http4s blaze up to 0.23.17/1.0.0-M41 improper authorization

Summaryinfo

A vulnerability was found in http4s blaze up to 0.23.17/1.0.0-M41. It has been rated as critical. Affected by this issue is some unknown functionality. The manipulation leads to improper authorization. This vulnerability is documented as CVE-2026-73495. The attack can be initiated remotely. There is not any exploit available.

Detailsinfo

A vulnerability was found in http4s blaze up to 0.23.17/1.0.0-M41 and classified as critical. This issue affects an unknown function. The manipulation with an unknown input leads to a improper authorization vulnerability. Using CWE to declare the problem leads to CWE-285. The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-73495 since 08/12/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. The technical details are unknown and an exploit is not publicly available. The attack technique deployed by this issue is T1548.002 according to MITRE ATT&CK.

Upgrading to version 0.23.18 or 1.0.0-M42 eliminates this vulnerability.

Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Vendor

• http4s

Name

• blaze

Version

• 0.23.0
• 0.23.1
• 0.23.2
• 0.23.3
• 0.23.4
• 0.23.5
• 0.23.6
• 0.23.7
• 0.23.8
• 0.23.9
• 0.23.10
• 0.23.11
• 0.23.12
• 0.23.13
• 0.23.14
• 0.23.15
• 0.23.16
• 0.23.17
• 1.0.0-M41

Website

• Product: https://github.com/http4s/blaze/

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Discussion