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"I'm an enthusiastic supporter of the CERT Secure Coding Initiative. Programmers have lots of sources of advice on correctness, clarity, maintainability, performance, and even safety. Advice on how specific language features affect security has been missing. The CERT (R) C Secure Coding Standard fills this need."-Randy Meyers, Chairman of ANSI C"For years we have relied upon the CERT/CC to publish advisories documenting an endless stream of security problems. Now CERT has embodied the advice of leading technical experts to give programmers and managers the practical guidance needed to avoid those problems in new applications and to help secure legacy systems. Well done!"-Dr. Thomas Plum, founder of Plum Hall, Inc."Connectivity has sharply increased the need for secure, hacker-safe applications. By combining this CERT standard with other safety guidelines, customers gain all-round protection and approach the goal of zero-defect software."-Chris Tapp, Field Applications Engineer, LDRA Ltd."I've found this standard to be an indispensable collection of expert information on exactly how modern software systems fail in practice. It is the perfect place to start for establishing internal secure coding guidelines. You won't find this information elsewhere, and, when it comes to software security, what you don't know is often exactly what hurts you."-John McDonald, coauthor of The Art of Software Security AssessmentSoftware security has major implications for the operations and assets of organizations, as well as for the welfare of individuals. To create secure software, developers must know where the dangers lie. Secure programming in C can be more difficult than even many experienced programmers believe.This book is an essential desktop reference documenting the first official release of The CERT (R) C Secure Coding Standard. The standard itemizes those coding errors that are the root causes of software vulnerabilities in C and prioritizes them by severity, likelihood of exploitation, and remediation costs. Each guideline provides examples of insecure code as well as secure, alternative implementations. If uniformly applied, these guidelines will eliminate the critical coding errors that lead to buffer overflows, format string vulnerabilities, integer overflow, and other common software vulnerabilities.
Contents
Preface xviiAcknowledgments xxxiAbout the Author xxxiiiChapter 1: Using This Standard 1System Qualities 1Automatically Generated Code 2Compliance 3Chapter 2: Preprocessor (PRE) 5PRE00-C. Prefer inline or static functions to function-like macros 6PRE01-C. Use parentheses within macros around parameter names 11PRE02-C. Macro replacement lists should be parenthesized 13PRE03-C. Prefer type definitions to defines for encoding types 15PRE04-C. Do not reuse a standard header file name 16PRE05-C. Understand macro replacement when concatenating tokens or performing stringification 18PRE06-C. Enclose header files in an inclusion guard 21PRE07-C. Avoid using repeated question marks 22PRE08-C. Guarantee that header file names are unique 24PRE09-C. Do not replace secure functions with less secure functions 26PRE10-C. Wrap multistatement macros in a do-while loop 27PRE30-C. Do not create a universal character name through concatenation 29PRE31-C. Never invoke an unsafe macro with arguments containing assignment, increment, decrement, volatile access, or function call 30Chapter 3: Declarations and Initialization (DCL) 33DCL00-C. const-qualify immutable objects 35DCL01-C. Do not reuse variable names in subscopes 36DCL02-C. Use visually distinct identifiers 38DCL03-C. Use a static assertion to test the value of a constant expression 39DCL04-C. Do not declare more than one variable per declaration 42DCL05-C. Use type definitions to improve code readability 44DCL06-C. Use meaningful symbolic constants to represent literal values in program logic 45DCL07-C. Include the appropriate type information in function declarators 51DCL08-C. Properly encode relationships in constant definitions 54DCL09-C. Declare functions that return an errno error code with a return type of errno_t 57DCL10-C. Maintain the contract between the writer and caller of variadic functions 59DCL11-C. Understand the type issues associated with variadic functions 62DCL12-C. Implement abstract data types using opaque types 64DCL13-C. Declare function parameters that are pointers to values not changed by the function as const 66DCL14-C. Do not make assumptions about the order of global variable initialization across translation units 69DCL15-C. Declare objects that do not need