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Secure programming techniques(Based on: “Programming Secure Applications
for Unix-Like Systems,” David Wheeler)
Overview
Validate all input
Avoid buffer overflows
Program internals…
Careful calls to other resources
Send info back intelligently
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…
Validating input
Filenames– Disallow *, .., etc.
Html, URLs, cookies– Cf. cross-site scripting attacks
Command-line arguments– Even argv[0]…
Config files
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
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
Calling other resources
Use only “safe” library routines
Limit call parameters to valid values– Avoid metacharacters
Avoid calling the shell
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…
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…
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
Run-time checking: StackGuard
Embed “canaries” in stack frames and verify their integrity prior to function return
strretsfplocaltopof
stackcanarystrretsfplocal canary
Frame 1Frame 2
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
Canaries, continued…
StackGuard implemented as a GCC patch– Program must be recompiled
Minimal performance effects:
Not foolproof…
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
destret-addrsfptopof
stacksrc buf ret-addrsfp
libsafe main
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
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
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)
Code signing
Code producer signs code
Binary notion of trust
What if code producer compromised?
Lack of PKI => non-scalable approach
“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
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
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…
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.
Sandbox components
Verifier, Class loader, and Security Manager
If any of these fail, security may be compromised
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
Verifier…
Checks:– Byte code is well-formatted– No forged pointers– No violation of access restrictions– No incorrect typing
Of course, cannot be perfect…
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
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!
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
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
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…
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
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
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
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…
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
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
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…