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Secure Coding

James Walden Northern Kentucky University. Secure Coding. Topics. Error Handling Return Codes Exceptions Logging Memory Allocation Using and Storing Passwords Protecting Secrets in Memory. Security Impact of Error Handling. Information leakage Stack traces Database errors

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Secure Coding

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  1. James Walden Northern Kentucky University Secure Coding

  2. Topics • Error Handling • Return Codes • Exceptions • Logging • Memory Allocation • Using and Storing Passwords • Protecting Secrets in Memory

  3. CSC 666: Secure Software Engineering Security Impact of Error Handling Information leakage • Stack traces • Database errors Resource leakage • Return on error without de-allocation • Exceptions bypass de-allocation

  4. CSC 666: Secure Software Engineering Error Handling Techniques Return a neutral value: return a value that’s known to be harmless, i.e. a negative number, zero, or “”. Substitute the next piece of data: continue reading from hardware or file until a valid record is found. Return same answer as last time: don’t keep reading; instead return the last valid answer. Substitute closest legal value: if velocity has a range of 0..100, show a 0 when backing up. Log a warning message: Write a warning to a log, then continue on, perhaps using one of the other techniques. Terminate program: Terminate program execution. Return an error code: Report error by • Setting the value of a status variable (errno) • Return status as the function’s return value • Throw an exception

  5. CSC 666: Secure Software Engineering Return Codes Use function return code to indicate error. • Easy to ignore. Simply ignore return code. • Error handling logic is mixed with logic processing normal return codes. • No universal convention for error codes. Common return code patterns. • Negative values when nonnegative expected. • NULL values for pointer return codes.

  6. Example: character get functions fgetc(), getc(), getchar() read char, return int Use int to represent EOF error code. Incorrect example: return value is declared as a char char buf[BUFSIZ]; char c; inti = 0; while ( (c = getchar()) != '\n' && c != EOF ) if (i < BUFSIZ-1) { buf[i++] = c; } buf[i] = '\0'; /* terminate NTBS */ Correct example char buf[BUFSIZ]; int c; inti = 0; while (((c = getchar()) != '\n') && !feof(stdin) && !ferror(stdin)) if (i < BUFSIZ-1) { buf[i++] = c;} buf[i] = '\0'; /* terminate NTBS */

  7. CSC 666: Secure Software Engineering Resource Leaks Resources leak due to early returns • Memory • Filehandles Example char *getblock(intfd) { char *buf = (char *)malloc(1024); if (!buf) { return NULL; } if (read(fd, buf, 1024) != 1024) { return NULL; } return buf }

  8. CSC 666: Secure Software Engineering Using goto for error handling Problem: need to de-allocate resources on return. • Each return is different since • Different resources allocated at each point. Solution: single de-allocation point • Check if resource is allocated, then • De-allocate if it is, and • Return with appropriate error code. Why goto? • Avoids deep nesting. • Improves code readability. • Commonly used technique in kernel.

  9. CSC 666: Secure Software Engineering Fixed version with goto char *getblock(intfd) { char *buf = (char *)malloc(1024); if (!buf) { goto ERROR; } if (read(fd, buf, 1024) != 1024) { goto ERROR; } return buf; ERROR: if (buf) { free(buf); } return NULL; }

  10. CSC 666: Secure Software Engineering Exceptions Advantages of exceptions • Cannot be ignored by not checking for errors. • Separate main code from error code. Disadvantages of exceptions • Difficult to avoid resource leaks, as exceptions create many implicit control flow paths. • Can still ignore exceptions try { // code that can throw an exception } catch (AnException e) { // empty catch block }

  11. CSC 666: Secure Software Engineering Checked Exceptions Checked exceptions: Exceptions that the language requires client code to handle. • C++, C#: no checked exceptions • Java: exceptions that inherit from Exception Unchecked exceptions: Exceptions that can be ignored by client code. • C++, C#: all exceptions are unchecked • Java: exceptions that inherit from RuntimeException.

  12. CSC 666: Secure Software Engineering Exception Guarantees Levels of exception safety for a class. Basic Guarantee • No resources are leaked. Strong Guarantee • Exceptions leave state exactly as it was before the operation started. No Throw Guarantee • Component will handle all exceptions itself. No Exception Safety • Component may leak resources and leave object in an inconsistent unusable state.

  13. CSC 666: Secure Software Engineering Exception Safety Example void stack::push(int element) { top++; if( top == size-1 ) { int* buf = new int[size+=32]; if( buf == 0 ) throw “Out of memory”; for(inti = 0; i < top; i++) buf[i] = data[i]; delete [] data; data = buf; } data[top] = element; }

  14. CSC 666: Secure Software Engineering Catch-all Exception Handlers Ensure no information leakage at top level functions. doGet(), doPost(), web service entry points protected void doPost(HttpServletRequestreq, HttpServlet Response res) { try { /* function body */ } catch (Throwable t) { logger.error(“Top-level exception caught”, t); } } Do not do this in low level code. • Need to deal with individual error types separately, instead of ignoring them or handling generically.

