# CVE-2026-73495: http4s blaze HTTP/1.1 trailer header injection

Source: https://junglewise.ai/threats/cve-2026-73495-http4s-blaze-http-1-1-trailer-header-injection  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-12. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-73495 · Severity: high · CVSS 7.4 · Published 2026-08-12

blaze is a Scala networking library used by http4s applications to handle HTTP requests. A vulnerability in its HTTP/1.1 parser allows attackers to inject arbitrary headers via chunked transfer encoding trailers, bypassing proxy header filtering and enabling IP spoofing, authorization bypass, or connection termination. Applications behind proxies that strip or validate headers are at risk.

Vendors: [Http4s](https://junglewise.ai/threats/vendors/http4s), [Maven](https://junglewise.ai/threats/vendors/maven).

## Quick answers

**Is CVE-2026-73495 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-73495 being exploited in the wild.

**How severe is CVE-2026-73495?** High severity, with a CVSS score of 7.4, published 12 August 2026.

**Which products and versions are affected?** Http4s Blaze before 0.23.18 and before 1.0.0-M42; org.http4s:blaze-http_2.12 (Maven) ECOSYSTEM:introduced=0,fixed=0.23.18; 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.

**Is there a fix for CVE-2026-73495?** Yes. org.http4s:blaze-http_2.12 (Maven): upgrade to 0.23.18 or later; 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. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-73495 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-73495?** Check whether you run Http4s Blaze, org.http4s:blaze-http_2.12 (Maven), org.http4s:blaze-http_2.13 (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_2.12 (Maven): upgrade to 0.23.18 or later

  ```sh
  # set org.http4s:blaze-http_2.12 to 0.23.18 in your pom.xml or build.gradle
  ```

- org.http4s:blaze-http_2.13 (Maven): upgrade to 0.23.18 or later

  ```sh
  # 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

  ```sh
  # set org.http4s:blaze-http_3 to 0.23.18 in your pom.xml or build.gradle
  ```


## Technical details

blaze-server's HTTP/1.1 parser improperly merges trailer fields from chunked request bodies into Request.headers, treating them as regular request headers. Because trailers are attacker-controlled and arrive after proxy filtering, a remote unauthenticated client can inject headers like X-Forwarded-For, X-Real-IP, X-Forwarded-Host, or internal authorization headers that a fronting proxy has sanitized from the initial request. The parser also honors a malicious Connection: close trailer, allowing attacker-controlled termination of pooled backend connections. The vulnerability affects http4s applications using BlazeServerBuilder over HTTP/1.1 whose routes trust proxy-set headers. The fix routes trailer fields to a separate buffer instead of merging them into Request.headers, and is available in blaze 0.23.18 and 1.0.0-M42.

## Affected products

- http4s blaze before 0.23.18 and before 1.0.0-M42

## Identifiers

- CVE-2026-73495
- GHSA-46q4-43ph-c6fr

## Timeline

- 2026-08-12: disclosed: Published on NVD

## References

- [https://github.com/http4s/blaze/commit/ef3e666c146cfc16cb6603f1fc3c464daab4a24f](https://github.com/http4s/blaze/commit/ef3e666c146cfc16cb6603f1fc3c464daab4a24f)
- [https://github.com/http4s/blaze/releases/tag/v0.23.18](https://github.com/http4s/blaze/releases/tag/v0.23.18)

## Related threats

- [http4s blaze HTTP/1.1 parser conformance bypass](https://junglewise.ai/threats/cve-2026-73494-http4s-blaze-http-1-1-parser-conformance-bypass)