The principles of secure programming, such as input validation, avoiding buffer overflow, secure coding. Some briefs about source code scanners are also discussed.

1. CMSC 414
Computer and Network Security
Lecture 25
Jonathan Katz

2. Secure programming techniques
(Based on: “Programming Secure Applications
for Unix-Like Systems,” David Wheeler)

3. Overview
 Validate all input
 Avoid buffer overflows
 Program internals…
 Careful calls to other resources
 Send info back intelligently

4. Validating input
 Determine acceptable input, check for
match --- don’t just check against list of
“non-matches”
– Limit maximum length
– Watch out for special characters, escape chars.
 Check bounds on integer values
– E.g., sendmail bug…

5. Validating input
 Filenames
– Disallow *, .., etc.
 Html, URLs, cookies
– Cf. cross-site scripting attacks
 Command-line arguments
– Even argv[0]…
 Config files

6. Avoiding buffer overflows
 Use arrays instead of pointers (cf. Java)
 Avoid strcpy(), strcat(), etc.
– Use strncpy(), strncat(), instead
– Even these are not perfect… (e.g., no null
termination)
 Make buffers (slightly) longer than
necessary to avoid “off-by-one” errors

7. Program internals…
 Avoid race conditions
– E.g., authorizing file access, then opening file
 Watch out for temporary files in shared directories
(e.g., /tmp)
 Watch out for “spoofed” IP addresses/email
addresses
 Simple, open design; fail-safe defaults; completge
mediation; etc.
 Don’t write your own crypto algorithms
– Use crypto appropriately

8. Calling other resources
 Use only “safe” library routines
 Limit call parameters to valid values
– Avoid metacharacters
 Avoid calling the shell

9. User output
 Minimize feedback
– Don’t explain failures to untrusted users
– Don’t release version numbers…
– Don’t offer “too much” help (suggested
filenames, etc.)
 Don’t use printf(userInput)
– Use printf(“%s”, userInput) instead…

10. Source code scanners
 Used to check source code
– E.g., flawfinder, cqual
 “Static” analysis vs. “dynamic” analysis
– Not perfect
– Dynamic analysis can slow down execution,
lead to bloated code
– Will see examples of dynamic analysis later…

11. “Higher-level” techniques

12. Addressing buffer overflows
 Basic stack exploit can be prevented by marking
stack segment as non-executable, or randomizing stack
location.
– Code patches exist for Linux and Solaris.
– Some complications on x86.
 Problems:
– Does not defend against `return-to-libc’ exploit.
• Overflow sets ret-addr to address of libc function.
– Some apps need executable stack (e.g. LISP interpreters).
– Does not block more general overflow exploits:
• Overflow on heap: overflow buffer next to func pointer.
 Patch not shipped by default for Linux and Solaris

13. Run-time checking: StackGuard
 Embed “canaries” in stack frames and
verify their integrity prior to function return
str
ret
sfp
local
top
of
stack
canary
str
ret
sfp
local canary
Frame 1
Frame 2

14. Canary types
 Random canary: (used in Visual Studio 2003)
– Choose random string at program startup.
– Insert canary string into every stack frame.
– Verify canary before returning from function.
– To corrupt random canary, attacker must learn current
random string.
 Terminator canary:
Canary = 0, newline, linefeed, EOF
– String functions will not copy beyond terminator.
– Attacker cannot use string functions to corrupt stack

15. Canaries, continued…
 StackGuard implemented as a GCC patch
– Program must be recompiled
 Minimal performance effects:
 Not foolproof…

16. Run-time checking: Libsafe
 Intercepts calls to strcpy (dest, src)
– Validates sufficient space in current stack
frame:
|frame-pointer – dest| > strlen(src)
– If so, does strcpy.
Otherwise, terminates application
dest
ret-addr
sfp
top
of
stack
src buf ret-addr
sfp
libsafe main

17. More methods …
 Address obfuscation
– Encrypt return address on stack by XORing
with random string. Decrypt just before
returning from function.
– Attacker needs decryption key to set return
address to desired value.
 PaX ASLR: Randomize location of libc
– Attacker cannot jump directly to exec function

18. Software fault isolation
 Partition code into data and code segments
 Code inserted before each load/store/jump
– Verify that target address is safe
 Can be done at compiler, link, or run time
– Increases program size, slows down execution

19. Security for mobile code
 Mobile code is particularly dangerous!
 Sandboxing
– Limit the ability of code to do harmful things
 Code-signing
– Mechanism to decide whether code should be
trusted or not
 ActiveX uses code-signing, Java uses
sandboxing (plus code-signing)

