CWE-644 Variant Incomplete High likelihood

Improper Neutralization of HTTP Headers for Scripting Syntax

This vulnerability occurs when an application fails to properly sanitize or escape user-controlled data placed within HTTP response headers. Malicious scripting syntax can then be injected and…

Definition

What is CWE-644?

This vulnerability occurs when an application fails to properly sanitize or escape user-controlled data placed within HTTP response headers. Malicious scripting syntax can then be injected and executed by client-side components like Flash that process raw headers.
Attackers can exploit this weakness to launch cross-site scripting (XSS) attacks against users. When a browser or plugin processes a tainted HTTP header, embedded scripts run within the victim's browser context, potentially stealing session cookies, defacing websites, or performing actions on the user's behalf. Preventing this requires strict validation and encoding of all data before it is inserted into any HTTP response header, not just the body. Developers must account for various encoding schemes attackers might use to bypass filters. This control is essential for blocking both XSS and related HTTP response splitting attacks.
Real-world impact

Real-world CVEs caused by CWE-644

  • CVE-2006-3918

    Web server does not remove the Expect header from an HTTP request when it is reflected back in an error message, allowing a Flash SWF file to perform XSS attacks.

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 Java

In the following Java example, user-controlled data is added to the HTTP headers and returned to the client. Given that the data is not subject to neutralization, a malicious user may be able to inject dangerous scripting tags that will lead to script execution in the client browser.

Vulnerable Java 
response.addHeader(HEADER_NAME, untrustedRawInputData);
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-644

  • Architecture and Design Perform output validation in order to filter/escape/encode unsafe data that is being passed from the server in an HTTP response header.
  • Architecture and Design Disable script execution functionality in the clients' browser.
Detection signals

How to detect CWE-644

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-644 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-644?

This vulnerability occurs when an application fails to properly sanitize or escape user-controlled data placed within HTTP response headers. Malicious scripting syntax can then be injected and executed by client-side components like Flash that process raw headers.

How serious is CWE-644?

MITRE rates the likelihood of exploit as High — this weakness is actively exploited in the wild and should be prioritized for remediation.

What languages or platforms are affected by CWE-644?

MITRE lists the following affected platforms: Web Based.

How can I prevent CWE-644?

Perform output validation in order to filter/escape/encode unsafe data that is being passed from the server in an HTTP response header. Disable script execution functionality in the clients' browser.

How does Plexicus detect and fix CWE-644?

Plexicus's SAST engine matches the data-flow signature for CWE-644 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-644?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/644.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-644

CWE-116 Parent

Improper Encoding or Escaping of Output

This vulnerability occurs when an application builds a structured message—like a query, command, or request—for another component but…

CWE-117 Sibling

Improper Output Neutralization for Logs

This vulnerability occurs when an application creates log entries using unvalidated external data, allowing attackers to inject malicious…

CWE-838 Sibling

Inappropriate Encoding for Output Context

This vulnerability occurs when a system uses one type of encoding for its output, but the component receiving that data expects a…

Resources

Further reading

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