
void synchronized TransFunds(srcAcct, dstAcct, xfrAmt) {
if (xfrAmt < 0)
FatalError();
int srcAmt = srcAcct.GetBal();
if (srcAmt - xfrAmt < 0)
FatalError();
srcAcct.SetBal(srcAmt - xfrAmt);
dstAcct.SetBal(dstAcct.getBal() + xfrAmt);
}
37
Balances
Thread 1 Thread 2 Bob Ian
XfrFunds(Bob, Ian, 100) XfrFunds(Bob, Ian, 100) 100 0
In use? No, proceed
In use? Yes, wait.
srcAmt = 100
srcAmt – 100 < 0 ?
dst.SetBal(0 + 100) 100
srcAmt = 0
srcAmt – 100 < 0? Yes,
fail
time

39
Exception Vulnerabilities
• Exception are a nonlocal control flow
mechanism, usually used to propagate error
conditions in languages such as Java and C++.
try {
// code that generates exception
} catch (Exception e) {
// perform cleanup and error recovery
}
• Common Vulnerabilities include:
– Ignoring (program terminates)
– Suppression (catch, but do not handled)
– Information leaks (sensitive information in error
messages)

Proper Use of Exceptions
• Add proper exception handling
– Handle expected exceptions (i.e. check for errors)
– Don’t suppress:
• Do not catch just to make them go away
• Recover from the error or rethrow exception
– Include top level exception handler to avoid exiting:
catch, log, and restart
• Do not disclose sensitive information in messages
– Only report non-sensitive data
– Log sensitive data to secure store, return id of data
– Don't report unnecessary sensitive internal state
• Stack traces
• Variable values
• Configuration data
40

Exception Suppression
1. User sends malicious data
boolean Login(String user, String pwd){
boolean loggedIn = true;
String realPwd = GetPwdFromDb(user);
try {
if (!GetMd5(pwd).equals(realPwd))
{
loggedIn = false;
}
//this can not happen, ignore
}
return loggedIn;
}
user=“admin”,pwd=null
2. System grants access Login() returns true
41

Unusual or Exceptional
Conditions Mitigation
boolean Login(String user, String pwd){
boolean loggedIn = true;
String realPwd = GetPwdFromDb(user);
try {
if (!GetMd5(pwd).equals(realPwd))
{
loggedIn = false;
}
loggedIn = false;
}
return loggedIn;
}
2. System does not grant access Login() returns false
42
1. User sends malicious data user=“admin”,pwd=null

WTMI (Way Too Much Info)
43
Login(… user, … pwd) {
try {
ValidatePwd(user, pwd);
print("Login failed.n");
print(e + "n");
e.printStackTrace();
return;
}
}
void ValidatePwd(… user, … pwd)
throws BadUser, BadPwd {
realPwd = GetPwdFromDb(user);
if (realPwd == null)
throw BadUser("user=" + user);
if (!pwd.equals(realPwd))
throw BadPwd("user=" + user
+ " pwd=" + pwd
+ " expected=" + realPwd);
…
Login failed.
BadPwd: user=bob pwd=x expected=password
BadPwd:
at Auth.ValidatePwd (Auth.java:92)
at Auth.Login (Auth.java:197)
…
com.foo.BadFramework(BadFramework.java:71)
...
User exists Entered pwd
User's actual password ?!?
(passwords aren't hashed)
Reveals internal structure
(libraries used, call structure,
version information)

The Right Amount of Information
44
Login {
try {
ValidatePwd(user, pwd);
logId = LogError(e); // write exception and return log ID.
print("Login failed, username or password is invalid.n");
print("Contact support referencing problem id " + logId
+ " if the problem persists");
return;
}
}
void ValidatePwd(… user, … pwd) throws BadUser, BadPwd {
realPwdHash = GetPwdHashFromDb(user)
if (realPwdHash == null)
throw BadUser("user=" + HashUser(user));
if (!HashPwd(user, pwd).equals(realPwdHash))
throw BadPwdExcept("user=" + HashUser(user));
…
}
Log sensitive information
Generic error message
(id links sensitive information)
User and password are hashed
(minimizes damage if breached)

