Attack Chain

1. The attacker identifies a Netty-based HTTP server that uses HttpContentDecompressor or DelegatingDecompressorFrameListener with a configured maxAllocation value.
2. The attacker crafts a malicious compressed payload designed to expand dramatically upon decompression (a “decompression bomb”). For example, a small compressed file expands to gigabytes of zeros.
3. The attacker sets the Content-Encoding HTTP header to br, zstd, or snappy.
4. The attacker sends an HTTP POST request to the vulnerable server, including the malicious compressed payload in the request body.
5. The server receives the request and HttpContentDecompressor or DelegatingDecompressorFrameListener processes the request, detects the Content-Encoding, and attempts to decompress it using the corresponding decoder (BrotliDecoder, ZstdDecoder, or SnappyFrameDecoder).
6. Because the maxAllocation is not enforced for these decoders, decompression proceeds without memory limits.
7. The decoder allocates memory to store the decompressed data, which rapidly consumes available memory.
8. The server runs out of memory, causing a denial-of-service condition for legitimate users.

Impact

Successful exploitation of this vulnerability leads to a denial-of-service condition on the targeted Netty server. This can disrupt services, cause downtime, and impact legitimate users. Organizations using affected versions of Netty are vulnerable to this attack. Developers may have a false sense of security, believing that maxAllocation protects them from all decompression bombs, but are unknowingly exposed when using brotli, zstd, or snappy encodings. A trivial header modification bypasses the intended protection.

Recommendation

• Upgrade to a patched version of Netty that addresses CVE-2026-42587.
• Apply the recommended fix by passing maxAllocation to all decoder constructors, including BrotliDecoder, SnappyFrameDecoder, and ZstdDecoder, as outlined in the advisory.
• For BrotliDecoder and SnappyFrameDecoder, implement maxAllocation parameter with the same semantics as ZlibDecoder.prepareDecompressBuffer().
• For ZstdDecoder, ensure that when maxAllocation is set, total output across all buffers is bounded.
• Implement a network-level rule to limit the size of compressed requests based on Content-Encoding header and request size to mitigate potential decompression attacks even if the application is vulnerable.