CVE-2026-73495 blaze: Chunked-body trailer fields promoted into Request.headers in blaze-server (front-end header-sanitization bypass)
Attackers can bypass proxy header sanitization by injecting headers via HTTP chunked trailers, spoofing IPs or authorization in http4s applications.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-73495?
CVE-2026-73495 is a high-severity vulnerability in http4s blaze, published on 12 Aug 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 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.
Weakness: CWE-444
Which versions are affected by CVE-2026-73495?
10 open source packages ship an affected version, including org.http4s:blaze-http_2.13, org.http4s:blaze-http_2.12 and org.http4s:blaze-http_3.
| Package | Affected | Fixed in |
|---|---|---|
org.http4s:blaze-http_2.13 Maven | All versions | 0.23.18 |
org.http4s:blaze-http_2.13 Maven | All versions | 0.23.18 |
org.http4s:blaze-http_2.13 Maven | From 1.0.0-M1 | 1.0.0-M42 |
org.http4s:blaze-http_2.13 Maven | From 1.0.0-M1 | 1.0.0-M42 |
org.http4s:blaze-http_2.12 Maven | All versions | 0.23.18 |
org.http4s:blaze-http_2.12 Maven | All versions | 0.23.18 |
4 more packages
org.http4s:blaze-http_3 Maven | All versions | 0.23.18 |
org.http4s:blaze-http_3 Maven | All versions | 0.23.18 |
org.http4s:blaze-http_3 Maven | From 1.0.0-M1 | 1.0.0-M42 |
org.http4s:blaze-http_3 Maven | From 1.0.0-M1 | 1.0.0-M42 |
How do I fix CVE-2026-73495?
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-2026-73495 being exploited?
There is no known exploitation of CVE-2026-73495 in the wild and no public exploit code.
Sources
Also known as GHSA-46q4-43ph-c6fr.