45
Privilege, Sandboxing,
and Environment

46
Not Dropping Privilege
• Description
– When a program running with a privileged status
(running as root for instance), creates a process or
tries to access resources as another user
• General causes
– Running with elevated privilege
– Not dropping all inheritable process attributes such as
uid, gid, euid, egid, supplementary groups, open file
descriptors, root directory, working directory
– not setting close-on-exec on sensitive file descriptors

47
Not Dropping Privilege: chroot
• chroot changes the root directory for the
process, files outside cannot be accessed
• Only root can use chroot
• chdir needs to follow chroot, otherwise
relative pathnames are not restricted
• Need to recreate all support files used by
program in new root: /etc, libraries, …
Makes chroot difficult to use.

48
Insecure Permissions
• Set umask when using mkstemp or fopen
– File permissions need to be secure from
creation to destruction
• Don’t write sensitive information into
insecure locations (directories need to
have restricted permission to prevent
replacing files)
• Executables, libraries, configuration, data
and log files need to be write protected

49
Insecure Permissions
• If a file controls what can be run as a
privileged, users that can update the file
are equivalent to the privileged user
File should be:
– Owned by privileged user, or
– Owned by administrative account
• No login
• Never executes anything, just owns files
• DBMS accounts should be granted minimal
privileges for their task

50
Trusted Directory
• A trusted directory is one where only trusted
users can update the contents of anything in the
directory or any of its ancestors all the way to the
root
• A trusted path needs to check all components of
the path including symbolic links referents for
trust
• A trusted path is immune to TOCTOU attacks
from untrusted users
• This is extremely tricky to get right!
• safefile library
– http://www.cs.wisc.edu/mist/safefile
– Determines trust based on trusted users & groups

51
Directory Traversal
• Description
– When user data is used to create a pathname to a file
system object that is supposed to be restricted to a
particular set of paths or path prefixes, but which the
user can circumvent
• General causes
– Not checking for path components that are empty, "."
or ".."
– Not creating the canonical form of the pathname (there
is an infinite number of distinct strings for the same
object)
– Not accounting for symbolic links

52
Directory Traversal Mitigation
• Use realpath or something similar to
create canonical pathnames
• Use the canonical pathname when
comparing filenames or prefixes
• If using prefix matching to check if a path is
within directory tree, also check that the
next character in the path is the directory
separator or '0'

Directory Traversal
(Path Injection)
• User supplied data is used to create a path, and program security
requires but does not verify that the path is in a particular subtree of
the directory structure, allowing unintended access to files and
directories that can compromise the security of the system.
– Usually <program-defined-path-prefix> + "/" + <user-data>
• Mitigations
– Validate final path is in required directory using canonical paths
(realpath)
– Do not allow above patterns to appear in user supplied part (if
symbolic links exists in the safe directory tree, they can be used to
escape)
– Use chroot or other OS mechanisms
53
<user-data> Directory Movement
../ up
./ or empty string none
<dir>/ down

54
Successful Directory
Traversal Attack
1. Users requests File="....//etc/passwd"
2. Server deletes /etc/passwd
String path = request.getParameter("file");
path = "/safedir/" + path;
// remove ../'s to prevent escaping out of /safedir
Replace(path, "../", "");
File f = new File(path);
f.delete();
Before Replace path = "/safedir/….//etc/passwd"
After Replace path = "/safedir/../etc/passwd"
Moral: Don't try to fix user input, verify and reject instead

55
Mitigated Directory Traversal
1. Users requests file=“../etc/passwd”
2. Throws error /safedir/../etc/passwd is invalid
String path = request.getParameter(“file”);
if (file.length() == 0) {
throw new PathTraversalException(file + " is null");
}
File prefix = new File(new
File("/safedir").getCanonicalPath());
File path = new File(prefix, file);
if(!path.getAbsolutePath().equals(path.getCanonicalPath())){
throw new PathTraversalException(path + " is invalid");
}
path.getAbsolutePath().delete();

