The product uses a regular expression to perform neutralization, but the regular expression is not anchored and may allow malicious or malformed data to slip through.
When performing tasks such as validating against a set of allowed inputs (allowlist), data is examined and possibly modified to ensure that it is well-formed and adheres to a list of safe values. If the regular expression is not anchored, malicious or malformed data may be included before or after any string matching the regular expression. The type of malicious data that is allowed will depend on the context of the application and which anchors are omitted from the regular expression.

Primary

Regular expressions are typically used to match a pattern of text. Anchors are used in regular expressions to specify where the pattern should match: at the beginning, the end, or both (the whole input).

Implementation

Medium

Availability
Confidentiality
Access Control
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.

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.

Consider a web application that supports multiple languages. It selects messages for an appropriate language by using the lang parameter.

$dir = "/home/cwe/languages";$lang = $_GET['lang'];if (preg_match("/[A-Za-z0-9]+/", $lang)) {include("$dir/$lang");}else {echo "You shall not pass!\n";}

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, CWE-22. 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.

../../etc/passwd

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.

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

import subprocess
import re

def validate_ip_regex(ip: str):

ip_validator = re.compile(r"((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}")
if ip_validator.match(ip):

return ip

else:

raise ValueError("IP address does not match valid pattern.")

def run_ping_regex(ip: str):

validated = validate_ip_regex(ip)
# The ping command treats zero-prepended IP addresses as octal
result = subprocess.call(["ping", validated])
print(result)

Since the regular expression does not have anchors (CWE-777), 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 (CWE-1389). 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.

CVE-2022-30034
Chain: Web UI for a Python RPC framework does not use regex anchors to validate user login emails (CWE-777), potentially allowing bypass of OAuth (CWE-1390).
https://www.cve.org/CVERecord?id=CVE-2022-30034

Allowed
This CWE entry is at the Variant level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

CWE Content Team
MITRE
2009-06-30
1.5
2009-07-27

CWE Content Team
MITRE
2010-06-21
updated Description

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2017-11-08
updated Likelihood_of_Exploit

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2020-06-25
updated Common_Consequences, Description, Potential_Mitigations

CWE Content Team
MITRE
2022-10-13
updated Demonstrative_Examples, Observed_Examples

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Applicable_Platforms, Weakness_Ordinalities