The product uses a regular expression that does not sufficiently restrict the set of allowed values.

This effectively causes the regexp to accept substrings that match the pattern, which produces a partial comparison to the target. In some cases, this can lead to other weaknesses. Common errors include:

not identifying the beginning and end of the target string
using wildcards instead of acceptable character ranges
others

Primary

Implementation
This problem is frequently found when the regular expression is used in input validation or security features such as authentication.

Access Control
Bypass Protection Mechanism

Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
High

Implementation
When applicable, ensure that the regular expression marks beginning and ending string patterns, such as "/^string$/" for Perl.

The following code takes phone numbers as input, and uses a regular expression to reject invalid phone numbers.

$phone = GetPhoneNumber();if ($phone =~ /\d+-\d+/) {
# looks like it only has hyphens and digits
system("lookup-phone $phone");}
else {error("malformed number!");}

An attacker could provide an argument such as: "; ls -l ; echo 123-456" This would pass the check, since "123-456" is sufficient to match the "\d+-\d+" portion of the regular expression.

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-2021-22204
Chain: regex in EXIF processor code does not correctly determine where a string ends (CWE-625), enabling eval injection (CWE-95), as exploited in the wild per CISA KEV.
https://www.cve.org/CVERecord?id=CVE-2021-22204

CVE-2006-1895
".*" regexp leads to static code injection
https://www.cve.org/CVERecord?id=CVE-2006-1895

CVE-2002-2175
insertion of username into regexp results in partial comparison, causing wrong database entry to be updated when one username is a substring of another.
https://www.cve.org/CVERecord?id=CVE-2002-2175

CVE-2006-4527
regexp intended to verify that all characters are legal, only checks that at least one is legal, enabling file inclusion.
https://www.cve.org/CVERecord?id=CVE-2006-4527

CVE-2005-1949
Regexp for IP address isn't anchored at the end, allowing appending of shell metacharacters.
https://www.cve.org/CVERecord?id=CVE-2005-1949

CVE-2002-2109
Regexp isn't "anchored" to the beginning or end, which allows spoofed values that have trusted values as substrings.
https://www.cve.org/CVERecord?id=CVE-2002-2109

CVE-2006-6511
regexp in .htaccess file allows access of files whose names contain certain substrings
https://www.cve.org/CVERecord?id=CVE-2006-6511

CVE-2006-6629
allow load of macro files whose names contain certain substrings.
https://www.cve.org/CVERecord?id=CVE-2006-6629

IDS08-J
Sanitize untrusted data passed to a regex

Allowed
This CWE entry is at the Base 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
2007-05-07
Draft 6
2007-05-07

Eric Dalci
Cigital
2008-07-01
updated Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Applicable_Platforms, Description, Relationships, Observed_Example, Other_Notes, Weakness_Ordinalities

CWE Content Team
MITRE
2009-03-10
updated Description

CWE Content Team
MITRE
2009-05-27
updated Demonstrative_Examples

CWE Content Team
MITRE
2009-07-27
updated Relationships

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-05-11
updated Demonstrative_Examples, References, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-06-23
updated Modes_of_Introduction, Other_Notes

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2017-11-08
updated Demonstrative_Examples, Observed_Examples

CWE Content Team
MITRE
2019-01-03
updated Taxonomy_Mappings

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2021-03-15
updated Demonstrative_Examples

CWE Content Team
MITRE
2022-06-28
updated Observed_Examples

CWE Content Team
MITRE
2022-10-13
updated Demonstrative_Examples

CWE Content Team
MITRE
2023-04-27
updated Detection_Factors, Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes