CVE-2026-73494: HTTP4S Blaze

High severity, CVSS 7.4. EPSS: 0.6% chance of exploitation in the next 30 days.

blaze is a Scala library for building asynchronous pipelines, with a focus on network IO. Prior to 0.23.18 and from 1.0.0-M1 until 1.0.0-M42, five HTTP/1.1 conformance laxities in the hand-written Java parser under http/src/main/java/org/http4s/blaze/http/parser/ can cause blaze to derive a different request boundary than a stricter fronting intermediary. A default BlazeServerBuilder accepts invalid or valueless header field names that violate tchar syntax, obsolete folded field lines (obs-fold), unsupported Transfer-Encoding values, duplicate Content-Length fields, and requests containing both Transfer-Encoding and Content-Length. If a lenient or legacy proxy forwards the malformed bytes but interprets them differently, the disagreement can permit front-end authorization bypass, response-queue poisoning on pooled backend connections, or cache poisoning. Exploitation requires a pair of disagreeing parsers; no non-default blaze configuration is required. The affected checks are enforced in BodyAndHeaderParser and Http1ServerParser. This issue is fixed in versions 0.23.18 and 1.0.0-M42.

Affected products

• HTTP4S Blaze: before 0.23.18 (fixed in 0.23.18); from 1.0.0-M1, before 1.0.0-M42 (fixed in 1.0.0-M42)
• org.http4s Blaze-HTTP 2.13: before 0.23.18 (fixed in 0.23.18); from 1.0.0-M1, before 1.0.0-M42 (fixed in 1.0.0-M42)
• org.http4s Blaze-HTTP 3: before 0.23.18 (fixed in 0.23.18); from 1.0.0-M1, before 1.0.0-M42 (fixed in 1.0.0-M42)
• org.http4s HTTP4S-Blaze-Server 2.13: before 0.23.18 (fixed in 0.23.18)

Published 2026-09-14. Last modified 2026-09-30.