56
Command Line
• Description
– Convention is that argv[0] is the path to the
executable
– Shells enforce this behavior, but it can be set
to anything if you control the parent process
• General causes
– Using argv[0] as a path to find other files
such as configuration data
– Process needs to be setuid or setgid to be a
useful attack

Environment
• List of (name, value) string pairs
• Available to program to read
• Used by programs, libraries and runtime
environment to affect program behavior
• Mitigations:
– Clean environment to just safe names & values
– Don’t assume the length of strings
– Avoid PATH, LD_LIBRARY_PATH, and other
variables that are directory lists used to look
for execs and libs
57

59
Injection Attacks
• Description
– A string constructed with user input, that is then
interpreted by another function, where the string is not
parsed as expected
• Command injection (in a shell)
• Format string attacks (in printf/scanf)
• SQL injection
• Cross-site scripting or XSS (in HTML)
• General causes
– Allowing metacharacters
– Not properly quoting user data if metacharacters are
allowed

60
SQL Injections
• User supplied values used in SQL
command must be validated, quoted, or
prepared statements must be used
• Signs of vulnerability
– Uses a database mgmt system (DBMS)
– Creates SQL statements at run-time
– Inserts user supplied data directly into
statement without validation

61
SQL Injections:
attacks and mitigations
• Dynamically generated SQL without
validation or quoting is vulnerable
$u = " '; drop table t --";
$sth = $dbh->do("select * from t where u = '$u'");
Database sees two statements:
select * from t where u = ' '; drop table t --’
• Use prepared statements to mitigate
$sth = $dbh->do("select * from t where u = ?", $u);
– SQL statement template and value sent to
database
– No mismatch between intention and use

Successful SQL Injection Attack
boolean Login(String user, String pwd) {
boolean loggedIn = false;
conn = pool.getConnection( );
stmt = conn.createStatement();
rs = stmt.executeQuery("SELECT * FROM members"
+ "WHERE u='" + user
+ "' AND p='" + pwd + "'");
if (rs.next())
loggedIn = true;
}
user="admin"; pwd="'OR 'x'='x"
4. System grants access Login() returns true
62
SELECT * FROM members
WHERE u='admin' AND p='' OR 'x'='x'
2. DB Queried
3. Returns all row of table members

Mitigated SQL Injection Attack
boolean Login(String user, String pwd) {
boolean loggedIn = false;
conn = pool.getConnection( );
PreparedStatement pstmt = conn.prepareStatement(
"SELECT * FROM members WHERE u = ? AND p = ?");
pstmt.setString( 1, user);
pstmt.setString( 2, pwd);
ResultSet results = pstmt.executeQuery( );
if (rs.next())
loggedIn = true;
}
user="admin"; pwd="' OR 'x'='x"
4. System does not grant access Login() returns false
63
SELECT * FROM members WHERE u = ?1 AND p = ?2
?1 = "admin" ?2 = "' OR 'x'='x"
2. DB Queried 3. Returns null set

64
http://xkcd.com/327

65
Command Injections
• User supplied data used to create a string
that is the interpreted by command shell such
as /bin/sh
• Signs of vulnerability
– Use of popen, or system
– exec of a shell such as sh, or csh
– Argument injections, allowing arguments to begin
with "-" can be dangerous
• Usually done to start another program
– That has no C API
– Out of laziness

66
Command Injection Mitigations
• Check user input for metacharacters
• Neutralize those that can’t be eliminated or
rejected
– replace single quotes with the four characters, ''',
and enclose each argument in single quotes
• Use fork, drop privileges and exec for more
control
• Avoid if at all possible
• Use C API if possible

67
Command Argument Injections
• A string formed from user supplied input
that is used as a command line argument
to another executable
• Does not attack shell, attacks command line of
program started by shell
• Need to fully understand command line
interface
• If value should not be an option
– Make sure it doesn't start with a -
– Place after an argument of -- if supported

