CVE-2026-23941: Request smuggling via first-wins Content-Length parsing in inets httpd

Severity: critical (CVSS 9.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-13 · updated 2026-07-24
Weakness: CWE-444

Description

Inconsistent Interpretation of HTTP Requests ('HTTP Request Smuggling') vulnerability in Erlang OTP (inets httpd module) allows HTTP Request Smuggling. This vulnerability is associated with program files lib/inets/src/http_server/httpd_request.erl and program routines httpd_request:parse_headers/7. The server does not reject or normalize duplicate Content-Length headers. The earliest Content-Length in the request is used for body parsing while common reverse proxies (nginx, Apache httpd, Envoy) honor the last Content-Length value. This violates RFC 9112 Section 6.3 and allows front-end/back-end desynchronization, leaving attacker-controlled bytes queued as the start of the next request. This issue affects OTP from OTP 17.0 before OTP 28.4.1, OTP 27.3.4.9 and OTP 26.2.5.18, corresponding to inets from 5.10 before 9.6.1, 9.3.2.3 and 9.1.0.5.

Affected products

Erlang/Inets: from 5.10 before 9.1.0.5; from 9.3 before 9.3.2.3; from 9.6 before 9.6.1 — fixed in 9.1.0.5, 9.3.2.3, 9.6.1
Erlang OTP: from 17.0 before 26.2.5.18; from 27.0 before 27.3.4.9; from 28.0 before 28.4.1 — fixed in 26.2.5.18, 27.3.4.9, 28.4.1

How to fix

Upgrade to a fixed version: Erlang/Inets 9.1.0.5, 9.3.2.3, 9.6.1; Erlang OTP 26.2.5.18, 27.3.4.9, 28.4.1. Check the vendor advisory for mitigations.

References

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