CWE 797

Only Filtering Special Elements at an Absolute Position

Weakness ID: 797 (Weakness Variant)

Status: Incomplete

Description

Description Summary

The software receives data from an upstream component, but only accounts for special elements at an absolute position (e.g. "byte number 10"), 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 substring function to filter a 3-character "../" element located at the 0-index position 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();

if (substr($Username, 0, 3) eq '../') {

$Username = substr($Username, 3);

}

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

ReadAndSendFile($filename);

Since the if function is only looking for a substring of "../" between the 0 and 2 position, it only removes that specific "../" element. So an input value such as:

(Attack)

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

will have the first "../" filtered, 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