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.

Timeline

1. Aug 12, 2026, 9:08 PM UTCCVE published
2. Aug 13, 2026, 5:49 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Aug 13, 2026, 5:53 PM UTCRecord last updated

Affected versions

• http4s blaze

  fixed in 0.23.18; From 1.0.0-M1 · fixed in 1.0.0-M42

• org.http4s:http4s-blaze-server_2.13Maven

  fixed in 0.23.18

• org.http4s:http4s-blaze-server_2.13Maven

  From 1.0.0-M1 · fixed in 1.0.0-M42

• org.http4s:http4s-blaze-server_2.12Maven

  fixed in 0.23.18

• org.http4s:http4s-blaze-server_3Maven

  From 1.0.0-M1 · fixed in 1.0.0-M42

References 4