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Http4s Ember HTTP/2: unbounded inbound body bufferingGHSA-6m4x-pp6q-5jmm

HIGHFix: http4s/http4s@22d2335

GHSA-6m4x-pp6q-5jmm is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in org.http4s:http4s-ember-core_2.12. A fix is available for org.http4s:http4s-ember-core_2.12 — see the affected versions and patch details below.

Also known asCVE-2026-69202
Published
Updated
Affected
5 pkgs
Patched
5 / 5
Exploits
None indexed
Exploitation data as of Oct 27, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • 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 GHSA-6m4x-pp6q-5jmm.

EPSS Exploitation Probability

via FIRST.org ↗
0.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs50th percentile — riskier than 50% of all scored CVEsHighest risk
0.13%0.46%0.80%1.13%0.6%0.6%Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-6m4x-pp6q-5jmm by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 397,508 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

5 pkgs affected
☕org.http4s:http4s-ember-core_2.12☕org.http4s:http4s-ember-core_2.13☕org.http4s:http4s-ember-core_3☕org.http4s:http4s-ember-core_2.13☕org.http4s:http4s-ember-core_3

Real-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

Ember's HTTP/2 stack replenishes the inbound flow-control window based on bytes received off the wire, not bytes consumed by the application. Received DATA is buffered in an unbounded per-stream channel. Flow control therefore provides no backpressure: a peer can stream a large or unbounded body faster than the application drains it and the connection retains every payload in heap.

This is the read-path mirror of the outbound queue issue.

This affects an Ember receiving a request body and an Ember client receiving a response body from a hostile server.

Impact

Unauthenticated remote denial of service (OOM) against any Ember server built .withHttp2 for a non-draining or slow-draining route, and against an Ember client consuming from a hostile or compromised server.

Workarounds

  • Disable HTTP/2 to remove the vector entirely.
  • Apply an aggregate request-entity size limit (e.g. EntityLimiter middleware) on routes that consume the body.
  • Ensure handlers fully drain request bodies with aggressive idle timeouts.

Affected Packages

5 total 5 fixed
EcosystemPackageVulnerable rangeFix
☕Mavenorg.http4s:http4s-ember-core_2.12all versions0.23.35org.http4s:http4s-ember-core_2.12:0.23.35
☕Mavenorg.http4s:http4s-ember-core_2.13all versions0.23.35org.http4s:http4s-ember-core_2.13:0.23.35
☕Mavenorg.http4s:http4s-ember-core_3all versions0.23.35org.http4s:http4s-ember-core_3:0.23.35
☕Mavenorg.http4s:http4s-ember-core_2.13≥ 1.0.0-M1&&< 1.0.0-M471.0.0-M47org.http4s:http4s-ember-core_2.13:1.0.0-M47
☕Mavenorg.http4s:http4s-ember-core_3≥ 1.0.0-M1&&< 1.0.0-M471.0.0-M47org.http4s:http4s-ember-core_3:1.0.0-M47

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:http4s-ember-core_2.12, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update org.http4s:http4s-ember-core_2.12 to 0.23.35 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-6m4x-pp6q-5jmm is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

Ember's HTTP/2 stack replenishes the inbound flow-control window based on bytes received off the wire, not bytes consumed by the application. Received DATA is buffered in an unbounded per-stream channel. Flow control therefore provides no backpressure: a peer can stream a large or unbounded body faster than the application drains it and the connection retains every payload in heap. This is the read-path mirror of the [outbound queue issue](https://github.com/http4s/http4s/security/advisories/GHSA-8f3q-3jmv-7prw). This affects an Ember receiving a request body and an Ember client receiving
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Http4s Ember HTTP/2: unbounded inbound body buffering