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Http4s Ember HTTP/2 does not enforce SETTINGS_MAX_CONCURRENT_STREAMS

High severity GitHub Reviewed Published Jul 6, 2026 in http4s/http4s • Updated Sep 15, 2026

Package

maven org.http4s:http4s-ember-core_2.12 (Maven)

Affected versions

<= 0.23.34

Patched versions

0.23.35
maven org.http4s:http4s-ember-core_2.13 (Maven)
<= 0.23.34
>= 1.0.0-M1, <= 1.0.0-M46
0.23.35
1.0.0-M47
maven org.http4s:http4s-ember-core_3 (Maven)
<= 0.23.34
>= 1.0.0-M1, <= 1.0.0-M46
0.23.35
1.0.0-M47

Description

An ember server with HTTP/2 enabled (.withHttp2) does not enforce SETTINGS_MAX_CONCURRENT_STREAMS on streams opened by the peer. A single unauthenticated connection can open an unbounded number of concurrent streams, each of which allocates per-stream server state that is never released, exhausting the heap.

Impact

Unauthenticated remote denial of service (memory exhaustion) against any Ember server built .withHttp2. This is the resource-exhaustion class of the HTTP/2 "Rapid Reset" family (CVE-2023-44487).

The same unchecked allocation path is reachable on the client via server-initiated PUSH_PROMISE frames, so a malicious or compromised server can exhaust an ember-client's heap the same way.

Preconditions

  • Server: with .withHttp2 enabled.
  • Client: makes HTTP/2 requests to malicious or compromised sites. enablePush is not enforced.

Workarounds

  • Disable HTTP/2 on EmberServerBuilder or EmberClientBuilder (default)
  • Client only: avoid HTTP/2 to untrusted servers until patched.

References

@rossabaker rossabaker published to http4s/http4s Jul 6, 2026
Published to the GitHub Advisory Database Sep 15, 2026
Reviewed Sep 15, 2026
Last updated Sep 15, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(49th percentile)

Weaknesses

Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource. Learn more on MITRE.

Allocation of Resources Without Limits or Throttling

The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. Learn more on MITRE.

CVE ID

CVE-2026-69203

GHSA ID

GHSA-9vwc-pc8p-253q

Source code

Credits

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