CVE-2026-73494: blaze: Multiple HTTP/1.1 request-smuggling primitives in blaze's Java wire parser

Severity: high (CVSS 7.4 v3.1)
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: 2026-09-14 · updated 2026-09-30
Weakness: CWE-444

Description

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

org.http4s blaze-http_2.13: before 0.23.18; from 1.0.0-M1, < 1.0.0-M42
org.http4s blaze-http_3: before 0.23.18; from 1.0.0-M1, < 1.0.0-M42
org.http4s http4s-blaze-server_2.13: before 0.23.18
http4s blaze: before 0.23.18; from 1.0.0-M1, < 1.0.0-M42

References

Get alerted when CVE-2026-73494 or the next vulnerability affects your stack