CVE-2021-39154

Q: What is the severity of CVE-2021-39154?

CVE-2021-39154 has a CVSS score of 8.5 (HIGH). CVE-2021-39154 is a remote code execution vulnerability in XStream library that allows attackers to execute arbitrary code by manipulating XML input streams. Only users who haven't implemented XStream's security framework with a minimal type whitelist are affected. The vulnerability stems from insufficient input validation during XML deserialization.

8.5 HIGH

Remote Code Execution Deserialization

📋 TL;DR

CVE-2021-39154 is a remote code execution vulnerability in XStream library that allows attackers to execute arbitrary code by manipulating XML input streams. Only users who haven't implemented XStream's security framework with a minimal type whitelist are affected. The vulnerability stems from insufficient input validation during XML deserialization.

💻 Affected Systems

Products:

XStream

Versions: All versions before 1.4.18

Operating Systems: All operating systems running Java applications using XStream

Default Config Vulnerable: ⚠️ Yes

Notes: Applications using XStream for XML deserialization without proper security configuration are vulnerable. The vulnerability affects the default configuration in versions before 1.4.18.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Full system compromise with remote attacker gaining complete control over the affected system, potentially leading to data theft, ransomware deployment, or lateral movement within the network.

🟠

Likely Case

Remote code execution leading to application compromise, data exfiltration, or deployment of malware/backdoors on vulnerable systems.

🟢

If Mitigated

No impact for users who have properly configured XStream's security framework with a minimal type whitelist as recommended.

🌐 Internet-Facing: HIGH - This vulnerability can be exploited remotely without authentication, making internet-facing systems particularly vulnerable to attack.

🏢 Internal Only: MEDIUM - Internal systems are still at risk from insider threats or compromised internal systems, though attack surface is reduced compared to internet-facing systems.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires sending specially crafted XML to vulnerable endpoints. Public proof-of-concept code exists in the security advisory.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 1.4.18

Vendor Advisory: https://github.com/x-stream/xstream/security/advisories/GHSA-6w62-hx7r-mw68

Restart Required: Yes

Instructions:

1. Update XStream dependency to version 1.4.18 or later. 2. For Maven projects: update pom.xml to <version>1.4.18</version>. 3. For Gradle projects: update build.gradle to implementation 'com.thoughtworks.xstream:xstream:1.4.18'. 4. Rebuild and redeploy application. 5. Restart affected services.

🔧 Temporary Workarounds

Implement XStream Security Framework

all

Configure XStream with a minimal type whitelist to restrict deserialization to only necessary classes

XStream xstream = new XStream();
xstream.allowTypes(new Class[]{YourAllowedClass1.class, YourAllowedClass2.class});

Input Validation and Sanitization

all

Implement strict input validation and sanitization for all XML input before processing with XStream

// Validate XML input against schema
// Sanitize input before deserialization
// Implement XML parsing with security features enabled

🧯 If You Can't Patch

Implement network segmentation to isolate systems using XStream from untrusted networks
Deploy web application firewall (WAF) rules to detect and block malicious XML payloads targeting XStream deserialization

🔍 How to Verify

Check if Vulnerable:

Check application dependencies for XStream version. For Java applications: check pom.xml, build.gradle, or manifest files for XStream version < 1.4.18.

Check Version:

For Maven: mvn dependency:tree | grep xstream. For Gradle: gradle dependencies | grep xstream. For general Java: check classpath or manifest.

Verify Fix Applied:

Verify XStream version is 1.4.18 or later in dependencies. Test with known malicious payloads from advisory to ensure they are rejected.

📡 Detection & Monitoring

Log Indicators:

Unusual XML parsing errors
Stack traces containing XStream deserialization errors
Unexpected class loading attempts in logs
Increased error rates in XML processing endpoints

Network Indicators:

Unusual XML payloads to application endpoints
Requests containing Java class names in XML
Traffic patterns suggesting XML deserialization attacks

SIEM Query:

source="application.logs" AND ("XStream" OR "deserialization") AND (error OR exception OR "class not found")

📊 Metadata

CVE ID: CVE-2021-39154

CVSS v3 Score: 8.5 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H

CWE: CWE-434

Published: August 23, 2021

Last Updated: May 23, 2025

🔗 References

https://github.com/x-stream/xstream/security/advisories/GHSA-6w62-hx7r-mw68 Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/09/msg00017.html Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/22KVR6B5IZP3BGQ3HPWIO2FWWCKT3DHP/ Mailing List
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/PVPHZA7VW2RRSDCOIPP2W6O5ND254TU7/ Mailing List
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QGXIU3YDPG6OGTDHMBLAFN7BPBERXREB/ Mailing List
https://security.netapp.com/advisory/ntap-20210923-0003/ Third Party Advisory
https://www.debian.org/security/2021/dsa-5004 Third Party Advisory
https://www.oracle.com/security-alerts/cpuapr2022.html Patch,Third Party Advisory
https://www.oracle.com/security-alerts/cpujan2022.html Patch,Third Party Advisory
https://www.oracle.com/security-alerts/cpujul2022.html Third Party Advisory
https://x-stream.github.io/CVE-2021-39154.html Exploit,Third Party Advisory
https://github.com/x-stream/xstream/security/advisories/GHSA-6w62-hx7r-mw68 Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/09/msg00017.html Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/22KVR6B5IZP3BGQ3HPWIO2FWWCKT3DHP/ Mailing List
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/PVPHZA7VW2RRSDCOIPP2W6O5ND254TU7/ Mailing List
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QGXIU3YDPG6OGTDHMBLAFN7BPBERXREB/ Mailing List
https://security.netapp.com/advisory/ntap-20210923-0003/ Third Party Advisory
https://www.debian.org/security/2021/dsa-5004 Third Party Advisory
https://www.oracle.com/security-alerts/cpuapr2022.html Patch,Third Party Advisory
https://www.oracle.com/security-alerts/cpujan2022.html Patch,Third Party Advisory
https://www.oracle.com/security-alerts/cpujul2022.html Third Party Advisory
https://x-stream.github.io/CVE-2021-39154.html Exploit,Third Party Advisory

📋 TL;DR

💻 Affected Systems

📦 What is this software?

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Fedora by Fedoraproject

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Retail Xstore Point Of Service by Oracle

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Snapmanager by Netapp

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Utilities Framework by Oracle

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Webcenter Portal by Oracle

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Xstream by Xstream

⚠️ Risk & Real-World Impact

Worst Case

Likely Case

If Mitigated

🎯 Exploit Status

🛠️ Fix & Mitigation

✅ Official Fix

Instructions:

🔧 Temporary Workarounds

Implement XStream Security Framework

Input Validation and Sanitization

🧯 If You Can't Patch

🔍 How to Verify

Check if Vulnerable:

Check Version:

Verify Fix Applied:

📡 Detection & Monitoring

Log Indicators:

Network Indicators:

SIEM Query:

📊 Metadata

🔗 References

📤 Share & Export

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