blaze: Chunked-body trailer fields promoted into Request.headers in blaze-server (front-end header-sanitization bypass)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.4 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

• CVE-2026-73495: 0.48% estimated exploitation activity in the next 30 days; percentile 41.45 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary blaze-server can merge HTTP/1.1 chunked-body trailer fields into Request.headers. Because trailer fields are attacker-controlled, an unauthenticated remote client can inject arbitrary header names/values (e.g. X-Forwarded-For, internal-auth headers) that a fronting proxy sanitized from the request-header section, bypassing header-based trust decisions in the application.

Impact Any http4s application using BlazeServerBuilder over HTTP/1.1 whose routes or middleware trust proxy-set headers (e.g., X-Forwarded-For, X-Real-IP, X-Forwarded-Host, org-internal auth headers) is affected. Where a fronting proxy strips/normalizes those headers but forwards chunked bodies with trailers intact, an attacker can spoof client IP for allow-lists/rate-limits/audit, forge the https scheme, or inject internal-auth headers. A promoted Connection: close trailer is also honored, allowing attacker-controlled termination of pooled backend connections.

Workarounds Deploy behind a proxy that…

Published by the source

24 Jul 2026, 22:26 UTC

Last modified by the source

12 Aug 2026, 21:12 UTC

Source last checked

26 Oct 2026, 16:54 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-46q4-43ph-c6fr

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# Patch Next advisory brief: CVE-2026-73495

Advisory: blaze: Chunked-body trailer fields promoted into Request.headers in blaze-server (front-end header-sanitization bypass)
IDs: CVE-2026-73495, GHSA-46q4-43ph-c6fr
Record: https://patchnext.com/advisories/cve-2026-73495 (revision 1, source checked 26 Oct 2026, 16:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: org.http4s:blaze-http_3 1.0.0-M42, 0.23.18; org.http4s:blaze-http_2.13 1.0.0-M42, 0.23.18; org.http4s:blaze-http_2.12 0.23.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.http4s:blaze-http_3: affected <= 0.23.17; >= 1.0.0-M1, < 1.0.0-M42; fixed in 1.0.0-M42, 0.23.18
- Maven org.http4s:blaze-http_2.13: affected <= 0.23.17; >= 1.0.0-M1, <= 1.0.0-M41; fixed in 1.0.0-M42, 0.23.18
- Maven org.http4s:blaze-http_2.12: affected <= 0.23.17; fixed in 0.23.18

## Suggested inspection (read-only)
- Maven org.http4s:blaze-http_3: check the installed version with your package manager.
- Maven org.http4s:blaze-http_2.13: check the installed version with your package manager.
- Maven org.http4s:blaze-http_2.12: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.http4s:blaze-http_3, org.http4s:blaze-http_2.13, org.http4s:blaze-http_2.12.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-46q4-43ph-c6fr (GitHub-reviewed advisory GHSA-46q4-43ph-c6fr lists affected ranges: Maven org.http4s:blaze-http_2.13 >=0 =1.0.0-M1 =0 =0 =1.0.0-M1 <1.0.0-M42)

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