This rule finds calls to functions that are dangerous to
use. Currently, it checks for calls
gmtime. See Related rules
below for rules that identify other dangerous functions.
gets function is one of the vulnerabilities exploited by the Internet Worm of 1988, one of the first computer worms to spread through the Internet. The
gets function provides no way to limit the amount of data that is read and stored, so without prior knowledge of the input it is impossible to use it safely with any size of buffer.
gmtime function fills data into a
struct in shared memory and then returns a pointer to that struct. If
the function is called from multiple places in the same program, and
especially if it is called from multiple threads in the same program,
then the calls will overwrite each other's data.
Replace calls to
fgets, specifying the maximum length to copy. This will prevent the buffer overflow.
Replace calls to
gmtime_r, the application code manages allocation of
tm struct. That way, separate calls to the function
can use their own storage.
The following example checks the local time in two ways:
The first version uses
gmtime, so it is vulnerable to
its data being overwritten by another thread. Even if this code is not
used in a multi-threaded context right now, future changes may
make the program multi-threaded. The second version of the code
gmtime_r. Since it allocates a new
struct on every call, it is immune to other calls to
Other dangerous functions identified by CWE-676 ("Use of
Potentially Dangerous Function") include
strcat. Use of these functions is highlighted by
rules for the following CWEs:
- CWE-120 Classic Buffer Overflow
- CWE-131 Incorrect Calculation of Buffer Size
- Wikipedia: Morris worm.
- E. Spafford. The Internet Worm Program: An Analysis. Purdue Technical Report CSD-TR-823, (online), 1988.
- SEI CERT C Coding Standard: CON33-C. Avoid race conditions when using library functions.
- Common Weakness Enumeration: CWE-242.