CWE-797 Variant Incomplete

Only Filtering Special Elements at an Absolute Position

This vulnerability occurs when software checks for dangerous characters or patterns only at a fixed, hardcoded location in input data. Because it ignores these same elements if they appear anywhere…

Definition

What is CWE-797?

This vulnerability occurs when software checks for dangerous characters or patterns only at a fixed, hardcoded location in input data. Because it ignores these same elements if they appear anywhere else, attackers can bypass the filter by simply moving the malicious content to a different position.
Imagine a filter that only removes a semicolon if it's exactly the 10th character, believing this will prevent SQL injection. An attacker can easily bypass this by adding a few harmless characters at the start, moving the malicious semicolon to the 11th position. The filter sees nothing wrong at its one checked spot and lets the tainted data pass through, leading to a successful attack. This flaw stems from a misunderstanding of input validation. Effective security requires checking all input comprehensively, not just sampling a single location. Developers should use allowlists or context-aware encoding that processes the entire data stream, ensuring dangerous elements are neutralized no matter where they appear.
Auswirkungen in der Praxis

Real-world CVEs caused by CWE-797

Bisher sind in MITREs Katalog keine öffentlichen CVE-Referenzen mit dieser CWE verknüpft.

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

  1. 1

    The following code takes untrusted input and uses a substring function to filter a 3-character "../" element located at the 0-index position of the input string. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.

  2. 2

    Since the if function is only looking for a substring of "../" between the 0 and 2 position, it only removes that specific "../" element. So an input value such as:

  3. 3

    will have the first "../" filtered, resulting in:

  4. 4

    This value is then concatenated with the /home/user/ directory:

  5. 5

    which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-22).

Verwundbares Codebeispiel

Vulnerable Perl

The following code takes untrusted input and uses a substring function to filter a 3-character "../" element located at the 0-index position of the input string. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.

Verwundbar Perl 
my $Username = GetUntrustedInput();
  if (substr($Username, 0, 3) eq '../') {
  	$Username = substr($Username, 3);
  }
  my $filename = "/home/user/" . $Username;
  ReadAndSendFile($filename);
Angreifer-Payload

Since the if function is only looking for a substring of "../" between the 0 and 2 position, it only removes that specific "../" element. So an input value such as:

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-797

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Erkennungssignale

How to detect CWE-797

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-797 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-797?

This vulnerability occurs when software checks for dangerous characters or patterns only at a fixed, hardcoded location in input data. Because it ignores these same elements if they appear anywhere else, attackers can bypass the filter by simply moving the malicious content to a different position.

Wie gravierend ist CWE-797?

MITRE hat für diese Schwachstelle keine Exploit-Wahrscheinlichkeit veröffentlicht. Behandle sie als mittlere Auswirkung, bis dein Threat Model anderes belegt.

Welche Sprachen oder Plattformen sind von CWE-797 betroffen?

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

Wie kann ich CWE-797 verhindern?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

Wie erkennt und behebt Plexicus CWE-797?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-797 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-797?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/797.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-797

CWE-795 Parent

Only Filtering Special Elements at a Specified Location

This vulnerability occurs when a security filter only checks for dangerous input patterns at specific, predefined locations within the…

CWE-796 Sibling

Only Filtering Special Elements Relative to a Marker

This vulnerability occurs when software filters dangerous inputs or characters, but only checks for them in specific, expected locations…

Ressourcen

Further reading

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