external linkage with the storage-class specifier static 70DCL30-C. Declare objects with appropriate storage durations 72DCL31-C. Declare identifiers before using them 74DCL32-C. Guarantee that mutually visible identifiers are unique 78DCL33-C. Ensure that restrict-qualified source and destination pointers in function arguments do not reference overlapping objects 80DCL34-C. Use volatile for data that cannot be cached 82DCL35-C. Do not convert a function using a type that does not match the function definition 84DCL36-C. Do not declare an identifier with conflicting linkage classifications 87Chapter 4: Expressions (EXP) 91EXP00-C. Use parentheses for precedence of operation 93EXP01-C. Do not take the size of a pointer to determine the size of the pointed-to type 95EXP02-C. Be aware of the short-circuit behavior of the logical AND and OR operators 96EXP03-C. Do not assume the size of a structure is the sum of the sizes of its members 98EXP04-C. Do not perform byte-by-byte comparisons between structures 100EXP05-C. Do not cast away a const qualification 102EXP06-C. Operands to the sizeof operator should not contain side effects 104EXP07-C. Do not diminish the benefits of constants by assuming their values in expressions 105EXP08-C. Ensure pointer arithmetic is used correctly 107EXP09-C. Use sizeof to determine the size of a type or variable 109EXP10-C. Do not depend on the order of evaluation of subexpressions or the order in which side effects take place 111EXP11-C. Do not apply operators expecting one type to data of an incompatible type 114EXP12-C. Do not ignore values returned by functions 118EXP30-C. Do not depend on order of evaluation between sequence points 119EXP31-C. Avoid side effects in assertions 122EXP32-C. Do not cast away a volatile qualification 123EXP33-C. Do not reference uninitialized memory 124EXP34-C. Ensure a null pointer is not dereferenced 128EXP35-C. Do not access or modify the result of a function call after a subsequent sequence point 129EXP36-C. Do not convert pointers into more strictly aligned pointer types 131EXP37-C. Call functions with the arguments intended by the API 133EXP38-C. Do not call offsetof() on bit-field members or invalid types 135Chapter 5: Integers (INT) 139INT00-C. Understand the data model used by your implementation(s) 141INT01-C. Use rsize_t or size_t for all integer values representing the size of an object 145INT02-C. Understand integer conversion rules 149INT03-C. Use a secure integer library 153INT04-C. Enforce limits on integer values originating from untrusted sources 155INT05-C. Do not use input functions to convert character data if they cannot handle all possible inputs 157INT06-C. Use strtol() or a related function to convert a string token to an integer 159INT07-C. Use only explicitly signed or unsigned char type for numeric values 162INT08-C. Verify that all integer values are in range 164INT09-C. Ensure enumeration constants map to unique values 167INT10-C. Do not assume a positive remainder when using the % operator 168INT11-C. Take care when converting from pointer to integer or integer to pointer 170INT12-C. Do not make assumptions about the type of a plain int bit-field when used in an expression 172INT13-C. Use bitwise operators only on unsigned operands 174INT14-C. Avoid performing bitwise and arithmetic operations on the same data 175INT15-C. Use intmax_t or uintmax_t for formatted I/O on programmer-defined integer types 178INT30-C. Ensure that unsigned integer operations do not wrap 181INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data 186INT32-C. Ensure that operations on signed integers do not result in overflow 191INT33-C. Ensure that division and modulo operations do not result in divide-by-zero errors 201INT34-C. Do not shift a negative number of bits or more bits than exist in the operand 203INT35-C. Evaluate integer expressions in a larger size before comparing or assigning to that size 207Chapter 6: Floating Point (FLP) 211FLP00-C. Understand the limitations of floating-point numbers 212FLP01-C. Take care in rearranging floating-point expressions 214FLP02-C. Consider avoiding floating-point numbers when precise computation is needed 215FLP03-C. Detect and handle floating-point errors 218FLP30-C. Do not use floating-point variables as loop counters 224FLP31-C. Do not call functions expecting real values with complex values 226FLP32-C. Prevent or detect domain and range errors in math functions 227FLP33-C. Convert integers to floating point for floating-point operations 234FLP34-C. Ensure that floating-point conversions are within range of the new type 236Chapter 7: Arrays (ARR) 241ARR00-C. Understand