CVE-2016-4000
📋 TL;DR
CVE-2016-4000 is a critical deserialization vulnerability in Jython that allows remote attackers to execute arbitrary code by sending a malicious serialized PyFunction object. This affects any application using vulnerable Jython versions for Python code execution within Java environments. Attackers can achieve complete system compromise through this remote code execution flaw.
💻 Affected Systems
- Jython
📦 What is this software?
Jython by Jython Project
⚠️ Risk & Real-World Impact
Worst Case
Complete system takeover with attacker gaining full control over the affected server, enabling data theft, lateral movement, and persistent backdoor installation.
Likely Case
Remote code execution leading to application compromise, data exfiltration, and potential pivot to other systems in the network.
If Mitigated
Limited impact if proper network segmentation, least privilege, and input validation are implemented, though exploitation risk remains high.
🎯 Exploit Status
Exploitation is straightforward with publicly available proof-of-concept code. The vulnerability requires only the ability to send serialized data to the vulnerable application.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Jython 2.7.1rc1 and later
Vendor Advisory: http://bugs.jython.org/issue2454
Restart Required: Yes
Instructions:
1. Download Jython 2.7.1rc1 or later from official sources. 2. Replace existing Jython installation with patched version. 3. Restart all Java applications using Jython. 4. Verify no applications are using old Jython libraries.
🔧 Temporary Workarounds
Input validation and filtering
allImplement strict input validation to reject serialized PyFunction objects at application boundaries.
Implement custom ObjectInputStream with restricted class loading
Use whitelisting for deserialized classes
Network controls
linuxRestrict network access to vulnerable applications using firewalls and segmentation.
iptables -A INPUT -p tcp --dport [APP_PORT] -s [TRUSTED_NETWORK] -j ACCEPT
iptables -A INPUT -p tcp --dport [APP_PORT] -j DROP
🧯 If You Can't Patch
- Isolate vulnerable systems in separate network segments with strict firewall rules
- Implement application-level monitoring for suspicious deserialization attempts and PyFunction usage
🔍 How to Verify
Check if Vulnerable:
Check Jython version with: java -jar jython.jar --version or examine JAR file metadata. Versions before 2.7.1rc1 are vulnerable.Check Version:
java -jar jython.jar --version 2>&1 | grep -i versionVerify Fix Applied:
Confirm Jython version is 2.7.1rc1 or later and test with known exploit payloads to ensure they are rejected.📡 Detection & Monitoring
Log Indicators:
- Java deserialization errors
- ClassNotFoundException for PyFunction
- Unexpected process execution from Java applications
Network Indicators:
- Serialized Java objects sent to application ports
- Base64-encoded serialized data in HTTP requests
SIEM Query:
source="application.log" AND ("PyFunction" OR "ObjectInputStream" OR "readObject") AND severity=ERROR🔗 References
- http://bugs.jython.org/issue2454
- http://www.debian.org/security/2017/dsa-3893
- http://www.oracle.com/technetwork/security-advisory/cpuoct2018-4428296.html
- http://www.securityfocus.com/bid/105647
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=864859
- https://hg.python.org/jython/file/v2.7.1rc1/NEWS
- https://hg.python.org/jython/rev/d06e29d100c0
- https://lists.apache.org/thread.html/0919ec1db20b1022f22b8e78f355667df74d6142b463ff17d03ad533%40%3Cdevnull.infra.apache.org%3E
- https://security-tracker.debian.org/tracker/CVE-2016-4000
- https://security.gentoo.org/glsa/201710-28
- https://snyk.io/vuln/SNYK-JAVA-ORGPYTHON-31451
- https://www.oracle.com/security-alerts/cpuapr2020.html
- https://www.oracle.com/security-alerts/cpujan2020.html
- https://www.oracle.com/security-alerts/cpujul2020.html
- https://www.oracle.com/technetwork/security-advisory/cpuapr2019-5072813.html
- https://www.oracle.com/technetwork/security-advisory/cpujan2019-5072801.html
- https://www.oracle.com/technetwork/security-advisory/cpuoct2019-5072832.html
- http://bugs.jython.org/issue2454
- http://www.debian.org/security/2017/dsa-3893
- http://www.oracle.com/technetwork/security-advisory/cpuoct2018-4428296.html
- http://www.securityfocus.com/bid/105647
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=864859
- https://hg.python.org/jython/file/v2.7.1rc1/NEWS
- https://hg.python.org/jython/rev/d06e29d100c0
- https://lists.apache.org/thread.html/0919ec1db20b1022f22b8e78f355667df74d6142b463ff17d03ad533%40%3Cdevnull.infra.apache.org%3E
- https://security-tracker.debian.org/tracker/CVE-2016-4000
- https://security.gentoo.org/glsa/201710-28
- https://snyk.io/vuln/SNYK-JAVA-ORGPYTHON-31451
- https://www.oracle.com/security-alerts/cpuapr2020.html
- https://www.oracle.com/security-alerts/cpujan2020.html
- https://www.oracle.com/security-alerts/cpujul2020.html
- https://www.oracle.com/technetwork/security-advisory/cpuapr2019-5072813.html
- https://www.oracle.com/technetwork/security-advisory/cpujan2019-5072801.html
- https://www.oracle.com/technetwork/security-advisory/cpuoct2019-5072832.html
