Uncontrolled format string

Uncontrolled format string is a type of code injection vulnerability discovered around 1989 that can be used in security exploits. Originally thought harmless, format string exploits can be used to crash a program or to execute harmful code. The problem stems from the use of unchecked user input as the format string parameter in certain C functions that perform formatting, such as printf() . A malicious user may use the %s and %x format tokens, among others, to print data from the call stack or possibly other locations in memory. One may also write arbitrary data to arbitrary locations using the %n format token, which commands printf() and similar functions to write the number of bytes formatted to an address stored on the stack.

Uncontrolled format string is a type of code injection vulnerability discovered around 1989 that can be used in security exploits. Originally thought harmless, format string exploits can be used to crash a program or to execute harmful code. The problem stems from the use of unchecked user input as the format string parameter in certain C functions that perform formatting, such as printf() . A malicious user may use the %s and %x format tokens, among others, to print data from the call stack or possibly other locations in memory. One may also write arbitrary data to arbitrary locations using the %n format token, which commands printf() and similar functions to write the number of bytes formatted to an address stored on the stack. ==Details== A typical exploit uses a combination of these techniques to take control of the instruction pointer (IP) of a process, for example by forcing a program to overwrite the address of a library function or the return address on the stack with a pointer to some malicious shellcode. The padding parameters to format specifiers are used to control the number of bytes output and the %x token is used to pop bytes from the stack until the beginning of the format string itself is reached. The start of the format string is crafted to contain the address that the %n format token can then overwrite with the address of the malicious code to execute. This is a common vulnerability because format bugs were previously thought harmless and resulted in vulnerabilities in many common tools. MITRE's CVE project lists roughly 500 vulnerable programs as of June 2007, and a trend analysis ranks it the 9th most-reported vulnerability type between 2001 and 2006. Format string bugs most commonly appear when a programmer wishes to output a string containing user supplied data (either to a file, to a buffer, or to the user). The programmer may mistakenly write printf(buffer) instead of printf("%s", buffer) . The first version interprets buffer as a format string, and parses any formatting instructions it may contain. The second version simply prints a string to the screen, as the programmer intended. Both versions behave identically in the absence of format specifiers in the string, which makes it easy for the mistake to go unnoticed by the developer. Format bugs arise because C's argument passing conventions are not type-safe. In particular, the varargs mechanism allows functions to accept any number of arguments (e.g. printf ) by "popping" as many arguments off the call stack as they wish, trusting the early arguments to indicate how many additional arguments are to be popped, and of what types. Format string bugs can occur in other programming languages besides C, such as Perl, although they appear with less frequency and usually cannot be exploited to execute code of the attacker's choice. Many compilers can statically check format strings and produce warnings for dangerous or suspect formats. In the GNU Compiler Collection, the relevant compiler flags are, -Wall , -Wformat , -Wno-format-extra-args , -Wformat-security , -Wformat-nonliteral , and -Wformat=2 . Most of these are only useful for detecting bad format strings that are known at compile-time. If the format string may come from the user or from a source external to the application, the application must validate the format string before using it. Care must also be taken if the application generates or selects format strings on the fly. If the GNU C library is used, the -D_FORTIFY_SOURCE=2 parameter can be used to detect certain types of attacks occurring at run-time. The -Wformat-nonliteral check is more stringent. ==See also== Cross-application scripting exploits a similar kind of programming error Cross-site scripting printf scanf syslog Improper input validation SQL injection is a similar attack that succeeds when input is not filtered ==References== ==Further reading== (vii+663 pages) ==External links== Introduction to format string exploits 2013-05-02, by Alex Reece scut / team-TESO Exploiting Format String Vulnerabilities v1.2 2001-09-09 WASC Threat Classification - Format String Attacks CERT Secure Coding Standards CERT Secure Coding Initiative Known vulnerabilities at MITRE's CVE project. Secure Programming with GCC and GLibc (2008), by Marcel Holtmann

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