how arrays work 242ARR01-C. Do not apply the sizeof operator to a pointer when taking the size of an array 245ARR02-C. Explicitly specify array bounds, even if implicitly defined by an initializer 248ARR30-C. Guarantee that array indices are within the valid range 250ARR31-C. Use consistent array notation across all source files 251ARR32-C. Ensure size arguments for variable length arrays are in a valid range 254ARR33-C. Guarantee that copies are made into storage of sufficient size 255ARR34-C. Ensure that array types in expressions are compatible 258ARR35-C. Do not allow loops to iterate beyond the end of an array 259ARR36-C. Do not subtract or compare two pointers that do not refer to the same array 261ARR37-C. Do not add or subtract an integer to a pointer to a non-array object 263ARR38-C. Do not add or subtract an integer to a pointer if the resulting value does not refer to a valid array element 265Chapter 8: Characters and Strings (STR) 271STR00-C. Represent characters using an appropriate type 273STR01-C. Adopt and implement a consistent plan for managing strings 275STR02-C. Sanitize data passed to complex subsystems 276STR03-C. Do not inadvertently truncate a null-terminated byte string 280STR04-C. Use plain char for characters in the basic character set 282STR05-C. Use pointers to const when referring to string literals 284STR06-C. Do not assume that strtok() leaves the parse string unchanged 286STR07-C. Use TR 24731 for remediation of existing string manipulation code 288STR08-C. Use managed strings for development of new string manipulation code 291STR30-C. Do not attempt to modify string literals 293STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator 294STR32-C. Null-terminate byte strings as required 299STR33-C. Size wide character strings correctly 303STR34-C. Cast characters to unsigned types before converting to larger integer sizes 305STR35-C. Do not copy data from an unbounded source to a fixed-length array 307STR36-C. Do not specify the bound of a character array initialized with a string literal 312STR37-C. Arguments to character-handling functions must be representable as an unsigned char 314Chapter 9: Memory Management (MEM) 317MEM00-C. Allocate and free memory in the same module at the same level of abstraction 319MEM01-C. Store a new value in pointers immediately after free() 322MEM02-C. Immediately cast the result of a memory allocation function call into a pointer to the allocated type 324MEM03-C. Clear sensitive information stored in reusable resources returned for reuse 328MEM04-C. Do not perform zero-length allocations 332MEM05-C. Avoid large stack allocations 335MEM06-C. Ensure that sensitive data is not written out to disk 338MEM07-C. Ensure that the arguments to calloc(), when multiplied, can be represented as a size_t 342MEM08-C. Use realloc() only to resize dynamically allocated arrays 343MEM09-C. Do not assume memory allocation routines initialize memory 346MEM10-C. Use a pointer validation function 348MEM30-C. Do not access freed memory 351MEM31-C. Free dynamically allocated memory exactly once 353MEM32-C. Detect and handle memory allocation errors 355MEM33-C. Use the correct syntax for flexible array members 358MEM34-C. Only free memory allocated dynamically 360MEM35-C. Allocate sufficient memory for an object 362Chapter 10: Input/Output (FIO) 367FIO00-C. Take care when creating format strings 370FIO01-C. Be careful using functions that use file names for identification 372FIO02-C. Canonicalize path names originating from untrusted sources 374FIO03-C. Do not make assumptions about fopen() and file creation 383FIO04-C. Detect and handle input and output errors 386FIO05-C. Identify files using multiple file attributes 389FIO06-C. Create files with appropriate access permissions 394FIO07-C. Prefer fseek() to rewind() 398FIO08-C. Take care when calling remove() on an open file 399FIO09-C. Be careful with binary data when transferring data across systems 401FIO10-C. Take care when using the rename() function 403FIO11-C. Take care when specifying the mode parameter of fopen() 407FIO12-C. Prefer setvbuf() to setbuf() 408FIO13-C. Never push back anything other than one read character 409FIO14-C. Understand the difference between text mode and binary mode with file streams 411FIO15-C. Ensure that file operations are performed in a secure directory 413FIO16-C. Limit access to files by creating a jail 418FIO30-C. Exclude user input from format strings 421FIO31-C. Do not simultaneously open the same file multiple times 424FIO32-C. Do not perform operations on devices that are only appropriate for files 426FIO33-C. Detect and handle