AsyncHttpClient: Unbounded WebSocket permessage-deflate decompression enables a decompression-bomb denial of service when compression is enabled

Impact

With WebSocket compression enabled, the client inflates permessage-deflate messages with no limit on the decompressed size. It installed Netty's shared WebSocketClientCompressionHandler.INSTANCE, whose inflater is unbounded, and webSocketMaxFrameSize and webSocketMaxBufferSize only bound the compressed bytes, because the frame aggregator sits in front of the inflater.

A malicious or compromised WebSocket server, or anyone on the path of a ws:// connection, can therefore send a message of about 2 MiB that inflates to about 2 GiB, the most a Netty buffer can hold. The client then copies the inflated message again to hand it to the listener. That exhausts the heap of a typically sized JVM. Netty catches the resulting OutOfMemoryError and closes that connection, but while the buffer is live any other allocation in the process can fail too, and a server that keeps sending such messages, on one connection or several, keeps the client at heap exhaustion.

Who is Impacted

Only applications that enable WebSocket compression with setEnablewebSocketCompression(true), which is off by default, and connect to a WebSocket server that is untrusted, compromised, or reached over cleartext ws://.

Affected versions

• 3.x: up to and including 3.0.13
• 2.x: from 2.2.0, when WebSocket compression was added, up to and including 2.16.1

Patches

Fixed in 3.0.14. A new setting, webSocketMaxDecompressedFrameSize (setWebSocketMaxDecompressedFrameSize, or the org.asynchttpclient.webSocketMaxDecompressedFrameSize property), bounds how far a message may inflate, and a message that would go past it fails the connection. It defaults to 128000000 bytes, the same as webSocketMaxBufferSize, so a message is bounded alike whether or not it was compressed; a compressed message that inflates past that, which was accepted before, now fails the connection. With aggregateWebSocketFrameFragments turned off, the bound applies to each frame instead, and fragments are delivered one at a time. Set it lower if you enable compression and do not expect large messages. 0 disables the limit.

The 2.x line is end of life and will not receive a fix. Upgrade to 3.0.14.

Workarounds

Leave WebSocket compression disabled, which is the default.

Details

After the handshake the inbound pipeline is ws-decoder, ws-aggregator, PerMessageDeflateDecoder, ahc-ws: the aggregator, which enforces webSocketMaxBufferSize, sees each message before it is inflated. WebSocketClientCompressionHandler.INSTANCE is built with maxAllocation = 0, which Netty treats as unbounded, and Netty has deprecated it in favour of a constructor that takes a limit. RFC 6455 Section 10.4 asks an implementation to limit the size of a message after reassembly, and under RFC 7692 Section 6.2 the message delivered to the application is the decompressed payload.

This is a different path from the HTTP response decompression fixed under CVE-2026-85721, which never reached the WebSocket pipeline.

Attribution

AI-assisted tools were used to support discovery and analysis.