CVE-2026-66357: inets,httpd:HTTP Request Smuggling via obs-fold Header Continuation

Severity: high (CVSS 8.3 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-01 · updated 2026-09-08
Weakness: CWE-444

Description

httpd has never implemented obs-fold (RFC 2616 §2.2 / RFC 7230 §3.2.4 header continuation lines). Every CRLF followed by a non-CRLF octet unconditionally starts a new header. This missing feature became a security concern as the understanding of HTTP request smuggling attacks evolved. This issue affects OTP from OTP 17.0 before OTP 27.3.4.17, from OTP 28.0 before OTP 28.5.0.6, and from OTP 29.0 before OTP 29.0.6, corresponding to inets from 5.10 before 9.3.2.7, from 9.4 before 9.6.2.3, and from 9.7 before 9.7.2. Whether OTP before OTP 17.0, corresponding to inets before 5.10, is affected is unknown.

Affected products

Erlang OTP: from 17.0 before 27.3.4.17; from 28.0 before 28.5.0.6; from 29.0 before 29.0.6; from 5.10 before 9.3.2.7; from 9.4 before 9.6.2.3; from 9.7 before 9.7.2 — fixed in 27.3.4.17, 28.5.0.6, 29.0.6, 9.3.2.7, 9.6.2.3, 9.7.2

How to fix

Upgrade to a fixed version: Erlang OTP 27.3.4.17, 28.5.0.6, 29.0.6, 9.3.2.7. Check the vendor advisory for mitigations.

References

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