68
Command Argument
Injection Example
• Example
snprintf(s, sSize, "/bin/mail -s hi %s", email);
M = popen(s, "w");
fputs(userMsg, M);
pclose(M);
• If email is -I , turns on interactive mode …
• … so can run arbitrary code by if userMsg
includes: ~!cmd

69
Perl Command Injection
Danger Signs
• open(F, $filename)
– Filename is a tiny language besides opening
• Open files in various modes
• Can start programs
• dup file descriptors
– If $userFile is "rm -rf /|", you probably
won’t like the result
– Use separate mode version of open to
eliminate vulnerability

71
Examples
• open(CMD, "|/bin/mail -s $sub $to");
– Bad if $to is "badguy@evil.com; rm -rf /"
• open(CMD, “|/bin/mail -s '$sub' '$to'");
– Bad if $to is "badguy@evil.com'; rm -rf /'"
• ($qSub = $sub) =~ s/'/'''/g;
($qTo = $to) =~ s/'/'''/g;
open(CMD, "|/bin/mail -s '$qSub' '$qTo'");
– Safe from command injection
• open(cmd, "|-", "/bin/mail", "-s", $sub, $to);
– Safe and simpler: use this whenever possible.

72
Eval Injections
• A string formed from user supplied input that is
used as an argument that is interpreted by the
language running the code
• Usually allowed in scripting languages such as
Perl, sh and SQL
• In Perl eval($s) and s/$pat/$replace/ee
– $s and $replace are evaluated as perl code

Successful OS Injection Attack
3. System executes nslookup x.com;rm –rf /*
73
String rDomainName(String hostname) {
…
String cmd = "/usr/bin/nslookup" + hostname;
Process p = Runtime.getRuntime().exec(cmd);
…
hostname="x.com;rm –rf /*"
2. Application uses nslookup to get DNS records
4. All files possible are deleted

Mitigated OS Injection Attack
3. System returns error "Invalid host name"
74
String rDomainName(String hostname) {
…
if (hostname.matches("[A-Za-z][A-Za-z0-9.-]*")) {
String cmd = "/usr/bin/nslookup " + hostname);
Process p = Runtime.getRuntime().exec(cmd);
} else {
System.out.println(“Invalid host name”);
…
hostname="x.com;rm –rf /*"
2. Application uses nslookup only if input validates

75
Format String Injections
• User supplied data used to create format strings
in scanf or printf
• printf(userData) is insecure
– %n can be used to write memory
– large field width values can be used to create a denial
of service attack
– Safe to use printf("%s", userData) or
fputs(userData, stdout)
• scanf(userData, …) allows arbitrary writes to
memory pointed to by stack values
• ISO/IEC 24731 does not allow %n

Code Injection
• Cause
– Program generates source code from template
– User supplied data is injected in template
– Failure to neutralized user supplied data
• Proper quoting or escaping
• Only allowing expected data
– Source code compiled and executed
• Very dangerous – high consequences for
getting it wrong: arbitrary code execution
76

77
Code Injection Vulnerability
%data = ReadLogFile('logfile');
PH = open("|/usr/bin/python");
print PH "import LogItn";w
while (($k, $v) = (each %data)) {
if ($k eq 'name') {
print PH "LogIt.Name('$v')";
}
2. Perl log processing code – uses Python to do real work
name = John Smith
name = ');import os;os.system('evilprog');#
1. logfile – name's value is user controlled
import LogIt;
LogIt.Name('John Smith')
LogIt.Name('');import os;os.system('evilprog');#')
3. Python source executed – 2nd LogIt executes arbitrary code
Start Python,
program sent
on stdin
Read
logfile

