Netty Mqtt Decoder MqttDecoder.java memory allocation

Summaryinfo

A vulnerability identified as problematic has been detected in Netty. This vulnerability affects the function io.netty.handler.codec.mqtt.MqttDecoder.decodeProperties of the file MqttDecoder.java of the component Mqtt Decoder. The manipulation leads to memory allocation. This vulnerability is referenced as CVE-2026-93575. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability was found in Netty (affected version not known). It has been rated as problematic. This issue affects the function io.netty.handler.codec.mqtt.MqttDecoder.decodeProperties of the file MqttDecoder.java of the component Mqtt Decoder. The manipulation with an unknown input leads to a memory allocation vulnerability. Using CWE to declare the problem leads to CWE-789. The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. Impacted is availability. The summary by CVE is:

### 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. #

The advisory is shared at access.redhat.com. The identification of this vulnerability is CVE-2026-93575 since 09/18/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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• Netty

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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