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Regular Expression without Anchors

Incomplete Variant
Structure: Simple
Description

This vulnerability occurs when a regular expression used for validation or sanitization lacks anchors, allowing unintended characters or malicious payloads to bypass security checks.

Extended Description

Anchors like ^ (start of string) and $ (end of string) are critical for security-focused regular expressions. Without them, your pattern only needs to match a portion of the input, not the entire string. This means an attacker can prepend or append malicious content to an otherwise valid value, effectively sneaking dangerous data past your filter. The impact depends on the missing anchor and the application's context. For example, omitting the start anchor allows payloads before a valid pattern, while omitting the end anchor permits trailing malicious code. Always anchor regex patterns used for input validation, allowlisting, or sanitization to enforce a complete match from start to finish.
Common Consequences 1
Scope: AvailabilityConfidentialityAccess Control

Impact: Bypass Protection Mechanism

An unanchored regular expression in the context of an allowlist will possibly result in a protection mechanism failure, allowing malicious or malformed data to enter trusted regions of the program. The specific consequences will depend on what functionality the allowlist was protecting.
Potential Mitigations 1
Phase: Implementation
Be sure to understand both what will be matched and what will not be matched by a regular expression. Anchoring the ends of the expression will allow the programmer to define an allowlist strictly limited to what is matched by the text in the regular expression. If you are using a package that only matches one line by default, ensure that you can match multi-line inputs if necessary.
Demonstrative Examples 2
Consider a web application that supports multiple languages. It selects messages for an appropriate language by using the lang parameter.

Code Example:

Bad
PHP
php
The previous code attempts to match only alphanumeric values so that language values such as "english" and "french" are valid while also protecting against path traversal, Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'). However, the regular expression anchors are omitted, so any text containing at least one alphanumeric character will now pass the validation step. For example, the attack string below will match the regular expression.

Code Example:

Attack
bash
If the attacker can inject code sequences into a file, such as the web server's HTTP request log, then the attacker may be able to redirect the lang parameter to the log file and execute arbitrary code.

ID : DX-154

This code uses a regular expression to validate an IP string prior to using it in a call to the "ping" command.

Code Example:

Bad
Python
python

The ping command treats zero-prepended IP addresses as octal*

python
Since the regular expression does not have anchors (Regular Expression without Anchors), i.e. is unbounded without ^ or $ characters, then prepending a 0 or 0x to the beginning of the IP address will still result in a matched regex pattern. Since the ping command supports octal and hex prepended IP addresses, it will use the unexpectedly valid IP address (Incorrect Parsing of Numbers with Different Radices). For example, "0x63.63.63.63" would be considered equivalent to "99.63.63.63". As a result, the attacker could potentially ping systems that the attacker cannot reach directly.
Observed Examples 1
CVE-2022-30034Chain: Web UI for a Python RPC framework does not use regex anchors to validate user login emails (Regular Expression without Anchors), potentially allowing bypass of OAuth (Weak Authentication).
Likelihood of Exploit

Medium

Modes of Introduction
Implementation
Related Weaknesses
ChildOf: Permissive Regular Expression (CWE-625)