78
Code Injection Mitigated
%data = ReadLogFile('logfile');
PH = open("|/usr/bin/python");
print PH "import LogItn";w
while (($k, $v) = (each %data)) {
if ($k eq 'name') {
$q = QuotePyString($v);
print PH "LogIt.Name($q)";
}
2. Perl log processing code – use QuotePyString to safely create string literal
name = John Smith
name = ');import os;os.system('evilprog');#
1. logfile – name's value is user controlled
import LogIt;
LogIt.Name('John Smith')
LogIt.Name('');import os;os.system('evilprog');#')
3. Python source executed – 2nd LogIt is now safe
sub QuotePyString {
my $s = shift;
$s =~ s///g; # 
$s =~ s/n/n/g; # NL  n
return "'$s'"; # add quotes
}

80
Cross Site Scripting (XSS)
• Injection into an HTML page
– HTML tags
– JavaScript code
• Reflected (from URL) or
persistent (stored from prior attacker visit)
• Web application fails to neutralize special characters in
user supplied data
• Mitigate by preventing or encoding/escaping special
characters
• Special characters and encoding depends on context
– HTML text
– HTML tag attribute
– HTML URL

81
Reflected Cross Site Scripting
(XSS)
•••
String query = request.getParameter("q");
if (query != null) {
out.writeln("You searched for:n" + query);
}
•••
<html>
•••
You searched for:
widget
•••
</html>
http://example.com?q=widget
3. Generated HTML displayed by browser
1. Browser sends request to web server
2. Web server code handles request

82
Reflected Cross Site Scripting
(XSS)
•••
}
•••
<html>
•••
You searched for:
<script>alert('Boo!')</script>
•••
</html>
http://example.com?q=<script>alert('Boo!')</script>
2. Web server code handles request

83
XSS Mitigation
•••
if (query.matches("^w*$")) {
} else {
out.writeln("Invalid query");
}
}
•••
<html>
•••
Invalid query
•••
</html>
http://example.com?q=<script>alert('Boo!')</script>
2. Web server code correctly handles request

84
Cross Site Request Forgery (CSRF)
• CSRF is when loading a web pages causes a malicious
request to another server
• Requests made using URLs or forms (also transmits any
cookies for the site, such as session or auth cookies)
– http://bank.com/xfer?amt=1000&toAcct=joe HTTP GET method
– <form action=/xfer method=POST> HTTP POST method
<input type=text name=amt>
<input type=text name=toAcct>
</form>
• Web application fails to distinguish between a user
initiated request and an attack
• Mitigate by using a large random nonce

85
Cross Site Request Forgery (CSRF)
1. User loads bad page from web server
– XSS – Fake server
– Bad guy’s server – Compromised server
2. Web browser makes a request to the victim web server
directed by bad page
– Tags such as
<img src=‘http://bank.com/xfer?amt=1000&toAcct=evil37’>
– JavaScript
3. Victim web server processes request and assumes
request from browser is valid
– Session IDs in cookies are automatically sent along
SSL does not help – channel security is not an issue here

86
Successful CSRF Attack
•••
String id = response.getCookie(“user”);
userAcct = GetAcct(id);
If (userAcct != null) {
deposits.xfer(userAcct, toAcct, amount);
}
<html>
•••
<img src=‘http://bank.com/xfer?amt=1000&toAcct=evil37’>
•••
</html>
http://bank.com/xfer?amt=1000&toAcct=evil37
2. evil.com returns HTML
3. Browser sends attack
4. bank.com server code handles request
http://evil.com
1. User visits evil.com

87
CSRF Mitigation
•••
String nonce = (String)session.getAttribute(“nonce”);
String id = response.getCookie(“user”);
if (Utils.isEmpty(nonce)
|| !nonce.equals(getParameter(“nonce”) {
Login(); // no nonce or bad nonce, force login
return; // do NOT perform request
} // nonce added to all URLs and forms
userAcct = GetAcct(id);
if (userAcct != null) {
deposits.xfer(userAcct, toAcct, amount);
}
2. evil.com returns HTML
3. Browser sends attack
4. bank.com server code correctly handles request
1. User visits evil.com Very unlikely
attacker will
provide correct
nonce

