CVE-2026-73494: http4s blaze HTTP/1.1 parser conformance bypass

CVE-2026-73494 · Severity: high · CVSS 7.4 · Published 2026-09-14

blaze is a Scala library for building asynchronous network applications. The HTTP/1.1 parser accepts malformed HTTP requests that violate standards, potentially allowing attackers to bypass authorization rules, poison response caches, or inject requests on pooled connections when used behind a stricter proxy or intermediary.

Technologies: org.http4s:http4s-blaze-server_2.13 (Maven). Vendors: Http4s, Maven.

Quick answers

Is CVE-2026-73494 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-73494 being exploited in the wild.
How severe is CVE-2026-73494?
High severity, with a CVSS score of 7.4, published 14 September 2026.
Which products and versions are affected?
Http4s Blaze < 0.23.18, 1.0.0-M1 through 1.0.0-M41; org.http4s:blaze-http_2.13 (Maven) ECOSYSTEM:introduced=0,fixed=0.23.18; org.http4s:blaze-http_3 (Maven) ECOSYSTEM:introduced=0,fixed=0.23.18; org.http4s:http4s-blaze-server_2.13 (Maven) ECOSYSTEM:introduced=0,fixed=0.23.18.
Is there a fix for CVE-2026-73494?
Yes. org.http4s:blaze-http_2.13 (Maven): upgrade to 0.23.18 or later; org.http4s:blaze-http_3 (Maven): upgrade to 0.23.18 or later; org.http4s:http4s-blaze-server_2.13 (Maven): upgrade to 0.23.18 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-73494 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-73494?
Check whether you run Http4s Blaze, org.http4s:blaze-http_2.13 (Maven), org.http4s:blaze-http_3 (Maven) or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • org.http4s:blaze-http_2.13 (Maven): upgrade to 0.23.18 or later
    # set org.http4s:blaze-http_2.13 to 0.23.18 in your pom.xml or build.gradle
  • org.http4s:blaze-http_3 (Maven): upgrade to 0.23.18 or later
    # set org.http4s:blaze-http_3 to 0.23.18 in your pom.xml or build.gradle
  • org.http4s:http4s-blaze-server_2.13 (Maven): upgrade to 0.23.18 or later
    # set org.http4s:http4s-blaze-server_2.13 to 0.23.18 in your pom.xml or build.gradle

Technical details

The hand-written Java parser in blaze (BodyAndHeaderParser and Http1ServerParser) implements five HTTP/1.1 parsing laxities: acceptance of invalid header field names violating tchar syntax, obsolete folded field lines (obs-fold), unsupported Transfer-Encoding values, duplicate Content-Length fields, and requests with both Transfer-Encoding and Content-Length. An attacker can exploit this by sending malformed requests that a lenient blaze parser accepts but a stricter upstream proxy rejects or interprets differently, leading to HTTP request smuggling, cache poisoning, or authorization bypass. The vulnerability requires a pair of disagreeing parsers (blaze and a stricter intermediary); no non-default configuration is needed to trigger it. Patches are available in versions 0.23.18 and 1.0.0-M42.

Affected products

  • http4s blaze < 0.23.18, 1.0.0-M1 through 1.0.0-M41

Identifiers

  • CVE-2026-73494
  • GHSA-mhvj-jhpq-885v

Timeline

  • 2026-09-14: disclosed
  • 2026-09-14: patched: Versions 0.23.18 and 1.0.0-M42

References

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