CVE-2026-73495 is a high-severity (CVSS 7.4) CWE-444 vulnerability in org.http4s:blaze-http_2.13. A fix is available for org.http4s:blaze-http_2.13 — see the affected versions and patch details below.
blaze: Chunked-body trailer fields promoted into Request.headers in blaze-server (front-end header-sanitization bypass)
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-73495.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-73495 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 379,842 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.http4s:blaze-http_2.13 | all versions | 0.23.18org.http4s:blaze-http_2.13:0.23.18 |
| ☕Maven | org.http4s:blaze-http_2.12 | all versions | 0.23.18org.http4s:blaze-http_2.12:0.23.18 |
| ☕Maven | org.http4s:blaze-http_3 | all versions | 0.23.18org.http4s:blaze-http_3:0.23.18 |
| ☕Maven | org.http4s:blaze-http_3 | ≥ 1.0.0-M1&&< 1.0.0-M42 | 1.0.0-M42org.http4s:blaze-http_3:1.0.0-M42 |
| ☕Maven | org.http4s:blaze-http_2.13 | ≥ 1.0.0-M1&&< 1.0.0-M42 | 1.0.0-M42org.http4s:blaze-http_2.13:1.0.0-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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-73495 can be triaged on real exposure rather than presence alone.
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 Security finds CVE-2026-73495 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.