String data passed to complex subsystems may contain special characters that can trigger commands or actions, resulting in a software vulnerability. As a result it is necessary to sanatize all string data passed to complex subsystems including, but not limited to:
- command processor via a call to
system()or similar function - relational databases
- third-party COTS components (e.g.,
The white listing approach to data sanatization is to define a list of acceptable characters and remove any character that is not acceptable. The list of valid input values is typically a predictable, well-defined set of manageable size. The following example, based on the tcp_wrappers package written by Wietse Venema, illustrates white listing approach:
static char ok_chars[] = "abcdefghijklmnopqrstuvwxyz\
ABCDEFGHIJKLMNOPQRSTUVWXYZ\
1234567890_-.@";
char user_data[] = "Bad char 1:} Bad char 2:{";
char *cp; /* cursor into string */
for (cp = user_data; \*(cp \+= strspn(cp, ok_chars)); )
*cp = '_';
The benefit of white listing is that a programmer can be certain that a
string contains only characters that are considered safe by the programmer.
White listing is recommended over black listing because, instead of having
to trap all unacceptable characters, the programmer only needs to ensure that
acceptable characters are identified. As a result, the programmer can be less
concerned about which characters an attacker may try in an attempt to bypass
security checks.
References
7.20.4.6 The system function