Session Hijacking
• Session IDs identify a user’s session in
web applications.
• Obtaining the session ID allows
impersonation
• Attack vectors:
– Intercept the traffic that contains the ID value
– Guess a valid ID value (weak randomness)
– Discover other logic flaws in the sessions
handling process
88

Good Session ID Properties
• Hard to guess
– Large entropy (big random number)
– No patterns in IDs issued
• No reuse
89
http://xkcd.com/221

Session Hijacking Mitigation
• Create new session id after
– Authentication
– switching encryption on
– other attributes indicate a host change (IP address
change)
• Encrypt to prevent obtaining session ID through
eavesdropping
• Expire IDs after short inactivity to limit exposure of
guessing or reuse of illicitly obtained IDs
• Entropy should be large to prevent guessing
• Invalidate session IDs on logout and provide logout
functionality
90

Session Hijacking Example
1. An insecure web application accepts and
reuses a session ID supplied to a login page.
2. Attacker tricked user visits the web site
using attacker chosen session ID
3. User logs in to the application
4. Application creates a session using attacker
supplied session ID to identify the user
5. The attacker uses session ID to impersonate
the user
91

92
Successful Hijacking Attack
1. Tricks user to visit
if(HttpServletRequest.getRequestedSessionId() == null)
{
HttpServletRequest.getSession(true);
}
...
http://bank.com/login;JSESSIONID=123
3. Creates the
session
HTTP/1.1 200 OK
Set-Cookie:
JSESSIONID=123
2. User Logs In
http://bank.com/home
Cookie: JSESSIONID=123
4. Impersonates the user

93
Mitigated Hijacking Attack
1. Tricks user to visit
HttpServletRequest.invalidate();
HttpServletRequest.getSession(true);
...
3. Creates the
session
HTTP/1.1 200 OK
Set-Cookie:
JSESSIONID=XXX
2. User Logs In
4. Impersonates the user
http://bank.com/home
Cookie: JSESSIONID=123

Open Redirect
(AKA: URL Redirection to Untrusted Site, and Unsafe URL Redirection)
• Description
– Web app redirects user to malicious site chosen
by attacker
• URL parameter (reflected)
http://bank.com/redir?url=http://evil.com
• Previously stored in a database (persistent)
– User may think they are still at safe site
– Web app uses user supplied data in redirect URL
• Mitigations
– Use white list of tokens that map to acceptable
redirect URLs
– Present URL and require explicit click to navigate
to user supplied URLs
94

Open Redirect Example
1. User receives phishing e-mail with URL
http://www.bank.com/redir?url=http://evil.com
2. User inspects URL, finds hostname valid for
their bank
3. User clicks on URL
4. Bank’s web server returns a HTTP redirect
response to malicious site
5. User’s web browser loads the malicious site
that looks identical to the legitimate one
6. Attacker harvests user’s credentials or other
information
95

96
Successful Open Redirect Attack
String url = request.getParameter("url");
if (url != null) {
response.sendRedirect( url );
}
http://bank.com/redir?url=http://evil.com2. Opens
3. Web server redirects Location: http://evil.com
5. Browser displays
forgery
<h1>Welcome to bank.com<h1>
Please enter your PIN ID:
<from action="login">
•••
4. Browser requests http://evil.com
Dear bank.com costumer,
Because of unusual number of invalid login
attempts...
<a href="http://bank.com/redir?url=http://evil.com">
Sign in to verify</a>
1. User receives phishing e-mail

97
Open Redirect Mitigation
http://bank.com/redir?url=http://evil.com2. Opens
3. bank.com server code correctly handles
request
boolean isValidRedirect(String url) {
List<String> validUrls = new ArrayList<String>();
validUrls.add("index");
validUrls.add("login");
return (url != null && validUrls.contains(url));
}
•••
if (!isValidRedirect(url)){
response.sendError(response.SC_NOT_FOUND, "Invalid URL");
•••
404 Invalid
URL
Dear bank.com costumer,
•••
1. User receives phishing e-mail

