---
id: CVE-2026-73493
title: >-
  Http4s (http4s-blaze-server) is a minimal, idiomatic Scala interface for HTTP
  services
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 cou…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-770
published: '2026-08-12'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T20:30:11.423'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-73493'
references:
  - url: >-
      https://github.com/http4s/blaze/commit/173e8ca820a0d12110bfe409c72e9b9c3d28d471
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/blaze/releases/tag/v0.23.18'
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/blaze/releases/tag/v1.0.0-M42'
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/blaze/security/advisories/GHSA-7ppr-r889-mcf2'
    label: security-advisories@github.com
tags:
  - nvd
epss: 0.0063
epssPercentile: 0.47965
ingestedAt: '2026-09-10T21:05:53.596Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.