20. Code signing
 Code producer signs code
 Binary notion of trust
 What if code producer compromised?
 Lack of PKI => non-scalable approach

21. “Proof-carrying code”
 Input: code, safety policy of client
 Output: safety proof for code
 Proof generation expensive
– Proof verification cheaper
– Prove once, use everywhere (with same policy)
 Prover/compiler need not be trusted
– Only need to trust the verifier

22. Sandboxing in Java
 Focus on preventing system modification
and violations of user privacy
– Denial of service attacks much harder to
prevent, and not handled all that well
 We will discuss some of the basics, but not
all the most up-to-date variants of the Java
security model

23. Sandboxing
 A default sandbox applied to untrusted code
 Users can change the defaults…
– Can also define “larger” sandboxes for
“partially trusted” code
– Trust in code determined using code-signing…

24. Some examples…
 Default sandbox prevents:
– Reading/writing/deleting files on client system
– Listing directory contents
– Creating new network connections to other
hosts (other than originating host)
– Etc.

25. Sandbox components
 Verifier, Class loader, and Security Manager
 If any of these fail, security may be
compromised

26. Verifier
 Java program is compiled to platform-
independent Java byte code
 This code is verified before it is run
– Prevents, for example, malicious “hand-
written” byte code
 Efficiency gains by checking code before it
is run, rather than constantly checking it
while running

27. Verifier…
 Checks:
– Byte code is well-formatted
– No forged pointers
– No violation of access restrictions
– No incorrect typing
 Of course, cannot be perfect…

28. Class loader
 Helps prevent “spoofed” classes from being
loaded
– E.g., external class claiming to be the security
manager
 Whenever a class needs to be loaded, this is
done by a class loader
– The class loader decides where to obtain the
code for the class

29. Security manager
 Restricts the way an applet uses Java API
calls
– All calls to the OS are mediated by the security
manager
 Security managers are browser-dependent!

30. System call monitoring
 Monitor all system calls
– Enforce particular policy
– Policy may be loaded in kernel
 Hand-tune policy for individual applications
 Similar to Java security manager
– Difference in where implemented

31. Viruses/malicious code

32. Viruses/malicious code
 Virus – passes malicious code to other non-
malicious programs
– Or documents with “executable” components
 Trojan horse – software with unintended
side effects
 Worm – propagates via network
– Typically stand-alone software, in contrast to
viruses which are attached to other programs

33. Viruses
 Can insert themselves before program, can
surround program, or can be interspersed
throughout program
– In the last case, virus writer needs to know
about the specifics of the other program
 Two ways to “insert” virus:
– Insert virus in memory at (old) location of
original program
– Change pointer structure…

34. Viruses…
 Boot sector viruses
– If a virus is loaded early in the boot process,
can be very difficult (impossible?) to detect
 Memory-resident viruses
– Note that virus might complicate its own
detection
– E.g., removing virus name from list of active
programs, or list of files on disk

35. Some examples
 BRAIN virus
– Locates itself in upper memory; resets the
upper memory bound below itself
– Traps “disk reads” so that it can handle any
requests to read from the boot sector
– Not inherently malicious, although some
variants were

36. Morris worm (1988)
 Resource exhaustion (unintended)
– Was supposed to have only one copy running, but did
not work correctly…
 Spread in three ways
– Exploited buffer overflow flaw in fingerd
– Exploited flaw in sendmail debug mode
– Guessing user passwords(!) on current network
 Bootstrap loader would be used to obtain the rest
of the worm

37. Chernobyl virus (1998)
 When infected program run, virus becomes
resident in memory of machine
– Rebooting does not help
 Virus writes random garbage to hard drive
 Attempts to trash FLASH BIOS
– Physically destroys the hardware…

38. Melissa virus/worm (1999)
 Word macro…
– When file opened, would create and send
infected document to names in user’s Outlook
Express mailbox
– Recipient would be asked whether to disable
macros(!)
• If macros enabled, virus would launch

39. Code red (2001)
 Propagated itself on web server running
Microsoft’s Internet Information Server
– Infection using buffer overflow…
– Propagation by checking IP addresses on port
80 of the PC to see if they are vulnerable

40. Detecting viruses
 Can try to look for “signatures”
– Unreliable unless up-to-date
– Encrypted viruses
– Polymorphic viruses
 Examine storage
– Sizes of files, “jump” instruction at beginning of code
– Can be hard to distinguish from normal software
 Check for (unusual) execution patterns
– Hard to distinguish from normal software…