CVE-2020-9493: Apache Chainsaw RCE Vulnerability

CVE-2020-9493 Overview

CVE-2020-9493 is an insecure deserialization vulnerability in Apache Chainsaw, a GUI-based log viewer historically bundled with Apache Log4j 1.x. Versions prior to 2.1.0 process untrusted serialized Java objects without validation, allowing attackers to trigger malicious code execution. The flaw is classified under CWE-502: Deserialization of Untrusted Data. Because Chainsaw can receive log events over the network, an unauthenticated remote attacker can deliver a crafted serialized payload and execute arbitrary code in the context of the Chainsaw process.

Critical Impact

Unauthenticated attackers can achieve remote code execution on hosts running Apache Chainsaw versions earlier than 2.1.0 by sending crafted serialized Java objects.

Affected Products

• Apache Chainsaw versions prior to 2.1.0
• Apache Log4j 1.x distributions bundling Chainsaw
• QOS.ch reload4j (legacy Log4j 1.x fork distributions)

Discovery Timeline

• 2021-06-16 - CVE-2020-9493 published to NVD via Openwall OSS-Security announcement
• 2022-01-18 - Follow-up advisory published via Openwall OSS-Security and Apache announce mailing list
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-9493

Vulnerability Analysis

Apache Chainsaw is a Java Swing-based log viewer that accepts log events from remote sources. Prior to version 2.1.0, the application deserialized incoming Java objects without restricting the allowed classes. An attacker who can reach a Chainsaw receiver port can deliver a gadget-chain payload built from classes already on the Chainsaw classpath. When ObjectInputStream.readObject() reconstructs the payload, side effects in the gadget chain trigger arbitrary code execution under the user account running Chainsaw.

The vulnerability is reachable across the network without authentication or user interaction. Successful exploitation yields full confidentiality, integrity, and availability impact on the host.

Root Cause

The root cause is unrestricted Java deserialization [CWE-502]. Chainsaw's log event receivers invoke readObject() on attacker-controlled byte streams with no allowlist, no type filtering, and no signature validation. Any class on the classpath that exposes a dangerous side effect during deserialization can be weaponized as a gadget to execute commands.

Attack Vector

The attack vector is network-based. An attacker locates a Chainsaw instance listening for remote log events, then transmits a serialized Java object containing a known gadget chain (for example, chains derived from ysoserial payloads using libraries present on the Chainsaw classpath). Upon deserialization, the gadget chain invokes runtime methods such as Runtime.exec() to execute attacker-supplied commands. Because Chainsaw is typically run interactively by administrators, the resulting code execution often occurs with elevated local privileges.

No verified public proof-of-concept code is referenced in the official advisories. For technical detail, refer to the Openwall OSS-Security thread and the Apache announce mailing list post.

Detection Methods for CVE-2020-9493

Indicators of Compromise

• Unexpected child processes (for example, cmd.exe, powershell.exe, bash, sh) spawned by a Java process running org.apache.log4j.chainsaw.Main or the Chainsaw JAR.
• Outbound network connections from the Chainsaw host to unfamiliar IP addresses immediately after receipt of a log event.
• Inbound TCP traffic carrying the Java serialization magic bytes 0xAC 0xED 0x00 0x05 directed at Chainsaw receiver ports.
• Presence of chainsaw-*.jar at versions earlier than 2.1.0 in deployed Log4j 1.x or reload4j installations.

Detection Strategies

• Inventory all Java applications and identify any bundled chainsaw-*.jar artifact below version 2.1.0 using software composition analysis.
• Inspect network telemetry for serialized Java payloads (0xAC 0xED header) on ports associated with Chainsaw SocketReceiver or XMLSocketReceiver listeners.
• Hunt for Java process trees that fork shell or scripting interpreters, a behavior pattern consistent with deserialization gadget execution.

Monitoring Recommendations

• Enable command-line auditing on hosts running Chainsaw and forward process creation events to a centralized analytics platform.
• Alert on any Chainsaw listener accepting external connections from outside the management network.
• Track JVM error and exception logs for java.io.InvalidClassException or unusual ObjectInputStream stack traces that may indicate exploitation attempts.

How to Mitigate CVE-2020-9493

Immediate Actions Required

• Upgrade Apache Chainsaw to version 2.1.0 or later, which removes the vulnerable deserialization paths.
• Remove or disable Chainsaw entirely if it is not actively required. Apache Log4j 1.x reached end of life in August 2015 and should not be used in production.
• Migrate from Log4j 1.x to a supported logging framework such as Log4j 2.x or reload4j releases that exclude Chainsaw.
• Block inbound traffic to any Chainsaw receiver ports at the host and network firewall.

Patch Information

The fix is delivered in Apache Chainsaw 2.1.0. Coordination details are published in the Apache announce mailing list message and the Openwall OSS-Security advisory. Operators relying on Log4j 1.x should also review the follow-up OSS-Security update for related deserialization issues.

Workarounds

• Run Chainsaw only on isolated, trusted analyst workstations and never expose its listeners to untrusted networks.
• Restrict Chainsaw to read pre-collected log files locally rather than accepting remote log events over the network.
• Apply host-based firewall rules to limit inbound connections to Chainsaw ports to specific management IP addresses.

# Example: block inbound Chainsaw default receiver ports on Linux
sudo iptables -A INPUT -p tcp --dport 4445 -j DROP
sudo iptables -A INPUT -p tcp --dport 4448 -j DROP

# Verify the installed Chainsaw version
unzip -p chainsaw.jar META-INF/MANIFEST.MF | grep -i version