CVE-2025-7339: on-headers Header Modification Vulnerability

CVE-2025-7339 Overview

CVE-2025-7339 is an Input Validation Error vulnerability in the on-headers Node.js middleware package. This middleware is used for listening to when a response writes headers. A bug in versions prior to 1.1.0 may result in response headers being inadvertently modified when an array is passed to response.writeHead(). This improper handling of data types (CWE-241: Improper Handling of Unexpected Data Type) can lead to unintended header modifications that could affect application behavior.

Critical Impact

Response headers may be inadvertently modified when arrays are passed to response.writeHead(), potentially causing unexpected application behavior and minor confidentiality or integrity impacts.

Affected Products

• on-headers Node.js middleware versions prior to 1.1.0
• Applications using the morgan Express.js logging middleware (which depends on on-headers)
• Node.js applications utilizing response.writeHead() with array parameters

Discovery Timeline

• July 17, 2025 - CVE-2025-7339 published to NVD
• July 17, 2025 - Last updated in NVD database

Technical Details for CVE-2025-7339

Vulnerability Analysis

The vulnerability exists in the on-headers middleware's handling of the response.writeHead() function. When developers pass an array to this function instead of an object, the middleware fails to properly process the header data, resulting in inadvertent modifications to response headers. This represents an Improper Handling of Unexpected Data Type issue (CWE-241).

The local attack vector requires privileged access to the affected system or application code to exploit. The vulnerability could result in minor confidentiality and integrity impacts through header manipulation, though availability is not affected.

Root Cause

The root cause stems from insufficient type checking in the on-headers middleware when processing parameters passed to response.writeHead(). The code does not properly distinguish between array and object data types when handling header information, leading to unexpected behavior when an array is provided where an object is expected.

Attack Vector

The attack vector is local, requiring the attacker to have high privileges on the affected system. An attacker with access to modify application code or influence the parameters passed to response.writeHead() could potentially cause response headers to be modified in unintended ways. This could lead to minor information leakage or integrity issues with HTTP headers.

The vulnerability manifests when an array is passed to response.writeHead() within applications using affected versions of the on-headers middleware. For technical implementation details, refer to the GitHub Security Advisory GHSA-76c9-3jph-rj3q and the fix commit.

Detection Methods for CVE-2025-7339

Indicators of Compromise

• Unexpected or malformed HTTP response headers in application logs
• Applications using on-headers versions below 1.1.0 in package.json or package-lock.json
• Code patterns showing arrays passed to response.writeHead() in conjunction with the on-headers middleware

Detection Strategies

• Audit package.json and package-lock.json files for on-headers dependency versions below 1.1.0
• Implement software composition analysis (SCA) tools to identify vulnerable npm packages
• Review application code for usage patterns involving response.writeHead() with array parameters
• Monitor HTTP response headers for unexpected modifications or anomalies

Monitoring Recommendations

• Enable detailed HTTP header logging to detect unexpected header modifications
• Configure npm audit or similar dependency scanning tools to run in CI/CD pipelines
• Set up alerts for applications using known vulnerable versions of on-headers
• Implement runtime application self-protection (RASP) to monitor header manipulation

How to Mitigate CVE-2025-7339

Immediate Actions Required

• Upgrade on-headers to version 1.1.0 or later immediately
• Audit application code for instances where arrays are passed to response.writeHead()
• Review and update any applications using the morgan logging middleware, which depends on on-headers
• Verify that the upgrade is reflected in package-lock.json to prevent transitive dependency issues

Patch Information

The vulnerability has been patched in on-headers version 1.1.0. Users should upgrade to this version or later to receive the fix. The patch properly handles the data type checking when processing parameters passed to response.writeHead().

For more information, see the GitHub Security Advisory and the OpenJS Foundation Security Advisories.

Workarounds

• Pass an object to response.writeHead() rather than an array until the upgrade can be applied
• Refactor application code to use the object-based syntax for header specification
• Implement input validation to ensure only objects are passed to response.writeHead()

# Upgrade on-headers to patched version
npm update on-headers@1.1.0

# Verify the installed version
npm list on-headers

# Audit for additional vulnerabilities
npm audit