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⇱ CWE - CWE-625: Permissive Regular Expression (4.20)

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Home > CWE List > CWE-625: Permissive Regular Expression (4.20)  
CWE Glossary Definition
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CWE-625: Permissive Regular Expression

Weakness ID: 625
Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
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Description
The product uses a regular expression that does not sufficiently restrict the set of allowed values.
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Extended Description

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
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Common Consequences
👁 Section Help
This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Impact Details

Bypass Protection Mechanism

Scope: Access Control
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Potential Mitigations
Phase(s) Mitigation

Implementation
When applicable, ensure that the regular expression marks beginning and ending string patterns, such as "/^string$/" for Perl.
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 Relationships
👁 Section Help
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
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Relevant to the view "Research Concepts" (View-1000)
Nature Type ID Name
ChildOf 👁 Class
Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 		185 Incorrect Regular Expression
ParentOf 👁 Variant
Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource. 		777 Regular Expression without Anchors
PeerOf 👁 Base
Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 		183 Permissive List of Allowed Inputs
PeerOf 👁 Base
Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 		184 Incomplete List of Disallowed Inputs
PeerOf 👁 Variant
Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource. 		187 Partial String Comparison
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Relevant to the view "Software Development" (View-699)
Nature Type ID Name
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic. 		19 Data Processing Errors
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Modes Of Introduction
👁 Section Help
The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
Phase Note
Implementation This problem is frequently found when the regular expression is used in input validation or security features such as authentication.
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Applicable Platforms
👁 Section Help
This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.
Languages

Perl (Undetermined Prevalence)

PHP (Undetermined Prevalence)

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Demonstrative Examples

Example 1


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

(bad code)
Example Language: Perl 
$phone = GetPhoneNumber();
if ($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.



Example 2


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

(bad code)
Example Language: Python 
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.



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Selected Observed Examples

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

Reference Description
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.
".*" regexp leads to static code injection
insertion of username into regexp results in partial comparison, causing wrong database entry to be updated when one username is a substring of another.
regexp intended to verify that all characters are legal, only checks that at least one is legal, enabling file inclusion.
Regexp for IP address isn't anchored at the end, allowing appending of shell metacharacters.
Regexp isn't "anchored" to the beginning or end, which allows spoofed values that have trusted values as substrings.
regexp in .htaccess file allows access of files whose names contain certain substrings
allow load of macro files whose names contain certain substrings.
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Weakness Ordinalities
Ordinality Description
Primary
(where the weakness exists independent of other weaknesses)
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Detection Methods
Method Details

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.)

Effectiveness: High
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Memberships
👁 Section Help
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Nature Type ID Name
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 845 The CERT Oracle Secure Coding Standard for Java (2011) Chapter 2 - Input Validation and Data Sanitization (IDS)
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 990 SFP Secondary Cluster: Tainted Input to Command
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 1397 Comprehensive Categorization: Comparison
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Vulnerability Mapping Notes
Usage ALLOWED
(this CWE ID may be used to map to real-world vulnerabilities)
Reason Acceptable-Use

Rationale

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.

Comments

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.
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Taxonomy Mappings
Mapped Taxonomy Name Node ID Fit Mapped Node Name
The CERT Oracle Secure Coding Standard for Java (2011) IDS08-J Sanitize untrusted data passed to a regex
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References
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 8, "Character Stripping Vulnerabilities", Page 437. 1st Edition. Addison Wesley. 2006.
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Content History
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Submissions
Submission Date Submitter Organization
2007-05-07
(CWE Draft 6, 2007-05-07) 		CWE Content Team MITRE
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Modifications
Modification Date Modifier Organization
2023-06-29
(CWE 4.12, 2023-06-29) 		CWE Content Team MITRE
updated Mapping_Notes
2023-04-27
(CWE 4.11, 2023-04-27) 		CWE Content Team MITRE
updated Detection_Factors, Relationships
2022-10-13
(CWE 4.9, 2022-10-13) 		CWE Content Team MITRE
updated Demonstrative_Examples
2022-06-28
(CWE 4.8, 2022-06-28) 		CWE Content Team MITRE
updated Observed_Examples
2021-03-15
(CWE 4.4, 2021-03-15) 		CWE Content Team MITRE
updated Demonstrative_Examples
2020-02-24
(CWE 4.0, 2020-02-24) 		CWE Content Team MITRE
updated Relationships
2019-01-03
(CWE 3.2, 2019-01-03) 		CWE Content Team MITRE
updated Taxonomy_Mappings
2017-11-08
(CWE 3.0, 2017-11-08) 		CWE Content Team MITRE
updated Demonstrative_Examples, Observed_Examples
2014-07-30
(CWE 2.8, 2014-07-31) 		CWE Content Team MITRE
updated Relationships
2014-06-23
(CWE 2.7, 2014-06-23) 		CWE Content Team MITRE
updated Modes_of_Introduction, Other_Notes
2012-10-30
(CWE 2.3, 2012-10-30) 		CWE Content Team MITRE
updated Potential_Mitigations
2012-05-11
(CWE 2.2, 2012-05-15) 		CWE Content Team MITRE
updated Demonstrative_Examples, References, Relationships, Taxonomy_Mappings
2011-06-01
(CWE 1.13, 2011-06-01) 		CWE Content Team MITRE
updated Common_Consequences, Relationships, Taxonomy_Mappings
2009-07-27
(CWE 1.5, 2009-07-27) 		CWE Content Team MITRE
updated Relationships
2009-05-27
(CWE 1.4, 2009-05-27) 		CWE Content Team MITRE
updated Demonstrative_Examples
2009-03-10
(CWE 1.3, 2009-03-10) 		CWE Content Team MITRE
updated Description
2008-09-08
(CWE 1.0, 2008-09-09) 		CWE Content Team MITRE
updated Applicable_Platforms, Description, Relationships, Observed_Example, Other_Notes, Weakness_Ordinalities
2008-07-01
(CWE 1.0, 2008-09-09) 		Eric Dalci Cigital
updated Time_of_Introduction
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Page Last Updated: April 30, 2026

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