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http4s-blaze-server: Unbounded WebSocket message aggregation

Type: advisorySources: OSV

Http4s (http4s-blaze-server) is a minimal, idiomatic Scala interface for HTTP services. Prior to 0.23.18 and 1.0.0-M42, http4s-blaze-server aggregates fragments of an incoming WebSocket message with no limit on total size or fragment count. A client that completes a WebSocket handshake can send an unterminated fragmented message and drive unbounded heap growth in the server JVM, resulting in denial of service through OutOfMemoryError. Any http4s application serving WebSocket routes over BlazeServerBuilder is affected, no non-default configuration is required, and maxWebSocketBufferSize does not bound the aggregate because it bounds only individual frames. A single connection sending continuation frames that never set FIN forces the server to buffer every fragment until the heap is exhausted, terminating the JVM with OutOfMemoryError on the blaze selector thread. Small fragments amplify the cost through per-frame object overhead, so a modest volume of wire bytes is sufficient. This issue is fixed in versions 0.23.18 and 1.0.0-M42.

Products
Not specified
Affected versions
Not specified
Fixed versions
Not specified
CVEs
CVE-2026-73493
Confirmed fixed CVEs
Not specified
Updated in this feed
6 Nov 2026, 00:09 UTC · Revision 1

Source evidence

http4s-blaze-server: Unbounded WebSocket message aggregation

core-osv · Original source

Published 12 Aug 2026, 21:08 UTC

Http4s (http4s-blaze-server) is a minimal, idiomatic Scala interface for HTTP services. Prior to 0.23.18 and 1.0.0-M42, http4s-blaze-server aggregates fragments of an incoming WebSocket message with no limit on total size or fragment count. A client that completes a WebSocket handshake can send an unterminated fragmented message and drive unbounded heap growth in the server JVM, resulting in denial of service through OutOfMemoryError. Any http4s application serving WebSocket routes over BlazeServerBuilder is affected, no non-default configuration is required, and maxWebSocketBufferSize does not bound the aggregate because it bounds only individual frames. A single connection sending continuation frames that never set FIN forces the server to buffer every fragment until the heap is exhausted, terminating the JVM with OutOfMemoryError on the blaze selector thread. Small fragments amplify the cost through per-frame object overhead, so a modest volume of wire bytes is sufficient. This issue is fixed in versions 0.23.18 and 1.0.0-M42.

osv:CVE-2026-73493
Complete source record
{
  "id": "048cc7cd40aa3d45f964f632e6b5265719d0980d96efefbc78937e0ec7ed6133",
  "url": "https://osv.dev/vulnerability/CVE-2026-73493",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "http4s-blaze-server: Unbounded WebSocket message aggregation",
    "summary": "Http4s (http4s-blaze-server) is a minimal, idiomatic Scala interface for HTTP services. Prior to 0.23.18 and 1.0.0-M42, http4s-blaze-server aggregates fragments of an incoming WebSocket message with no limit on total size or fragment count. A client that completes a WebSocket handshake can send an unterminated fragmented message and drive unbounded heap growth in the server JVM, resulting in denial of service through OutOfMemoryError. Any http4s application serving WebSocket routes over BlazeServerBuilder is affected, no non-default configuration is required, and maxWebSocketBufferSize does not bound the aggregate because it bounds only individual frames. A single connection sending continuation frames that never set FIN forces the server to buffer every fragment until the heap is exhausted, terminating the JVM with OutOfMemoryError on the blaze selector thread. Small fragments amplify the cost through per-frame object overhead, so a modest volume of wire bytes is sufficient. This issue is fixed in versions 0.23.18 and 1.0.0-M42.",
    "evidence": "osv:CVE-2026-73493",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-73493",
      "aliases": [
        "CVE-2026-73493",
        "GHSA-7ppr-r889-mcf2"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/http4s/blaze",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "2ae13a74d55209b6573d5228d1aa94f0361a75d0"
                },
                {
                  "fixed": "b2cb8e8591391409bb1df242b45433891251b1b1"
                },
                {
                  "fixed": "173e8ca820a0d12110bfe409c72e9b9c3d28d471"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "0.23.18"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "introduced": "1.0.0-M1"
                      },
                      {
                        "fixed": "1.0.0-M42"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "fixed": "173e8ca820a0d12110bfe409c72e9b9c3d28d471"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "2ae13a74d55209b6573d5228d1aa94f0361a75d0"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "v0.23.18"
                  },
                  {
                    "range": [
                      {
                        "fixed": "b2cb8e8591391409bb1df242b45433891251b1b1"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "v1.0.0-M42"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v1.0.0-M41",
            "v0.23.17",
            "v1.0.0-M40",
            "v0.23.16",
            "v1.0.0-M39",
            "v0.23.15",
            "v0.23.14",
            "v1.0.0-M38",
            "v1.0.0-M37",
            "v0.23.13",
            "v1.0.0-M36",
            "v1.0.0-M35",
            "v1.0.0-M34",
            "v1.0.0-M33",
            "v0.23.12",
            "v0.15.3",
            "v0.15.2",
            "v0.15.1",
            "v0.15.0",
            "v0.15.0-M4",
            "v0.15.0-M3",
            "v0.15.0-M2",
            "v0.15.0-M1",
            "v0.14.14",
            "v0.14.13",
            "v0.14.12",
            "v0.14.10",
            "v0.14.8",
            "v0.14.7",
            "v0.14.6",
            "v0.14.5",
            "v0.14.4",
            "v0.14.2",
            "v0.14.0",
            "v0.14.0-M12",
            "v0.14.0-M11",
            "v0.14.0-M10",
            "v0.14.0-M9",
            "v0.14.0-M7",
            "v0.14.0-M3",
            "v0.14.0-M2",
            "v0.13.0",
            "v0.12.0",
            "v0.11.0",
            "v0.10.0",
            "v0.9.0",
            "v0.8.1",
            "v0.8.0",
            "v0.7.0",
            "v0.6.0",
            "v0.5.0",
            "v0.4.0",
            "v0.3.0",
            "v0.2.0",
            "v0.1.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-73493.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:45.133819903Z",
      "provider": "osv",
      "references": [
        "https://github.com/http4s/blaze/releases/tag/v0.23.18",
        "https://github.com/http4s/blaze/releases/tag/v1.0.0-M42",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/73xxx/CVE-2026-73493.json",
        "https://github.com/http4s/blaze/security/advisories/GHSA-7ppr-r889-mcf2",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-73493",
        "https://github.com/http4s/blaze/commit/173e8ca820a0d12110bfe409c72e9b9c3d28d471"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-770"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/73xxx/CVE-2026-73493.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-73493",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 513352,
  "published_at": "2026-08-12T21:08:34.981+00:00"
}