99
General Software Engineering
• Don’t trust user data
– You don’t know where that data has been
• Don’t trust your own client software either
– It may have been modified, so always revalidate data at the
server.
• Don’t trust your operational configuration either
– If your program can test for unsafe conditions, do so and quit
• Don’t trust your own code either
– Program defensively with checks in high and low level functions
• KISS - Keep it simple, stupid
– Complexity kills security, its hard enough assessing simple code

100
Denial of Service
• Description
– Programs becoming unresponsive due to over
consumption of a limited resource or unexpected
termination.
• General causes
– Not releasing resources
– Crash causing bugs
– Infinite loops or data causing algorithmic complexity to
consume excessive resources
– Failure to limit data sizes
– Failure to limit wait times
– Leaks of scarce resources (memory, file descriptors)

101
Information Leaks
• Description
– Inadvertent divulgence of sensitive information
• General causes
– Reusing buffers without completely erasing
– Providing extraneous information that an
adversary may not be able to otherwise obtain
• Generally occurs in error messages
• Give as few details as possible
• Log full details to a database and return id to user,
so admin can look up details if needed

102
Information Leaks
• General causes (cont.)
– Timing attacks where the duration of the
operation depends on secret information
– Lack of encryption when using observable
channels
– Allowing secrets on devices where they can't
be erased such as swap space (mlock
prevents this) or backups

103
General Software Engineering
• Don’t trust user data
– You don’t know where that data has been
• Don’t trust your own client software either
– It may have been modified, so always revalidate data
at the server.
• Don’t trust your own code either
– Program defensively with checks in high and low level
functions
• KISS - Keep it simple, stupid
– Complexity kills security, its hard enough assessing
simple code

104
Let the Compiler Help
• Turn on compiler warnings and fix problems
• Easy to do on new code
• Time consuming, but useful on old code
• Use lint, multiple compilers
• -Wall is not enough!
gcc: -Wall, -W, -O2, -Werror, -Wshadow,
-Wpointer-arith, -Wconversion, -Wcast-qual,
-Wwrite-strings, -Wunreachable-code and many
more
– Many useful warning including security related
warnings such as format strings and integers

105
Let the Perl Compiler Help
• -w - produce warning about suspect code
and runtime events
• use strict - fail if compile time
• use Fatal - cause built-in function to raise
an exception on error instead of returning
an error code
• use diagnostics - better diagnostic
messages

106
Perl Taint Mode
• Taint mode (-T) prevents data from untrusted
sources from being used in dangerous ways
• Untrusted sources
– Data read from a file descriptor
– Command line arguments
– Environment
– User controlled fields in password file
– Directory entries
– Link referents
– Shared memory
– Network messages
• Environment sanitizing required for exec
– IFS PATH CDPATH ENV BASH_ENV

107
Books
• Viega, J. & McGraw, G. (2002). Building Secure Software:
How to Avoid Security Problems the Right Way. Addison-
Wesley.
• Seacord, R. C. (2005). Secure Coding in C and C++.
Addison-Wesley.
• Seacord, R. C. (2009). The CERT C Secure Coding
Standard, Addison-Wesley.
• McGraw, G. (2006). Software security: Building Security
In. Addison-Wesley.
• Dowd, M., McDonald, J., & Schuh, J. (2006). The Art of
Software Assessment: Identifying and Preventing
Software Vulnerabilities. Addison-Wesley.

108
Would you like this tutorial (and related
ones) taught at your site?
Tutorials for users, developers,
administrators and managers:
– Security Risks
– Secure Programming
– Vulnerability Assessment
Contact us!
Barton P. Miller
bart@cs.wisc.edu
Elisa Heymann

109
Secure Coding Practices
for Middleware
Barton P. Miller
James A. Kupsch
bart@cs.wisc.edu
Elisa Heymann
http://www.cs.wisc.edu/mist/
http://www.cs.wisc.edu/mist/papers/VAshort.pdf

110
Questions?
http://www.cs.wisc.edu/mist

Secure Coding Practices for Middleware

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