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Unlike gets() and scanf(), strcpy() operates on data already stored in a program variable, which makes it a less obvious security risk Because strcpy() copies the contents of one buffer into another until a null byte is encountered in the source buffer, it can be used safely if the code surrounding it correctly ensures that the contents of the source buffer are guaranteed to be no larger than the capacity of the destination buffer The combination of the lack of a direct connection to user input and the possibility of safe behavior means the use of strcpy() is more frequently tolerated than the use of gets() In practice, the conditions that must be met to use strcpy() safely are often too dif cult to meet, primarily because they are inherently distinct from the invocation of strcpy() Functions that mimic the behavior of strcpy(), such as wcscpy() and lstrcpy(), are equally dangerous Table 63 summarizes the strcpy()function
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Table 63 Function prototype and description for strcpy() [ISO C99, 2005] Function Prototype
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char strcpy(char *DST, const char *SRC)
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Description The strcpy() function copies the string pointed to by s2 (including the terminating null character) into the array pointed to by s1
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The code in Example 613 is from the phpcgi program in Version 20beta10 of PHP/FI [Network Associates PHP, 1997] The filename parameter to FixFilename() is user controlled and can be as large as 8KB, which the code copies into a 128-byte buffer, causing a buffer over ow This vulnerability has been remotely exploited to gain root privileges
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Example 613 Code from the phpcgi program in PHP/FI 20beta10 that is vulnerable to a remote buffer over ow caused by an unsafe call to strcpy()
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char *FixFilename(char *filename, int cd, int *ret) { char fn[128], user[128], *s; s = strrchr(filename,'/'); if(s) { strcpy(fn,s+1);
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To use sprintf() safely, you must ensure that the destination buffer can accommodate the combination of all the source arguments, the string size of which could vary, depending on what conversions are performed as they are formatted, and the nonformat specifier components of the format string In the same way that scanf() can be used safely if proper widthlimiters are used, a carefully calculated format string with the appropriate width-limiters can make a call to sprintf() safe However, the likelihood of error is even worse than with scanf() because here the calculation must accommodate many variables and formatting options Functions that mimic the behavior of sprintf(), such as fprintf() and swprintf(), are equally dangerous See Table 64 for a summary of the sprintf() class of functions
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Table 64 Function prototype and description for sprintf() [ISO C99, 2005] Function Prototype
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int sprintf(char *STR, const char *FORMAT [, ARG, ])
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Description The sprintf() function writes output to the string pointed to by STR, under control of the string pointed to by format that speci es how subsequent arguments are converted for output
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The code in Example 614 contains a buffer over ow caused by the unsafe use of sprintf() in Version 10 of the Kerberos 5 Telnet daemon [Network Associates, 1997] If an attacker supplies a large value of TERM, the code will over ow the speed buffer Because the daemon runs with root privileges and can be invoked by a remote user under certain con gurations, this vulnerability has been exploited remotely to gain root privileges
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Example 614 Code from Version 10 of the Kerberos 5 Telnet daemon that contains a buffer over ow because the length of the TERM environment variable is never validated
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char speed[128]; sprintf(speed, "%s/%d", (cp = getenv("TERM")) cp : "", (def_rspeed > 0) def_rspeed : 9600);
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Risks of Reimplementation Functions that are considered dangerous today were created because they provided functionality that programmers found useful The desirable traits of functions are usually easy to identify; it s the risks that take some thought and security awareness to uncover The same needs that led to the creation of gets(), scanf(), strcpy(), sprintf(), and other dangerous functions still exist today, which often leads developers to reimplement both the functionality and the vulnerabilities When the behavior of a dangerous function is replicated in proprietary code, the overall security of the program is worse off than if the dangerous function were used directly because it can no longer be identi ed by name alone The code in Example 615 is part of csv2xml Version 061 The programmer who coded the method get_csv_token() has unwittingly duplicated the dangerous behavior of gets(), effectively introducing the same class of vulnerability as the standard library function
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Example 615 A function from csv2xml that replicates the dangerous interface of gets()
int get_csv_token(char *token) { int c; int quoted; int len; len=0; quoted=0; while(c=getchar()) { if(c==-1) { break; } if(len==0 && c=='"' && quoted==0) { quoted=1; continue; } if(c=='"') { quoted=0; continue; } if(quoted==0 && c==',') { *token='\0'; return(1); } if(c==10) { line++; } if(quoted==0 && c==10) { *token='\0'; return(0); } *token=c; len++; token++; } *token='\0'; return(-1); }