Netty: Denial of Service in XmlFrameDecoder via CPU Exhaustion

Overview

Summary

An attacker can cause Denial of Service by sending a specially crafted malicious XML payload (e.g., repeated </ characters) to a Netty server utilizing XmlFrameDecoder, causing the server's EventLoop thread to exhaust CPU resources and become unresponsive.

Details

io.netty.handler.codec.xml.XmlFrameDecoder suffers from a vulnerability resulting in CPU exhaustion. When < followed by / is encountered, the decoder scans the remaining buffer for a closing >. Because the parser state is not saved between decode() invocations, an attacker can trickle-feed a payload of </ characters. This forces the decoder to repeatedly rescan the entire accumulated buffer. A 1MB maxFrameLength is enough to completely hang a server's thread while it loops endlessly.

Impact

Denial of Service via CPU Exhaustion. Any application utilizing Netty's XmlFrameDecoder is impacted. An unauthenticated remote attacker can exploit this flaw by sending a modest amount of malformed XML data to an exposed port.

Affected packages

• io.netty:netty-codec-xml >= 4.2.0.Final, <= 4.2.15.Final
• io.netty:netty-codec-xml <= 4.1.135.Final

Remediation

Upgrade to a patched release:

• io.netty:netty-codec-xml 4.2.16.Final
• io.netty:netty-codec-xml 4.1.136.Final