Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-777 Variant Incomplete
Regular Expression without Anchors
This vulnerability occurs when a regular expression used for validation or sanitization lacks anchors, allowing unintended characters or malicious payloads to bypass security checks.
Definition
What is CWE-777?
This vulnerability occurs when a regular expression used for validation or sanitization lacks anchors, allowing unintended characters or malicious payloads to bypass security checks.
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.
Real-world impact
Real-world CVEs caused by CWE-777
-
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).
How attackers exploit it
Step-by-step attacker path
- 1
Consider a web application that supports multiple languages. It selects messages for an appropriate language by using the lang parameter.
- 2
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.
- 3
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.
- 4
This code uses a regular expression to validate an IP string prior to using it in a call to the "ping" command.
- 5
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 PHP
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";
} Attacker payload
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 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-777
- 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.
Detection signals
How to detect CWE-777
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
Plexicus auto-detects CWE-777 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-777?
How serious is CWE-777?
What languages or platforms are affected by CWE-777?
How can I prevent CWE-777?
How does Plexicus detect and fix CWE-777?
Where can I learn more about CWE-777?
Frequently asked questions
What is CWE-777?
This vulnerability occurs when a regular expression used for validation or sanitization lacks anchors, allowing unintended characters or malicious payloads to bypass security checks.
How serious is CWE-777?
MITRE rates the likelihood of exploit as Medium — exploitation is realistic but typically requires specific conditions.
What languages or platforms are affected by CWE-777?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-777?
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.
How does Plexicus detect and fix CWE-777?
Plexicus's SAST engine matches the data-flow signature for CWE-777 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-777?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/777.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-777
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