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 or a new file has been created. This may lead to a program overwriting or accessing an unintended file.
Non-Compliant Code Example
In this example, a file name is supplied to fopen()
to create and open for writing. However, there is no guarantee that the file referenced by file_name
does not exist prior to calling fopen()
. This may cause an unintended file to be accessed or overwritten.
... FILE * fptr = fopen(file_name, "w"); if (!fptr) { /* Handle Error */ } ...
Compliant Solution 1
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]] 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 */ } ...
Compliant Solution 2
The function fdopen()
[[Open Group 04]] 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 */ } ...
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
- Open Group 04