CVE-2026-73495 is a high-severity (CVSS 7.4) CWE-444 vulnerability in org.http4s:blaze-http_2.13. O3 Security confirms whether CVE-2026-73495 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
blaze: Chunked-body trailer fields promoted into Request.headers in blaze-server (front-end header-sanitization bypass)
Real-World Exposure
org.http4s:blaze-http_2.13☕org.http4s:blaze-http_2.12☕org.http4s:blaze-http_3☕org.http4s:blaze-http_3☕org.http4s:blaze-http_2.13Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
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 removes trailer fields (or rejects requests that use trailers) before forwarding; until patched, avoid trust decisions based on headers that a proxy is relied upon to sanitize.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.http4s:blaze-http_2.13 | all versions | 0.23.18 |
| ☕Maven | org.http4s:blaze-http_2.12 | all versions | 0.23.18 |
| ☕Maven | org.http4s:blaze-http_3 | all versions | 0.23.18 |
| ☕Maven | org.http4s:blaze-http_3 | ≥ 1.0.0-M1&&< 1.0.0-M42 | 1.0.0-M42 |
| ☕Maven | org.http4s:blaze-http_2.13 | ≥ 1.0.0-M1&&< 1.0.0-M42 | 1.0.0-M42 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:blaze-http_2.13. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update org.http4s:blaze-http_2.13 to 0.23.18 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-73495 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-73495 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-73495. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-73495 in your dependencies?
O3 detects CVE-2026-73495 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.