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.)
CWE-625 Base Draft
Permissive Regular Expression
This weakness occurs when a regular expression is too permissive, failing to properly validate or sanitize input by allowing unintended values or patterns.
Definition
What is CWE-625?
This weakness occurs when a regular expression is too permissive, failing to properly validate or sanitize input by allowing unintended values or patterns.
A permissive regex often arises from forgetting to anchor the pattern to the start (^) and end ($) of the input string. This causes a partial match, where the system accepts any substring that fits the pattern, rather than validating the entire input. For example, a regex meant to validate a 5-digit ZIP code like \d{5} would incorrectly accept '12345' within 'abc12345def', leading to incomplete validation.
Other common mistakes include using overly broad wildcards (like .*) instead of specific character classes, or crafting patterns that fail to exclude dangerous or malformed data. This lax validation can open the door to data corruption, injection attacks, or logic flaws downstream, as the application processes input it assumed was already safe.
Real-world impact
Real-world CVEs caused by CWE-625
-
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.
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".*" 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.
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Regexp isn't "anchored" to the beginning or end, which allows spoofed values that have trusted values as substrings.
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regexp in .htaccess file allows access of files whose names contain certain substrings
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allow load of macro files whose names contain certain substrings.
How attackers exploit it
Step-by-step attacker path
- 1
The following code takes phone numbers as input, and uses a regular expression to reject invalid phone numbers.
- 2
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.
- 3
This code uses a regular expression to validate an IP string prior to using it in a call to the "ping" command.
- 4
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.
Vulnerable code example
Vulnerable 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 $phone");}
else {
```
error("malformed number!");
} Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-625
- Implementation When applicable, ensure that the regular expression marks beginning and ending string patterns, such as "/^string$/" for Perl.
Detection signals
How to detect CWE-625
Plexicus auto-detects CWE-625 and opens a fix PR in under 60 seconds.
Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.
Frequently asked questions What is CWE-625?
How serious is CWE-625?
What languages or platforms are affected by CWE-625?
How can I prevent CWE-625?
How does Plexicus detect and fix CWE-625?
Where can I learn more about CWE-625?
Frequently asked questions
What is CWE-625?
This weakness occurs when a regular expression is too permissive, failing to properly validate or sanitize input by allowing unintended values or patterns.
How serious is CWE-625?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-625?
MITRE lists the following affected platforms: Perl, PHP.
How can I prevent CWE-625?
When applicable, ensure that the regular expression marks beginning and ending string patterns, such as "/^string$/" for Perl.
How does Plexicus detect and fix CWE-625?
Plexicus's SAST engine matches the data-flow signature for CWE-625 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-625?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/625.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-625
CWE-185 Parent
Incorrect Regular Expression
This vulnerability occurs when a regular expression is written incorrectly, causing it to match or validate data in unintended and…
CWE-186 Sibling
Overly Restrictive Regular Expression
This weakness occurs when a regular expression is too narrow, failing to detect all the dangerous or unexpected input values it was…
CWE-187 Peer
Partial String Comparison
This weakness occurs when software checks only part of a string or token to determine a match, instead of comparing the entire value. This…
CWE-184 Peer
Incomplete List of Disallowed Inputs
This vulnerability occurs when a security filter or validation mechanism relies on a 'denylist'—a predefined list of forbidden inputs—but…
CWE-183 Peer
Permissive List of Allowed Inputs
This vulnerability occurs when an application's security filter uses an allowlist that is too broad, mistakenly permitting dangerous…
CWE-777 Child
Regular Expression without Anchors
This vulnerability occurs when a regular expression used for validation or sanitization lacks anchors, allowing unintended characters or…
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