CWE 796

Only Filtering Special Elements Relative to a Marker

Weakness ID: 796 (Weakness Variant)

Status: Incomplete

Description

Description Summary

The software receives data from an upstream component, but only accounts for special elements positioned relative to a marker (e.g. "at the beginning/end of a string; the second argument"), thereby missing remaining special elements that may exist before sending it to a downstream component.

Demonstrative Examples

Example 1

The following code takes untrusted input and uses a regular expression to filter a "../" element located at the beginning of the input string. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.

(Bad Code)

Example Language: Perl

my $Username = GetUntrustedInput();

$Username =~ s/^\.\.\///;

my $filename = "/home/user/" . $Username;

ReadAndSendFile($filename);

Since the regular expression is only looking for an instance of "../" at the beginning of the string, it only removes the first "../" element. So an input value such as:

(Attack)

../../../etc/passwd

will have the first "../" stripped, resulting in:

(Result)

../../etc/passwd

This value is then concatenated with the /home/user/ directory:

(Result)

/home/user/../../etc/passwd

which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-22).

Relationships