---
id: GHSA-7ppr-r889-mcf2
title: 'blaze: Unbounded WebSocket message aggregation in http4s-blaze-server'
summary: 'blaze: Unbounded WebSocket message aggregation in http4s-blaze-server'
severity: high
cvss: 7.5
cwe:
  - CWE-770
vendor: http4s
product: 'org.http4s:http4s-blaze-server_2.13'
ecosystem: maven
affected:
  - 'org.http4s:http4s-blaze-server_2.13 < 0.23.18'
  - 'org.http4s:http4s-blaze-server_2.13 >= 1.0.0-M1, < 1.0.0-M42'
  - 'org.http4s:http4s-blaze-server_2.12 < 0.23.18'
  - 'org.http4s:http4s-blaze-server_3 >= 1.0.0-M1, < 1.0.0-M42'
patched:
  - 'org.http4s:http4s-blaze-server_2.13 0.23.18'
  - 'org.http4s:http4s-blaze-server_2.13 1.0.0-M42'
  - 'org.http4s:http4s-blaze-server_2.12 0.23.18'
  - 'org.http4s:http4s-blaze-server_3 1.0.0-M42'
published: '2026-07-24'
updated: '2026-07-24'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-7ppr-r889-mcf2'
references:
  - url: 'https://github.com/http4s/blaze/security/advisories/GHSA-7ppr-r889-mcf2'
  - url: >-
      https://github.com/http4s/blaze/commit/173e8ca820a0d12110bfe409c72e9b9c3d28d471
  - url: >-
      https://github.com/http4s/blaze/commit/2ae13a74d55209b6573d5228d1aa94f0361a75d0
  - url: >-
      https://github.com/http4s/blaze/commit/fadbe6d0f7f59045425688d313c8d4804973d12f
  - url: 'https://github.com/http4s/blaze/releases/tag/v0.23.18'
  - url: 'https://github.com/http4s/blaze/releases/tag/v1.0.0-M42'
  - url: 'https://github.com/advisories/GHSA-7ppr-r889-mcf2'
tags:
  - ghsa
  - maven
ingestedAt: '2026-07-24T22:40:26.682Z'
---

## Overview

## Summary

`http4s-blaze-server` aggregates the 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 via `OutOfMemoryError`.

## Impact

Any http4s application serving WebSocket routes over
`BlazeServerBuilder` is affected; no non-default configuration is required,
and `maxWebSocketBufferSize` does not bound the aggregate (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. Where the
WebSocket endpoint is reachable without authentication the attacker is
unauthenticated and remote; where the handshake requires a principal, any
authenticated client can still trigger it.

## Workarounds

- No blaze-server configuration bounds the aggregate; `maxWebSocketBufferSize`
  is not a mitigation.
- Terminate/limit WebSocket traffic at a fronting layer that enforces
  message-size and fragment limits, or disable WebSocket routes.
- Longer term: blaze is EOL upstream; plan migration to a maintained
  backend (e.g. ember).

## Affected packages

- `org.http4s:http4s-blaze-server_2.13 < 0.23.18`
- `org.http4s:http4s-blaze-server_2.13 >= 1.0.0-M1, < 1.0.0-M42`
- `org.http4s:http4s-blaze-server_2.12 < 0.23.18`
- `org.http4s:http4s-blaze-server_3 >= 1.0.0-M1, < 1.0.0-M42`

## Remediation

Upgrade to a patched release:

- `org.http4s:http4s-blaze-server_2.13 0.23.18`
- `org.http4s:http4s-blaze-server_2.13 1.0.0-M42`
- `org.http4s:http4s-blaze-server_2.12 0.23.18`
- `org.http4s:http4s-blaze-server_3 1.0.0-M42`