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GHSA-cp4q-fqw9-4hf6

HighCVSS 7.5 / 10
Published Sep 15, 2026·Last modified Sep 15, 2026
Affected Components(5)
Maven logoorg.http4s/http4s-ember-core_3
< 0.23.35
Maven logoorg.http4s/http4s-ember-core_2.13
< 0.23.35
Maven logoorg.http4s/http4s-ember-core_3
1.0.0-M1 – 1.0.0-M47
1 / 2
Description

When Ember receives an HTTP/2 HEADERS or PUSH_PROMISE frame without the END_HEADERS flag, it buffers the header block fragment and waits for subsequent CONTINUATION frames. These accumulate unbounded until the connection closes.

Impact

A remote, unauthenticated peer can exhaust the heap on any Ember endpoint that has HTTP/2 enabled:

  • ember-server with .withHttp2: any HTTP/2 client can trigger this against any reachable path (including paths that return 404). No authentication is required because the attack completes before the request is decoded.
  • ember-client with .withHttp2: a malicious or compromised origin server can trigger this via the response header block. A single in-flight request is sufficient.

Memory consumption is bounded only by the attacker's upload bandwidth and the connection lifetime.

Prerequisites

  • EmberServerBuilder configured with .withHttp2.
    • For the server: the attacker can establish an HTTP/2 connection
    • For the client: the application makes a request to an attacker-controlled origin. HTTP/2 incoming headers exceeding the configured size limit are buffer in order to generate an informative HTTP 413 response. If a client does not stop sending headers, this leads to memory exhaustion.

Patches

The fix bounds the accumulated header-block size at SETTINGS_MAX_HEADER_LIST_SIZE (derived from EmberServerBuilder.maxHeaderSize / EmberClientBuilder.maxResponseHeaderSize). When a CONTINUATION frame would push the accumulated block over that limit, the connection is terminated with GOAWAY. The receiveHeadersTimeout additionally bounds how long an incomplete header block may remain open.

Workarounds

  • Disable HTTP/2 (.withHttp2) until upgraded (default). It is off by default.
  • If HTTP/2 must remain enabled, place ember behind a reverse proxy that terminates HTTP/2 and speaks HTTP/1.1 to ember.
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Risk Scores
Base Score
7.5

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the availability of the system.

Threat Intelligence
6.9

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.63%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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