Atomicorp WAF Research Note: CVE-2021-21351

XStream <1.4.16 - Remote Code Execution

CVE Details

• Already protected by existing Atomicorp WAF rules
• Protection status: Understanding Atomicorp WAF Protection Status
• CVE: CVE-2021-21351
• Name: XStream <1.4.16 - Remote Code Execution
• Severity: Critical
• CVSS: 9.1 (v3.1) (Critical)
• CVSS version: 3.1
• CVSS source: N/A
• CVSS vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H
  • Attack vector: Network
  • Attack complexity: Low
  • Privileges required: High
  • User interaction: None
  • Scope: Changed
  • Confidentiality impact: High
  • Integrity impact: High
  • Availability impact: High
• CWE: CWE-434 — Unrestricted Upload of File with Dangerous Type, CWE-502 — Deserialization of Untrusted Data
• Related CAPEC attack patterns (via MITRE CWE relationships): CAPEC-1 (opens in a new tab) , CAPEC-586 (opens in a new tab)
• Vendor: xstream_project
• Vulnerable products: oncommand insight , activemq , jmeter , xstream , debian linux , fedora , banking enterprise default management , banking platform , banking virtual account management , business activity monitoring , communications billing and revenue management elastic charging engine , communications policy management , communications unified inventory management , mysql server , retail xstore point of service , webcenter portal , xstream
• CVE published: 2021-03-23
• CVE last modified: 2026-06-17
• Affected versions (NVD configurations):
  • oncommand insight
  • activemq: < 5.15.14
  • activemq: 5.16.0
  • activemq: 5.16.1
  • jmeter: < 5.5
  • xstream: < 1.4.16
  • debian linux: 9.0
  • debian linux: 10.0
  • debian linux: 11.0
  • fedora: 33
  • fedora: 34
  • fedora: 35
  • banking enterprise default management: 2.10.0
  • banking enterprise default management: 2.12.0
  • banking platform: 2.4.0
  • banking platform: 2.7.1
  • banking platform: 2.9.0
  • banking platform: 2.12.0
  • banking virtual account management: 14.2.0
  • banking virtual account management: 14.3.0
  • banking virtual account management: 14.5.0
  • business activity monitoring: 11.1.1.9.0
  • business activity monitoring: 12.2.1.3.0
  • business activity monitoring: 12.2.1.4.0
  • communications billing and revenue management elastic charging engine: 12.0.0.3.0
  • communications policy management: 12.5.0
  • communications unified inventory management: 7.3.2
  • communications unified inventory management: 7.3.4
  • communications unified inventory management: 7.3.5
  • communications unified inventory management: 7.4.0
  • communications unified inventory management: 7.4.1
  • mysql server: <= 8.0.27
  • retail xstore point of service: 16.0.6
  • retail xstore point of service: 17.0.4
  • retail xstore point of service: 18.0.3
  • retail xstore point of service: 19.0.2
  • webcenter portal: 11.1.1.9.0
  • webcenter portal: 12.2.1.3.0
  • webcenter portal: 12.2.1.4.0
• EPSS: 0.8214; 99.6th percentile (FIRST (opens in a new tab) )
• Latest Update: 2026-09-06
• Selected Rule IDs observed: 344380
• Matching rules: 1; rules with blocking actions: 1
• Would-block Rule IDs: 344380
• Request surfaces inspected: JSON request data, Request arguments, Request body, SOAP request data

Vulnerability Description

XStream is a Java library to serialize objects to XML and back again. In XStream before version 1.4.16, there is a vulnerability may allow a remote attacker to load and execute arbitrary code from a remote host only by manipulating the processed input stream. No user is affected, who followed the recommendation to setup XStream’s security framework with a whitelist limited to the minimal required types. If you rely on XStream’s default blacklist of the Security Framework, you will have to use at least version 1.4.16.

Recommended Actions

1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (344380 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

   • Affected versions: x-stream xstream version < 1.4.16.
   • The CVE record does not include written upgrade or workaround instructions, but it does publish the following patch or mitigation reference:
     • Patch or mitigation information (opens in a new tab)
     • Patch or mitigation information (opens in a new tab)
     • Patch or mitigation information (opens in a new tab)

3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

• NVD (opens in a new tab)
• CVE Record (opens in a new tab)
• Patch or mitigation: http://x-stream.github.io/changes.html#1.4.16 (opens in a new tab)
• Exploit or technical research: https://github.com/x-stream/xstream/security/advisories/GHSA-hrcp-8f3q-4w2c (opens in a new tab)
• Exploit or technical research: https://lists.apache.org/thread.html/r8244fd0831db894d5e89911ded9c72196d395a90ae655414d23ed0dd%40%3Cusers.activemq.apache.org%3E (opens in a new tab)
• Exploit or technical research: https://lists.apache.org/thread.html/r9ac71b047767205aa22e3a08cb33f3e0586de6b2fac48b425c6e16b0%40%3Cdev.jmeter.apache.org%3E (opens in a new tab)
• Exploit or technical research: https://lists.debian.org/debian-lts-announce/2021/04/msg00002.html (opens in a new tab)
• Exploit or technical research: https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/22KVR6B5IZP3BGQ3HPWIO2FWWCKT3DHP/ (opens in a new tab)
• Exploit or technical research: https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/PVPHZA7VW2RRSDCOIPP2W6O5ND254TU7/ (opens in a new tab)
• Exploit or technical research: https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QGXIU3YDPG6OGTDHMBLAFN7BPBERXREB/ (opens in a new tab)
• Exploit or technical research: https://security.netapp.com/advisory/ntap-20210430-0002/ (opens in a new tab)
• Exploit or technical research: https://www.debian.org/security/2021/dsa-5004 (opens in a new tab)
• Exploit or technical research: https://www.oracle.com//security-alerts/cpujul2021.html (opens in a new tab)
• Vendor advisory: https://www.oracle.com/security-alerts/cpujan2022.html (opens in a new tab)
• Exploit or technical research: https://www.oracle.com/security-alerts/cpuoct2021.html (opens in a new tab)
• Exploit or technical research: https://x-stream.github.io/CVE-2021-21351.html (opens in a new tab)
• Patch or mitigation: https://x-stream.github.io/security.html#workaround (opens in a new tab)

Test or Research Source

• Source: http/cves/2021/CVE-2021-21351.yaml (opens in a new tab)
• Tags: cve, cve2021, deserialization, oast, rce, vulhub, vuln, xstream, xstream_project

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

• Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
• Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
• Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
• Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
• Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ (opens in a new tab) .