The ISO/IEC 9899-1999 C standard function fopen()
is typically used to open an existing file or create a new one. However, fopen()
does not indicate if an existing file has been opened for writing or a new file has been created. This may lead to a program overwriting or accessing an unintended file.
In this example, an attempt is made to check whether a file exists before opening it for writing by trying to open the file for reading.
... FILE *fp = fopen("foo.txt","r"); if( !fp ) { /* file does not exist */ fp = fopen("foo.txt","w"); ... fclose(fp); } else { /* file exists */ fclose(fp); } ... |
However, this code suffers from a _Time of Check, Time of Use_ (or _TOCTOU_) vulnerability (see \[[Seacord 05|AA. C++ References#Seacord 05]\] Section 7.2). On a shared multitasking system there is a window of opportunity between the first call of {{fopen()}} and the second call for a malicious attacker to, for example, create a link with the given filename to an existing file, so that the existing file is overwritten by the second call of {{fopen()}} and the subsequent writing to the file. |
The fopen_s()
function defined in ISO/IEC TR24731-2006 is designed to improve the security of the fopen()
function. However, like fopen()
, fopen_s()
provides no mechanism to determine if an existing file has been opened for writing or a new file has been created. The code below contains the same TOCTOU race condition as in
... FILE *fptr; errno_t res = fopen_s(&fptr,"foo.txt", "r"); if (res != 0) { /* file does not exist */ res = fopen_s(&fptr,"foo.txt", "w"); ... fclose(fptr); } else { fclose(fptr); } ... |
The {{fopen()}} function does not indicate if an existing file has been opened for writing or a new file has been created. However, the {{open()}} function as defined in the Open Group Base Specifications Issue 6 \[[Open Group 04|AA. C References#Open Group 04]\] is available on many platforms and provides such a mechanism. If the {{O_CREAT}} and {{O_EXCL}} flags are used together, the {{open()}} function fails when the file specified by {{file_name}} already exists. |
... int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode); if (fd == -1) { /* Handle Error */ } ... |
The function {{fdopen()}} \[[Open Group 04|AA. C References#Open Group 05]\] can be used in conjunction with {{open()}} to determine if a file is opened or created, and then associate a stream with the file descriptor. |
... FILE *fp; int fd; fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode); if (fd == -1) { /* Handle Error */ } fp = fdopen(fd,"w"); if (fp == NULL) { /* Handle Error */ } ... |
The ability to determine if an existing file has been opened, or a new file has been created provides greater assurance that the file accessed is the one that was intended.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
---|---|---|---|---|---|
FIO03-A |
3 (high) |
2 (probable) |
1 (high) |
P6 |
L2 |