Netty MQTT Decoder DoS via Property Length Exploit
CVE-2026-93575 Published on September 18, 2026
Netty: netty-codec-mqtt: io.netty/netty-codec-mqtt: netty: resource exhaustion in mqttdecoder
### Summary
Netty's fix for CVE-2026-44248 is incomplete. The decoder checks if the MQTT packet's `Remaining Length` exceeds `maxBytesInMessage`, but fails to validate the `Properties Length` against the `Remaining Length`. An attacker can bypass the size limit by sending a small `Remaining Length` but an enormous `Properties Length`. This forces Netty to buffer and parse millions of properties, allowing an unauthenticated remote attacker to trigger excessive memory and CPU consumption, leading to OutOfMemoryError.
### Details
In `io.netty.handler.codec.mqtt.MqttDecoder`, the `decodeProperties()` helper method reads `totalPropertiesLength` and attempts to parse that many bytes. If the buffer lacks the full length, a `Signal` is thrown. The `catch` block inside `decode()` only enforces `maxBytesInMessage` against `bytesRemainingBeforeVariableHeader` (the packet's `Remaining Length`).
By sending a `CONNECT` packet with a small `Remaining Length` but a huge `Properties Length`, the size check passes. `ReplayingDecoder` then buffers data from the network until the huge `Properties Length` is reached, parsing millions of `UserProperty` objects and exhausting CPU and memory.
#
Timeline
Reported to Red Hat.
Made public.
Weakness Type
OWASP Top Ten 2017 Category A9 - Using Components with Known Vulnerabilities
Weaknesses in this category are related to the A9 category in the OWASP Top Ten 2017.
Products Associated with CVE-2026-93575
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