CVE-2020-13942: Apache Unomi RCE Vulnerability

CVE-2020-13942 Overview

CVE-2020-13942 is a critical remote code execution vulnerability in Apache Unomi, an open-source Customer Data Platform. The vulnerability allows attackers to inject malicious OGNL (Object-Graph Navigation Language) or MVEL (MVFLEX Expression Language) scripts into the publicly accessible /context.json endpoint. This flaw enables unauthenticated remote attackers to execute arbitrary code on vulnerable Apache Unomi servers, potentially leading to complete system compromise.

The vulnerability represents a bypass of an incomplete fix implemented in version 1.5.1 for a previous similar issue. A new attack vector was discovered that circumvented the original security controls, prompting Apache to implement complete script filtering in version 1.5.2.

Critical Impact

Unauthenticated attackers can achieve remote code execution on Apache Unomi servers through OGNL/MVEL script injection via a publicly accessible endpoint, potentially leading to full system compromise.

Affected Products

• Apache Unomi versions prior to 1.5.2

Discovery Timeline

• November 24, 2020 - CVE-2020-13942 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-13942

Vulnerability Analysis

Apache Unomi exposes a /context.json endpoint that is publicly accessible and designed to handle user context data. The vulnerability stems from insufficient input validation and filtering of expression language scripts submitted to this endpoint. When processing incoming requests, the application fails to properly sanitize OGNL and MVEL expressions embedded within the request data.

OGNL and MVEL are powerful expression languages commonly used in Java applications for object manipulation. When these expression languages are not properly sandboxed or filtered, they can be leveraged to execute arbitrary Java code, including system commands. The attack requires no authentication, making any internet-exposed Apache Unomi instance a potential target.

This vulnerability is particularly severe because it represents a bypass of security measures implemented in version 1.5.1, where Apache attempted to address a similar script injection issue. The discovery of a new attack vector demonstrated that the original fix was incomplete, leaving systems vulnerable to exploitation through alternative injection techniques.

Root Cause

The root cause is improper input validation (CWE-20) combined with injection vulnerabilities (CWE-74). Apache Unomi failed to implement comprehensive filtering of OGNL and MVEL script constructs in user-supplied input to the /context.json endpoint. The expression language parsers would evaluate malicious scripts embedded in request parameters, enabling code execution within the application's security context.

Attack Vector

The attack is conducted over the network against the publicly accessible /context.json endpoint. An attacker crafts a malicious HTTP request containing OGNL or MVEL expression language payloads designed to execute arbitrary code. Since the endpoint requires no authentication, any attacker with network access to the Apache Unomi server can exploit this vulnerability.

The malicious payload typically leverages expression language capabilities to instantiate Java runtime objects and execute system commands. Upon successful exploitation, the attacker gains code execution privileges equivalent to the Apache Unomi application process, which often runs with elevated permissions on the host system.

Detection Methods for CVE-2020-13942

Indicators of Compromise

• Unusual HTTP POST requests to /context.json containing OGNL expressions such as # symbols, @java.lang, or Runtime.getRuntime()
• Requests containing MVEL expressions with Java class instantiation patterns
• Unexpected child processes spawned by the Apache Unomi Java process
• Network connections originating from the Unomi server to unknown external hosts

Detection Strategies

• Deploy web application firewall (WAF) rules to detect and block OGNL/MVEL injection patterns in requests to /context.json
• Monitor HTTP access logs for suspicious POST requests containing expression language syntax
• Implement intrusion detection signatures for known Apache Unomi exploitation patterns
• Enable application-level logging to capture detailed request payloads for forensic analysis

Monitoring Recommendations

• Configure alerts for unusual process execution patterns from the Apache Unomi service account
• Monitor network traffic for command-and-control communications originating from Unomi servers
• Implement file integrity monitoring on Apache Unomi installation directories
• Review authentication logs for privilege escalation attempts following potential exploitation

How to Mitigate CVE-2020-13942

Immediate Actions Required

• Upgrade Apache Unomi to version 1.5.2 or later immediately
• If immediate patching is not possible, restrict network access to the /context.json endpoint using firewall rules
• Place Apache Unomi behind a reverse proxy with request filtering capabilities
• Conduct incident response procedures if exploitation is suspected

Patch Information

Apache has released version 1.5.2 which implements complete script filtering to prevent OGNL and MVEL injection attacks. Organizations running affected versions should upgrade to the latest available version of the 1.5.x release line. The official security advisory is available at the Apache Unomi CVE-2020-13942 page. Additional technical details can be found in the Checkmarx Advisory CX-2020-4284.

Workarounds

• Implement network-level access controls to restrict access to the /context.json endpoint to trusted sources only
• Deploy a web application firewall with rules to detect and block expression language injection patterns
• Disable or remove the /context.json endpoint if it is not required for your deployment
• Run Apache Unomi with minimal privileges using a dedicated service account to limit post-exploitation impact

# Example: Restrict access to context.json using iptables
# Allow only internal network access to Unomi port
iptables -A INPUT -p tcp --dport 8181 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 8181 -j DROP