CVE-2026-1525

Summary

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

Affected Software

VendorProductVersion RangeStatus
undiciundici< 6.24.0; 7.0.0 < 7.24.0affected
undiciundici6.24.0: 7.24.0unaffected

Weaknesses

  • CWE-444: CWE-444 Inconsistent interpretation of HTTP requests ('HTTP Request/Response smuggling')

Workarounds

If upgrading is not immediately possible:

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

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: poc
    • Automatable: yes
    • Technical Impact: partial

References