---
id: CVE-2026-73495
title: >-
  blaze is a Scala library for building asynchronous pipelines, with a focus on
  network IO
summary: >-
  blaze is a Scala library for building asynchronous pipelines, with a focus on
  network IO. Prior to 0.23.18 and 1.0.0-M42, blaze-server can merge HTTP/1.1
  chunked-body trailer fields into Request.headers. Because trailer fields are
  attack…
severity: high
cvss: 7.4
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N'
cwe:
  - CWE-444
vendor: http4s
product: blaze
affected:
  - blaze < 0.23.18
  - 'blaze >= 1.0.0-M1, < 1.0.0-M42'
published: '2026-08-12'
updated: '2026-09-09'
sourceUpdated: '2026-09-09T21:02:22.660'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-73495'
references:
  - url: >-
      https://github.com/http4s/blaze/commit/ef3e666c146cfc16cb6603f1fc3c464daab4a24f
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/blaze/releases/tag/v0.23.18'
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/blaze/releases/tag/v1.0.0-M42'
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/blaze/security/advisories/GHSA-46q4-43ph-c6fr'
    label: security-advisories@github.com
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-08-13T13:02:55.958244Z'
ingestedAt: '2026-09-10T19:07:15.840Z'
epss: 0.0048
epssPercentile: 0.38713
---

## Overview

blaze is a Scala library for building asynchronous pipelines, with a focus on network IO. Prior to 0.23.18 and 1.0.0-M42, 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 and values, including X-Forwarded-For and internal authorization headers, that a fronting proxy sanitized from the request-header section, bypassing header-based trust decisions in the application. Any http4s application using BlazeServerBuilder over HTTP/1.1 whose routes or middleware trust proxy-set headers, including X-Forwarded-For, X-Real-IP, and X-Forwarded-Host, is affected. If a fronting proxy strips or normalizes those headers but forwards chunked bodies with trailers intact, an attacker can spoof client IP for allow-lists, rate limits, or auditing, forge the https scheme, or inject internal authorization headers. A promoted Connection: close trailer is also honored, allowing attacker-controlled termination of pooled backend connections. This issue is fixed in versions 0.23.18 and 1.0.0-M42.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.