CVE-2026-56818: Netty: RedisArrayAggregator max-elements failure leaves retained partial aggregate state

Summary

RedisArrayAggregator clears retained partial aggregate state when the maxNestedArrayDepth limit is exceeded, but it does not clear the same state when the sibling maxElements limit is exceeded. A peer can start a valid RESP array, send a bulk-string child, then send a nested array header longer than the configured maxElements. Netty throws a decoder exception, but the existing partial aggregate remains retained in the handler.

If the application leaves the channel alive after the exception, later messages are still consumed into the pre-error aggregate. The supplied PoV proves both the retained ByteBuf reference and the stale parser state continuation.

Technical Details

RedisArrayAggregator.decode(...) retains non-array messages before adding them to depths.peek().children. In decodeRedisArrayHeader(...), the header.length() > maxElements branch throws immediately:

java if (header.length() > maxElements) { throw new CodecException("this codec doesn't support longer length than " + maxElements); }

The immediately following nested-depth branch clears retained aggregate state before throwing:

java if (depths.size() >= maxNestedArrayDepth) { releaseAndClearDepths(); throw new CodecException("max nested array depth exceeded: " + maxNestedArrayDepth); }

The missing cleanup in the first branch leaves retained children and aggregate state reachable after the exception.

PoC

Place the supplied RedisArrayAggregatorIncompleteCleanupPovTest.java under:

codec-redis/src/test/java/io/netty/handler/codec/redis/

Run:

fish ./mvnw -pl codec-redis -am -Dtest=RedisArrayAggregatorIncompleteCleanupPovTest -Dsurefire.failIfNoSpecifiedTests=false -DskipNativeTests -DskipAutobahnTests test

The test suite includes:

- serialized RESP trigger through RedisDecoder, RedisBulkStringAggregator, and RedisArrayAggregator; - direct refcount proof that max-elements overflow does not release the retained child immediately; - post-exception continuation proof that the stale aggregate consumes a later message; - nested-depth controls that clear the same partial aggregate state.

All five tests pass on current 4.2, 4.2.15.Final, and 4.1.135.Final.

Impact

For Redis codec pipelines that continue after codec exceptions, an unauthenticated peer can keep attacker-controlled aggregate state alive across a security-limit exception. This can pin retained pooled buffers until channel close/removal or until a later message completes the stale aggregate.

RedisBulkStringAggregator permits bulk strings up to RedisConstants.REDISMESSAGEMAXLENGTH (512MB), so the retained child can be large in deployments that aggregate untrusted Redis streams.

Applications that always close the channel or remove the handler on decoder exceptions will trigger existing cleanup; the issue is the missing immediate cleanup on the max-elements failure path while the handler remains installed.

Suggested Fix

Call releaseAndClearDepths() before throwing from the max-elements branch. Consider applying the same cleanup to all unrecoverable decodeRedisArrayHeader(...) error exits that can occur while depths is non-empty.

Affected Package/Versions

io.netty:netty-codec-redis

Confirmed on:

- current 4.2 branch head 7bae566a93e69409697fe57fa807910ba5c9720e - 4.2.15.Final at a41f7b289ce1 - 4.1.135.Final at f05f765d8146

Advisory History

This differs from the public Redis codec advisories because it reproduces on their patched tags:

- GHSA-5w86-c3rq-vjj7 - GHSA-3244-j874-rhc2 / CVE-2026-44250 - GHSA-6jv9-x5w9-2ccm / CVE-2026-48006 - GHSA-6ghj-frrj-jjj3 / CVE-2026-44890

Why This Is Not Intended Behavior

The public API docs document RedisArrayAggregator as aggregating RedisMessage parts into ArrayRedisMessage and document a CodecException when an array header exceeds maxElements. They do not document preserving pre-exception partial aggregate state after that limit fires.

The adjacent nested-depth branch already calls releaseAndClearDepths() before throwing. The max-elements branch is the sibling aggregation-limit branch but throws without cleanup. Netty's later Redis lifecycle cleanup patch explicitly added release behavior for nested-array failure and handler removal, leaving the max-elements failure branch as a missed cleanup path.