GCVE Workshop - 22 September 2026 (14:00-18:00), Luxembourg Before The Vulnopticon Conference - Registration

GHSA-CP4Q-FQW9-4HF6

Vulnerability from github – Published: 2026-09-15 20:01 – Updated: 2026-09-15 20:01
VLAI
Summary
Http4s Ember HTTP/2: unbounded continuation frame accumulation
Details

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.
Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.23.34"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_2.12"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.23.35"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.23.34"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_2.13"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.23.35"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.23.34"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_3"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.23.35"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.0.0-M46"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_2.13"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.0.0-M1"
            },
            {
              "fixed": "1.0.0-M47"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.0.0-M46"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_3"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.0.0-M1"
            },
            {
              "fixed": "1.0.0-M47"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-69218"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-770"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-15T20:01:08Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "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. \n\n### Impact\n\nA remote, unauthenticated peer can exhaust the heap on any Ember endpoint that has HTTP/2 enabled:\n\n- **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.\n- **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.\n\nMemory consumption is bounded only by the attacker\u0027s upload bandwidth and the connection lifetime.\n\n### Prerequisites\n\n- `EmberServerBuilder` configured with `.withHttp2`.\n    - For the server: the attacker can establish an HTTP/2 connection\n    - For the client: the application makes a request to an attacker-controlled origin.\nHTTP/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.\n\n### Patches\n\nThe 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.\n\n### Workarounds\n\n- Disable HTTP/2 (`.withHttp2`) until upgraded (default).  It is off by default.\n- If HTTP/2 must remain enabled, place ember behind a reverse proxy that terminates HTTP/2 and speaks HTTP/1.1 to ember.",
  "id": "GHSA-cp4q-fqw9-4hf6",
  "modified": "2026-09-15T20:01:08Z",
  "published": "2026-09-15T20:01:08Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/http4s/http4s/security/advisories/GHSA-cp4q-fqw9-4hf6"
    },
    {
      "type": "WEB",
      "url": "https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/http4s/http4s"
    },
    {
      "type": "WEB",
      "url": "https://github.com/http4s/http4s/releases/tag/v0.23.35"
    },
    {
      "type": "WEB",
      "url": "https://github.com/http4s/http4s/releases/tag/v1.0.0-M47"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Http4s Ember HTTP/2: unbounded continuation frame accumulation"
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.