CWE-85 Variant Draft

Doubled Character XSS Manipulations

This vulnerability occurs when a web application fails to properly sanitize user input that contains doubled characters, allowing attackers to bypass filters and inject malicious scripts.

Definition

What is CWE-85?

This vulnerability occurs when a web application fails to properly sanitize user input that contains doubled characters, allowing attackers to bypass filters and inject malicious scripts.
Attackers exploit this weakness by submitting input where key characters in a script payload are doubled, such as turning `` into ``. Many basic security filters perform a single pass to remove or escape dangerous strings like `script`. When the filter removes the expected sequence, the remaining characters collapse back into a valid, executable script tag, effectively bypassing the protection. To prevent this, developers must implement robust, context-aware output encoding and validation that doesn't rely solely on simple string replacement. Input sanitization should be performed after decoding and before rendering, using well-tested libraries designed to neutralize such obfuscation techniques. This ensures the user input is treated as plain text data, not executable code, regardless of character manipulation attempts.
Real-world impact

Real-world CVEs caused by CWE-85

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 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.

Vulnerable pseudo 
// 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

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-85

  • Implementation Resolve all filtered input to absolute or canonical representations before processing.
  • Implementation Carefully check each input parameter against a rigorous positive specification (allowlist) defining the specific characters and format allowed. All input should be neutralized, not just parameters that the user is supposed to specify, but all data in the request, including tag attributes, hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often encounter data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended.
  • 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 With Struts, write all data from form beans with the bean's filter attribute set to true.
  • 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-85

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-85 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.

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Frequently asked questions

Frequently asked questions

What is CWE-85?

This vulnerability occurs when a web application fails to properly sanitize user input that contains doubled characters, allowing attackers to bypass filters and inject malicious scripts.

How serious is CWE-85?

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-85?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-85?

Resolve all filtered input to absolute or canonical representations before processing. Carefully check each input parameter against a rigorous positive specification (allowlist) defining the specific characters and format allowed. All input should be neutralized, not just parameters that the user is supposed to specify, but all data in the request, including tag attributes, hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS…

How does Plexicus detect and fix CWE-85?

Plexicus's SAST engine matches the data-flow signature for CWE-85 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-85?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/85.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-85

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-86 Sibling

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…

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-675 Peer

Multiple Operations on Resource in Single-Operation Context

This vulnerability occurs when a software component performs the same action on a resource multiple times, even though the action is…

Resources

Further reading

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