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.
Non-Compliant Code Example 1
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.
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...
FILE *fp = fopen("foo.txt","r");
if( !fp ) { /* file does not exist */
fp = fopen("foo.txt","w");
...
fclose(fp);
} else {
/* file exists */
fclose(fp);
}
...
|
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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. |
Non-Compliant Code Example 1
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
Non-Compliant Code Example 1.
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|---|---|---|
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...
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);
}
...
|
Compliant Solution 1
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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. |
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...
int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode);
if (fd == -1) {
/* Handle Error */
}
...
|
Compliant Solution 2
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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. |
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|---|---|---|
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...
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 */
}
...
|
Risk Assessment
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 |
References
- Seacord 05 Chapter 7, File I/O
- ISO/IEC 9899-1999 Sections 7.19.3, Files
- ISO/IEC 9899-1999 Sections 7.19.4, Operations on Files
- ISO/IEC TR 24731-2006 Section 6.5.2.1, The fopen_s function
- Open Group 04