CMSC 414 Computer and Network Security Lecture 25 Jonathan Katz

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/ 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…

“Higher-level” techniques

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 strretsfplocal top of stack canarystrretsfp local 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-addrsfp top of stack srcbufret-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…