CWE-777 Variant Incomplete Medium likelihood

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.
Auswirkungen in der Praxis

Real-world CVEs caused by CWE-777

  • 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).

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

  1. 1

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

  2. 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. 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. 4

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

  5. 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.

Verwundbares Codebeispiel

Vulnerable PHP

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

Verwundbar PHP 
$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";
  }
Angreifer-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.

Angreifer-Payload 
../../etc/passwd
Sicheres Codebeispiel

Secure pseudo

Sicher 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.
Präventions-Checkliste

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.
Erkennungssignale

How to detect CWE-777

SAST High

Führe statische Analyse (SAST) auf der Codebasis aus und suche im Datenfluss nach dem unsicheren Muster.

DAST Moderate

Führe dynamische Application-Security-Tests gegen den Live-Endpoint aus.

Runtime Moderate

Beobachte Runtime-Logs auf ungewöhnliche Exception-Traces, fehlerhafte Eingaben oder Versuche, Autorisierung zu umgehen.

Code review Moderate

Code Review: Markiere jeden neuen Code, der Eingaben von dieser Oberfläche ohne validierte Framework-Helper verarbeitet.

Plexicus Auto-Fix

Plexicus erkennt CWE-777 automatisch und öffnet in unter 60 Sekunden einen Fix-PR.

Codex Remedium scannt jeden Commit, identifiziert genau diese Schwachstelle und liefert einen reviewer-ready Pull Request mit dem Patch. Keine Tickets. Keine Hand-offs.

Demo anfordern Plexicus kostenlos testen
Häufig gestellte Fragen

Frequently asked questions

Was ist 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.

Wie gravierend ist CWE-777?

MITRE stuft die Exploit-Wahrscheinlichkeit als mittel ein — eine Ausnutzung ist realistisch, erfordert aber meist bestimmte Bedingungen.

Welche Sprachen oder Plattformen sind von CWE-777 betroffen?

MITRE hat für diese CWE keine betroffenen Plattformen spezifiziert — sie kann in den meisten Anwendungs-Stacks auftreten.

Wie kann ich CWE-777 verhindern?

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.

Wie erkennt und behebt Plexicus CWE-777?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-777 bei jedem Commit. Bei einem Treffer öffnet unser Codex-Remedium-Agent einen Fix-PR mit korrigiertem Code, Tests und einer einzeiligen Zusammenfassung für den Reviewer.

Wo erfahre ich mehr über CWE-777?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/777.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-777

CWE-625 Parent

Permissive Regular Expression

This weakness occurs when a regular expression is too permissive, failing to properly validate or sanitize input by allowing unintended…

Ressourcen

Further reading

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