CVE-2026-1525

Undici has an HTTP Request/Response Smuggling issue

Workarounds

If upgrading is not immediately possible:

1. Validate header names: Ensure no duplicate Content-Length headers (case-insensitive) are present before passing headers to undici
2. Use object format: Pass headers as a plain object ({ 'content-length': '123' }) rather than an array, which naturally deduplicates by key
3. Sanitize user input: If headers originate from user input, normalize header names to lowercase and reject duplicates

Undici allows duplicate HTTP Content-Length headers when they are provided in an array with case-variant names (e.g., Content-Length and content-length). This produces malformed HTTP/1.1 requests with multiple conflicting Content-Length values on the wire.

Who is impacted:

• Applications using undici.request(), undici.Client, or similar low-level APIs with headers passed as flat arrays
• Applications that accept user-controlled header names without case-normalization

Potential consequences:

• Denial of Service: Strict HTTP parsers (proxies, servers) will reject requests with duplicate Content-Length headers (400 Bad Request)
• HTTP Request Smuggling: In deployments where an intermediary and backend interpret duplicate headers inconsistently (e.g., one uses the first value, the other uses the last), this can enable request smuggling attacks leading to ACL bypass, cache poisoning, or credential hijacking

CVE-2026-1525 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.24.0, 7.24.0); upgrading removes the vulnerable code path.

Kodem intelligence

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Patched in the undici version v7.24.0 and v6.24.0. Users should upgrade to this version or later.

1. What is CVE-2026-1525? CVE-2026-1525 is a medium-severity security vulnerability in undici (npm), affecting versions < 6.24.0. It is fixed in 6.24.0, 7.24.0.
2. How severe is CVE-2026-1525? CVE-2026-1525 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of undici are affected by CVE-2026-1525? undici (npm) versions < 6.24.0 is affected.
4. Is there a fix for CVE-2026-1525? Yes. CVE-2026-1525 is fixed in 6.24.0, 7.24.0. Upgrade to this version or later.
5. Is CVE-2026-1525 exploitable, and should I be worried? Whether CVE-2026-1525 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-1525 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-1525?
   • Upgrade undici to 6.24.0 or later
   • Upgrade undici to 7.24.0 or later

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