  15. CSC 666: Secure Software Engineering Destructor De-Allocation Resource Acquisition Is Initialization design pattern • Resources acquired during initialization of object, before it can be used. • Resources are de-allocated by the object’s destructor, which occurs even via exceptions. Example file (const char* filename) { file_ = fopen(filename, “w+”); if (!file_) throw std::runtime_error("file open failure"); } ~file() { if (f) { fclose(file_); } }

  16. CSC 666: Secure Software Engineering Finally • Finally block executed regardless of whether an exception is caught or not. • Example Statement stmt = conn.createStatement(); try { stmt.execute(sqlString); } finally { if (stmt != null ) { stmt.close(); } }

  17. CSC 666: Secure Software Engineering Logging Frameworks Use a standard logging framework. • Provide single consistent view of system. • Facilitate changes, such as logging to a new system or to a database. Examples • syslog() • log4j • java.util.logging

  18. CSC 666: Secure Software Engineering Memory Allocation Strategies Static Buffer Allocation • Advantages: simple, easy to know bounds. • Disadvantages: inflexible, wastes memory. Dynamic Buffer Allocation • Advantages: flexible. • Disadvantages: must check for DoS attacks. Track Buffer Sizes typedefstruct { char* ptr; size_tbufsize; } buffer;

  19. Common Allocation Errors • Assuming that memory is zeroed. • Allocated memory contains junk, not zeros. • Failure to check that allocation succeeded. • Most C functions return a NULL pointer on failure. • new will throw std::bad_allocexception on failure. • Unless specify T* p = new(std::nothrow) T; • Use of invalid pointers. • Dereference NULLs, use after free, double free. • Failure to deallocate memory. • Memory leaks. • Zero-length allocations are implementation defined in C.

  20. CSC 666: Secure Software Engineering Inbound and Outbound Passwords Inbound Passwords • Used to authenticate users to application. • In cleartext only at point of user data entry. • Risks: online and offline password guessing. Outbound Passwords • Used to authenticate application to other systems, such as databases or CC processors. • Must be in cleartext to use. • Risks: information leakage.

  21. Securing Inbound Passwords

  22. Offline Password Cracking Password dictionary word = Next dictionary word wordhash= Hash(word) for each (username, hash) wordhash== hash Usernames + Hashed Passwords False True Store(usernames, word)

  23. CSC 666: Secure Software Engineering Hashing and Salting MD4 (Windows) • Unlimited password length. • Slow MD4 hash, no salt. SHA512crypt (Linux, Mac OS X) • Unlimited password length. • 5000 iterations of SHA-512 hash function. • 16 character salt. PBKDF2 (Password-Based Key Derivation Function 2) • Framework with configurable hash, iterations, salt. In .NET. Scrypt • Sequential, memory-hard hashing algorithm. • Defense against specialized hardware (GPUs, ASICs, FPGAs)

  24. CSC 666: Secure Software Engineering Outbound Passwords Used by app to auth to db, other systems. • Must be accessible in cleartext at some point. Solutions • Store in source code. • Easy to view in source or binary form. • Store cleartext in a configuration file. • Store encrypted in a configuration file. • Use a good, known algorithm like AES. • Limit ACLs so only app can access. • Require admin enter password on restart. • PCI 3.6.6 requires key be split among admins.

  25. CSC 666: Secure Software Engineering Secrets in Memory Attackers can obtain secrets from memory • Remote exploit: buffer overflow or fmt string • Physical attack: direct media access • Accidental leakage: core dumps or page files

  26. CSC 666: Secure Software Engineering Securing Secrets in Memory • Minimize time spent holding secrets. • Decrypt data just before use. • Overwrite data after use. • Share secrets sparingly. • Do not store secrets on the client. • Erase secrets securely. • Explicitly overwrite memory. • Prevent unnecessary duplication.

  27. CSC 666: Secure Software Engineering Locking Pages in Memory Prevent secrets from paging to disk. Does not prevent suspend or hibernate saving pages. Linux page locking mlock(const void *addr, size_tlen) munlock(const void *addr, size_tlen) Windows page locking VirtualLock(LPVOID lpAddress, SIZE_T dwSize); VirtualUnlock(LPVOID lpAddress, SIZE_T dwSize);

  28. CSC 666: Secure Software Engineering Erasing Secrets Securely Garbage collecting languages • Essentially impossible to ensure secrets are erased immediately. Low level languages • Compiler can optimize away code that overwrites a buffer if buffer contents are not used later. • Use memset_s() if compiler supports C11. • Use SecureZeroMemory() in Windows. • If neither function is available, use volatile pointers to prevent compiler from optimizing away memory overwrites. Some compilers may still cause problems.

  29. CSC 666: Secure Software Engineering Erasing Secrets Securely in C99 void auth_function() { char pass[32]; if (getpass(pass)) { // Do something with password } memset(pass, 0, sizeof(pass)); // Prevent memset from being optimized // away by using volatile pointers. *(volatile char *)pass = *(volatile char *)pass; }

  30. References • David Abrahams, Exception-Safety in Generic Components. Lecture Notes In Computer Science: 69-79, 2000. • Tom Cargill, Exception Handling: A False Sense of Security, C++ Report, Volume 6, Number 9, November-December 1994. • CERT, Error Handling, https://www.securecoding.cert.org/confluence/download/attachments/3524/error-handling.pdf, 2006. • Brian Chess and Jacob West, Secure Programming with Static Analysis, Addison-Wesley, 2007. • Robert J. Ellison et. al., Survivability: Protecting Your Critical Systems, IEEE Computer, 1999. • Fred Long, CERT Secure Coding Standards: Error Handling, https://www.securecoding.cert.org/confluence/display/cplusplus/12.+Error+Handling+(ERR), 2009. • Steve McConnell, Code Complete, 2nd edition, Microsoft Press, 2004.

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