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CVE-2026-66357 – HTTP Request Smuggling – Erlang/OTP inets httpd (versions 17.0–29.0.5)

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Summary

CVE-2026-66357 is an HTTP request smuggling vulnerability (CWE-444: Inconsistent Interpretation of HTTP Requests) affecting the httpd server component of the inets application bundled with Erlang/OTP. The root cause is that httpd has never implemented obs-fold (RFC 2616 §2.2 / RFC 7230 §3.2.4 header continuation lines): every CRLF sequence followed by a non-CRLF octet is unconditionally treated as the start of a new header rather than as a continuation of the previous one. The issue carries a CVSS score of 8.3 (High) and is remotely exploitable without authentication.

Technical details

  • Root cause: httpd (part of the inets OTP application) does not support obs-fold header continuation lines as defined in RFC 7230 §3.2.4. Any CRLF followed by a character that is not itself CRLF is unconditionally interpreted as beginning a new HTTP header, rather than as a folded continuation of the prior header line.
  • Trigger conditions: An attacker sends a specially crafted HTTP request containing folded/continuation-style header lines to a server using the httpd module. Because httpd‘s parsing diverges from how other HTTP intermediaries (proxies, load balancers, caches) interpret the same header continuation syntax, the two components disagree on where one request ends and the next begins.
  • Attack vector: Network — no authentication or user interaction is required. The flaw is exploitable by any party able to send HTTP requests to the affected httpd service, particularly when it sits behind a front-end proxy or load balancer.
  • Impact: This inconsistent interpretation enables classic HTTP request/response smuggling (CAPEC-33). An attacker can desynchronize the request stream between a front-end proxy and the httpd backend, potentially injecting or hijacking subsequent requests, poisoning caches, bypassing access controls enforced at the proxy layer, or interfering with the integrity/confidentiality of other users’ requests processed by the same connection.

Affected software

  • Erlang/OTP 17.0 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
  • Corresponding inets application versions: 5.10 before 9.3.2.7, 9.4 before 9.6.2.3, and 9.7 before 9.7.2
  • Whether OTP releases prior to 17.0 (inets before 5.10) are affected is unknown/unconfirmed by the vendor

Severity

  • CVSS v4.0 Base Score: 8.3 (High)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:H/VA:N/SC:N/SI:L/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade Erlang/OTP to a patched release:
    • OTP 27.x → 27.3.4.17 or later
    • OTP 28.x → 28.5.0.6 or later
    • OTP 29.x → 29.0.6 or later
  • If immediate patching is not feasible, consider taking httpd-based services out of any deployment topology involving front-end proxies/load balancers that terminate and re-forward HTTP connections, since request smuggling risk is amplified when parsing behavior differs between the proxy and httpd. Ensure any intermediary strictly rejects or normalizes ambiguous/obs-folded headers before forwarding to the httpd backend.
  • Monitor network traffic for anomalous or malformed HTTP header continuation patterns targeting services running the inets/httpd module.

References

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