Only Filtering Special Elements Relative to a Marker

Incomplete Variant

Structure: Simple

Description

The product 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.

Common Consequences 1

Scope: Integrity

Impact: Unexpected State

Demonstrative Examples 1

ID : DX-3

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.

Code Example:Bad
Perl
perl

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:

Code Example:Attack
bash

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

Code Example:Result
bash

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

Code Example:Result
bash

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 (Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')).

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Only Filtering Special Elements at a Specified Location (CWE-795)