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Example 630 A classic format string vulnerability from wuftpd 260
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while (fgets(buf, sizeof buf, f)) { lreply(200, buf); } void lreply(int n, char *fmt, ) { char buf[BUFSIZ]; vsnprintf(buf, sizeof buf, fmt, ap); }
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We explain format string vulnerabilities in detail, but rst a few words about the public history of format string vulnerabilities From a historical perspective, new varieties of vulnerabilities don t come around very often Unlike virus researchers, software security researchers do not get to identify new root-cause vulnerabilities every day or even every year, but when they do, it s likely to be relevant for years to come This was the case with format string vulnerabilities, which were rst identi ed in the wild starting in 1999 The industry had been talking about buffer over ow vulnerabilities for over a decade, but when format string vulnerabilities began to be widely exploited in 2000, a widespread weakness that had always been present in C and C++ suddenly came into the spotlight, and a lot of software was affected: Apache with PHP3 *BSD chpass IRIX telnetd Linux rpcstatd NLS / locale
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6 Buffer Over ow
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In these programs and others, an attacker supplies the vulnerable program with input that the program later includes in a format string argument To understand how formatted string functions are misused, consider the formatted output function printf() The format string that printf() accepts as its rst parameter is representative of most format strings The ISO C99 Standard describes the format string passed to printf() as follows:
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The format string is composed of zero or more directives: ordinary characters (not %), which are copied unchanged to the output stream; and conversion speci cations, each of which results in fetching zero or more subsequent arguments Each conversion speci cation is introduced by the character % and ends with a conversion speci er [ISO C99, 2005]
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String formatting functions in C and C++, such as printf(), are designed to be as flexible as possible Because valid format strings are not required to contain directives or conversion specifications, the format string argument can be used to process strings that require no formatting Any parameters beyond the format string that would correspond to format directives or conversion specifications must be optional This flexibility, which, at its core, is a type problem, leads programmers to take seemingly innocuous shortcuts, such as writing printf(str) instead of the more verbose printf("%s", str) Sometimes these shortcuts are so ingrained that programmers might not even realize that the function they are using expects a format string Although this often results in outwardly correct behavior because standard characters are simply passed through the format string unchanged, it is also the most common way that format string vulnerabilities occur If user input can influence the contents of the format string parameter, an attacker might be able to include malicious conversion specifications in a string that the programmer assumes will contain none In the most benign case, an attack will include conversion specifications designed to read arbitrary values off the stack and provide unauthorized access to sensitive information In the more serious and commonly exploited case, the
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Strings
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attacker uses the %n directive to write to arbitrary positions in memory (See the sidebar "A Classic Format String Attack" later in this section for an explanation of how attacks based on the %n directive work) When an attacker can alter values in memory, all the usual exploits for buffer overflow vulnerabilities become viable, which most often include overwriting the return address of the current stack frame, changing the value of a function pointer, or modifying other important values that govern the behavior of the program Although the complexity and variety of possible exploits is large, most format string vulnerabilities can be prevented by choosing the most restrictive of the following guidelines possible in your environment: Always pass a static format string to any function that accepts a format string argument If a single static format string is too restrictive, define a set of valid format strings and make selections from this safe set Accept the added program complexity of selecting from a fixed set of static format strings over the risk that a dynamically constructed string will include unchecked user input If a situation truly demands that a format string include input read from outside the program, perform rigorous whitelist-based input validation on any values read from outside the program that are included in the format string Example 631 demonstrates how the simpli ed wuftpd code from Example 630 could be rewritten to safely use a static format string
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