input output errors resulting in undefined behavior 431FIO34-C. Use int to capture the return value of character I/O functions 436FIO35-C. Use feof() and ferror() to detect end-of-file and file errors when sizeof(int) == sizeof(char) 438FIO36-C. Do not assume a new-line character is read when using fgets() 440FIO37-C. Do not assume character data has been read 442FIO38-C. Do not use a copy of a FILE object for input and output 443FIO39-C. Do not alternately input and output from a stream without an intervening flush or positioning call 444FIO40-C. Reset strings on fgets() failure 446FIO41-C. Do not call getc() or putc() with stream arguments that have side effects 448FIO42-C. Ensure files are properly closed when they are no longer needed 450FIO43-C. Do not create temporary files in shared directories 454FIO44-C. Only use values for fsetpos() that are returned from fgetpos() 464Chapter 11: Environment (ENV) 467ENV00-C. Do not store the pointer to the string returned by getenv() 468ENV01-C. Do not make assumptions about the size of an environment variable 474ENV02-C. Beware of multiple environment variables with the same effective name 475ENV03-C. Sanitize the environment when invoking external programs 478ENV04-C. Do not call system() if you do not need a command processor 482ENV30-C. Do notmodify the string returned by getenv() 487ENV31-C. Do not rely on an environment pointer following an operation that may invalidate it 489ENV32-C. No atexit handler should terminate in any way other than by returning 494Chapter 12: Signals (SIG) 499SIG00-C. Mask signals handled by noninterruptible signal handlers 500SIG01-C. Understand implementation-specific details regarding signal handler persistence 503SIG02-C. Avoid using signals to implement normal functionality 507SIG30-C. Call only asynchronous-safe functions within signal handlers 511SIG31-C. Do not access or modify shared objects in signal handlers 517SIG32-C. Do not call longjmp() from inside a signal handler 519SIG33-C. Do not recursively invoke the raise() function 523SIG34-C. Do not call signal() from within interruptible signal handlers 526Chapter 13: Error Handling (ERR) 531ERR00-C. Adopt and implement a consistent and comprehensive error-handling policy 533ERR01-C. Use ferror() rather than errno to check for FILE stream errors 535ERR02-C. Avoid in-band error indicators 537ERR03-C. Use runtime-constraint handlers when calling functions defined by TR 24731-1 541ERR04-C. Choose an appropriate termination strategy 544ERR05-C. Application-independent code should provide error detection without dictating error handling 549ERR06-C. Understand the termination behavior of assert() and abort() 556ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only after the function returns a value indicating failure 558ERR31-C. Do not redefine errno 563ERR32-C. Do not rely on indeterminate values of errno 564Chapter 14: Miscellaneous (MSC) 569MSC00-C. Compile cleanly at high warning levels 570MSC01-C. Strive for logical completeness 572MSC02-C. Avoid errors of omission 574MSC03-C. Avoid errors of addition 576MSC04-C. Use comments consistently and in a readable fashion 578MSC05-C. Do not manipulate time_t typed values directly 580MSC06-C. Be aware of compiler optimization when dealing with sensitive data 582MSC07-C. Detect and remove dead code 585MSC08-C. Library functions should validate their parameters 588MSC09-C. Character encoding: use subset of ASCII for safety 590MSC10-C. Character encoding: UTF-8-related issues 594MSC11-C. Incorporate diagnostic tests using assertions 597MSC12-C. Detect and remove code that has no effect 598MSC13-C. Detect and remove unused values 600MSC14-C. Do not introduce unnecessary platform dependencies 602MSC15-C. Do not depend on undefined behavior 604MSC30-C. Do not use the rand() function for generating pseudorandom numbers 607MSC31-C. Ensure that return values are compared against the proper type 610Appendix: POSIX (POS) 613POS00-C. Avoid race conditions with multiple threads 615POS01-C. Check for the existence of links 617POS02-C. Follow the principle of least privilege 620POS30-C. Use the readlink() function properly 623POS31-C. Do not unlock or destroy another thread's mutex 625POS32-C. Include a mutex when using bit-fields in a multithreaded environment 626POS33-C. Do not use vfork() 629POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument 631POS35-C. Avoid race conditions while checking for the existence of a symbolic link 633POS36-C. Observe correct revocation order while relinquishing privileges 636POS37-C. Ensure that privilege relinquishment is successful 637Glossary 643References 647Index 659
