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

What is CVE-2026-73494?

CVE-2026-73494 is a high-severity vulnerability in http4s blaze, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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.

Weakness: CWE-444

Which versions are affected by CVE-2026-73494?

10 open source packages ship an affected version, including org.http4s:blaze-http_2.13, org.http4s:blaze-http_3 and org.http4s:http4s-blaze-server_2.13.

How do I fix CVE-2026-73494?

Upgrade org.http4s:blaze-http_2.13 to a fixed version for your release line: 0.23.18 or 1.0.0-M42.

Is CVE-2026-73494 being exploited?

There is no known exploitation of CVE-2026-73494 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-mhvj-jhpq-885v.