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-86 Variant Draft
Improper Neutralization of Invalid Characters in Identifiers in Web Pages
This vulnerability occurs when an application fails to properly filter or escape invalid characters within web identifiers like HTML tag names or URI schemes, allowing malicious sequences to pass…
Definition
What is CWE-86?
This vulnerability occurs when an application fails to properly filter or escape invalid characters within web identifiers like HTML tag names or URI schemes, allowing malicious sequences to pass through.
Modern web browsers often interpret malformed identifiers in unexpected ways. For instance, a browser might silently strip out null bytes or other invalid characters, reconstructing a dangerous payload that the developer's filters missed. This creates a gap where your security checks see a harmless string, but the browser executes it as active code.
Attackers exploit this by embedding these invalid sequences—like null bytes or alternative encodings—within identifiers such as URI schemes. A common bypass involves encoding "javascript:" as something like `java%00script:`, which might bypass a simple blacklist filter but still render as executable JavaScript in the browser, leading to cross-site scripting (XSS) or other client-side attacks.
Auswirkungen in der Praxis
Real-world CVEs caused by CWE-86
-
XSS filter doesn't filter null characters before looking for dangerous tags, which are ignored by web browsers. Multiple Interpretation Error (MIE) and validate-before-cleanse.
Wie Angreifer es ausnutzen
Angreiferpfad Schritt für Schritt
- 1
Identifiziere einen Codepfad, der nicht vertrauenswürdige Eingaben ohne Validierung verarbeitet.
- 2
Erzeuge eine Payload, die das unsichere Verhalten auslöst — Injection, Traversal, Overflow oder Logik-Missbrauch.
- 3
Liefere die Payload über einen normalen Request aus und beobachte die Reaktion der Anwendung.
- 4
Iteriere, bis die Antwort Daten preisgibt, Angreifer-Code ausführt oder Berechtigungen eskaliert.
Verwundbares Codebeispiel
Vulnerable pseudo
MITRE hat kein Codebeispiel für diese CWE veröffentlicht. Das untenstehende Muster ist illustrativ — kanonische Referenzen findest du unter Ressourcen.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} Sicheres Codebeispiel
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.
Präventions-Checkliste
How to prevent CWE-86
- Implementation Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component. The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.
- Implementation To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XMLHTTPRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.
Erkennungssignale
How to detect CWE-86
Plexicus erkennt CWE-86 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.
Häufig gestellte Fragen Was ist CWE-86?
Wie gravierend ist CWE-86?
Welche Sprachen oder Plattformen sind von CWE-86 betroffen?
Wie kann ich CWE-86 verhindern?
Wie erkennt und behebt Plexicus CWE-86?
Wo erfahre ich mehr über CWE-86?
Frequently asked questions
Was ist CWE-86?
This vulnerability occurs when an application fails to properly filter or escape invalid characters within web identifiers like HTML tag names or URI schemes, allowing malicious sequences to pass through.
Wie gravierend ist CWE-86?
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-86 betroffen?
MITRE hat für diese CWE keine betroffenen Plattformen spezifiziert — sie kann in den meisten Anwendungs-Stacks auftreten.
Wie kann ich CWE-86 verhindern?
Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special,…
Wie erkennt und behebt Plexicus CWE-86?
Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-86 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-86?
MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/86.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.
Verwandte Schwachstellen
Weaknesses related to CWE-86
CWE-79 Parent
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
This vulnerability occurs when a web application fails to properly sanitize or encode user-supplied input before displaying it on a…
CWE-80 Sibling
Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)
This vulnerability, commonly known as Basic Cross-Site Scripting (XSS), occurs when a web application fails to properly sanitize user…
CWE-81 Sibling
Improper Neutralization of Script in an Error Message Web Page
This vulnerability occurs when an application fails to properly sanitize user-supplied input before displaying it within an error message…
CWE-83 Sibling
Improper Neutralization of Script in Attributes in a Web Page
This vulnerability occurs when a web application fails to properly sanitize or block JavaScript URIs (like 'javascript:') within HTML tag…
CWE-84 Sibling
Improper Neutralization of Encoded URI Schemes in a Web Page
This vulnerability occurs when a web application fails to properly sanitize user-supplied input that contains malicious scripts disguised…
CWE-85 Sibling
Doubled Character XSS Manipulations
This vulnerability occurs when a web application fails to properly sanitize user input that contains doubled characters, allowing…
CWE-87 Sibling
Improper Neutralization of Alternate XSS Syntax
This vulnerability occurs when an application fails to properly sanitize user-supplied input that uses alternative methods to execute…
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…
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