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CVE-2026-73276 – HTTP Request Smuggling – Erlang/OTP inets httpd Server

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Summary

CVE-2026-73276 is an HTTP Request Smuggling vulnerability (CWE-444: Inconsistent Interpretation of HTTP Requests) affecting the inets httpd HTTP server component bundled with Erlang/OTP. The flaw arises because httpd silently drops malformed HTTP headers that contain whitespace between the field name and the colon (e.g. Content-Length : 6) instead of rejecting them as required by RFC 7230 §3.2.4, which can cause the server to desynchronize request framing when deployed behind a tolerant reverse proxy. The issue carries a High severity rating (CVSS 8.3) and is remotely exploitable without authentication.

Technical details

  • Root cause: The httpd request-parsing "gracefulness" logic was intended to tolerate minor header formatting deviations, but it ignored a case that should have been rejected outright: headers with whitespace inserted before the colon separator. Rather than returning an error, httpd strips the malformed header entirely.
  • Trigger conditions: When the dropped header is Content-Length (submitted as Content-Length : <value>), httpd treats the request as having no body-length declaration and defaults the effective length to 0.
  • Attack vector: An attacker sends a crafted request containing a malformed Content-Length header to a front-end reverse proxy that tolerates the whitespace and correctly parses the real Content-Length value, while the backend Erlang/OTP httpd server drops the header and treats the request body as a new, separate ("smuggled") pipelined request — a classic CL.0 desynchronization.
  • Impact: Successful exploitation enables HTTP request smuggling, which can be leveraged to bypass front-end access controls, poison the request queue for other users, hijack or manipulate other users’ requests/responses passing through the same connection, and potentially exfiltrate sensitive data — consistent with the reported impact of low confidentiality loss and high integrity loss with no availability impact.
  • Prerequisites: Exploitation requires the vulnerable httpd server to be positioned behind a reverse proxy or intermediary that itself accepts and correctly interprets the same malformed/whitespace header, creating the front-end/back-end interpretation mismatch.

Affected software

  • Erlang/OTP 22.2 up to (but not including) 27.3.4.17
  • Erlang/OTP 28.0 up to (but not including) 28.5.0.6
  • Erlang/OTP 29.0 up to (but not including) 29.0.6
  • inets application 7.1.2 up to (but not including) 9.3.2.7
  • inets application 9.4 up to (but not including) 9.6.2.3
  • inets application 9.7 up to (but not including) 9.7.2

Severity

  • CVSS Score: 8.3 (High)
  • CVSS Vector (v4.0): CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N
  • Note: The Erlang Ecosystem Foundation CNA published this score using the CVSS v4.0 standard; no CVSS v3.1 vector has been issued for this CVE.

Mitigation and recommended actions

  • Immediate: Upgrade Erlang/OTP to a patched release — OTP 27.3.4.17, OTP 28.5.0.6, or OTP 29.0.6 (which include inets 9.3.2.7, 9.6.2.3, and 9.7.2 respectively) — depending on your currently deployed release line.
  • If immediate patching is not possible:
    • Ensure any reverse proxy or load balancer placed in front of httpd strictly enforces RFC 7230 header syntax and rejects requests containing whitespace between a header field name and its colon, rather than passing them through.
    • Disable HTTP request pipelining/keep-alive reuse at the proxy layer where feasible to reduce the impact of desynchronized framing.
    • Restrict and monitor direct network exposure of inets httpd instances, and audit proxy/backend chains for CL.0-style parsing inconsistencies.

References

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