CVE-2026-66357: Erlang httpd HTTP request smuggling via obs-fold header continuation

CVE-2026-66357 · Severity: info · CVSS 8.3 · Published 2026-09-01

Erlang's httpd HTTP server has never correctly implemented RFC-compliant header continuation lines (obs-fold), treating every line break as a new header instead. This oversight enables HTTP request smuggling attacks, where attackers craft malformed requests that are parsed differently by httpd and downstream proxies or application logic, potentially bypassing security controls or poisoning request interpretation. Systems using affected Erlang/OTP versions to serve HTTP content are vulnerable to request smuggling exploitation in the wild.

Technologies: Erlang Inets, Erlang OTP. Vendors: Erlang.

Quick answers

Is CVE-2026-66357 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-66357 being exploited in the wild.
How severe is CVE-2026-66357?
Informational severity, with a CVSS score of 8.3, published 1 September 2026.
Which products and versions are affected?
Erlang Inets 5.10 before 9.3.2.7, 9.4 before 9.6.2.3, 9.7 before 9.7.2; Erlang OTP 17.0 before 27.3.4.17, 28.0 before 28.5.0.6, 29.0 before 29.0.6.
Is there a fix for CVE-2026-66357?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-66357 attacked?
It is attacked over the network.
Am I affected by CVE-2026-66357?
Check whether you run Erlang Inets or Erlang OTP at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability is a CWE-444 HTTP request smuggling defect stemming from httpd's failure to implement obs-fold header continuation (RFC 2616 §2.2 and RFC 7230 §3.2.4). Instead of treating a CRLF followed by whitespace as a header continuation, httpd unconditionally begins a new header at each CRLF, creating a discrepancy in HTTP request parsing between httpd and compliant intermediaries. An attacker can send a crafted HTTP request with malformed headers over the network to a vulnerable httpd instance, causing smuggled content to be interpreted differently by proxies, WAFs, or backend application logic. This enables cache poisoning, request injection, or session hijacking depending on the deployment topology. Patches are available: OTP 27.3.4.17+, OTP 28.5.0.6+, and OTP 29.0.6+ (inets 9.3.2.7+, 9.6.2.3+, and 9.7.2+ respectively).

Affected products

  • Erlang OTP 17.0 before 27.3.4.17, 28.0 before 28.5.0.6, 29.0 before 29.0.6
  • Erlang inets 5.10 before 9.3.2.7, 9.4 before 9.6.2.3, 9.7 before 9.7.2

Timeline

  • 2026-09-01: disclosed
  • 2026-09-08: advisory: CNA advisory published

References

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