CVE-2026-73495 http4s CVE debrief

Who should care

Defenders of http4s applications using BlazeServerBuilder over HTTP/1.1 should assess exposure and verify the impact of injected headers on trust decisions. This includes operators, platform administrators, vulnerability management teams, and security teams who need to ensure that their applications are not exposed to this vulnerability.

Why it matters

CVE-2026-73495 allows unauthenticated remote clients to inject arbitrary headers, bypassing trust decisions in http4s applications using BlazeServerBuilder over HTTP/1.1. Defenders should verify exposure and assess the impact of injected headers on trust decisions.

• Bypassing header-based trust decisions
• Spoofing client IP for allow-lists, rate limits, or auditing
• Forging the https scheme or injecting internal authorization headers

Technical summary

The Blaze library, used for building asynchronous pipelines, has a vulnerability that allows unauthenticated remote clients to inject arbitrary headers into http4s applications using BlazeServerBuilder over HTTP/1.1. This vulnerability can lead to bypassing header-based trust decisions, spoofing client IP for allow-lists, rate limits, or auditing, forging the https scheme, or injecting internal authorization headers. The fix is available in versions 0.23.18 and 1.0.0-M42. Defenders should prioritize verifying exposure of http4s applications using BlazeServerBuilder over HTTP/1.1 and assessing the impact of injected headers on trust decisions.

Defensive priority

Defenders should prioritize verifying exposure of http4s applications using BlazeServerBuilder over HTTP/1.1 and assessing the impact of injected headers on trust decisions.

Recommended defensive actions

• Verify exposure of http4s applications using BlazeServerBuilder over HTTP/1.1
• Assess the impact of injected headers on trust decisions
• Update to versions 0.23.18 or 1.0.0-M42
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record and NVD entry provide details on the vulnerability in the Blaze library, which affects http4s applications using BlazeServerBuilder over HTTP/1.1. The vulnerability allows unauthenticated remote clients to inject arbitrary headers, bypassing trust decisions in http4s applications. The fix is available in versions 0.23.18 and 1.0.0-M42. Defenders should verify exposure and assess the impact of injected headers on trust decisions.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.