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.
Real-world impact
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.
How attackers exploit it
Step-by-step attacker path
- 1
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 4
Iterate until the response leaks data, executes attacker code, or escalates privileges.
Vulnerable code example
Vulnerable pseudo
MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} 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-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.
Detection signals
How to detect CWE-86
Plexicus auto-detects CWE-86 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-86?
How serious is CWE-86?
What languages or platforms are affected by CWE-86?
How can I prevent CWE-86?
How does Plexicus detect and fix CWE-86?
Where can I learn more about CWE-86?
Frequently asked questions
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.
How serious is CWE-86?
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-86?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-86?
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,…
How does Plexicus detect and fix CWE-86?
Plexicus's SAST engine matches the data-flow signature for CWE-86 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-86?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/86.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
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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