CriticalCVE-2021-39144

    XStream is vulnerable to a Remote Command Execution attack (CVE-2021-39144)

    Published In CISA KEV since Sources: SingCERT, Debian, CVE.org, OSV, Red Hat

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    What is affected, what to do

    XStream is a simple library to serialize objects to XML and back again. In affected versions this vulnerability may allow a remote attacker has sufficient rights to execute commands of the host only by manipulating the processed input stream.

    The vulnerability is being actively exploited; treat it as a priority. Update Xstream to version 1.4.18. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Xstream lt *..1.4.18
    Fixed in
    • Xstream 1.4.18

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    8.5

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    8.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: SingCERT, Debian, CVE.org, OSV, Red Hat

    98.1 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.91

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    222.36 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: CSA/SingCERT How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.5 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    8.5
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    98.1 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.91
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 6 February 2025
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-94, CWE-502

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This CVE is on KEV because XStream ships as a transitive deserializer inside many enterprise Java stacks (VMware Cloud Foundation, Jenkins and Confluence plugins, NetApp SnapManager), and a network-reachable attacker with only minimal privileges reaches full RCE in the JVM context. Exploitation likelihood sits among the highest tracked by EPSS, and public PoCs have been available since 2021. Treat as Critical for any externally reachable VMware SDDC Manager and for Java apps that parse XML from untrusted sources; High for internally segmented services. CISA has not flagged ransomware campaign use, but hypervisor-management RCE still makes this a strategic pivot risk for the entire virtualisation stack.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Upgrade XStream to 1.4.18 or later across all Java applications that embed it (identify via mvn dependency:tree, gradle dependencies, or find / -name "xstream-*.jar").
      • Patch VMware Cloud Foundation (SDDC Manager) per VMware advisory VMSA-2021-0030; if immediate patching is not possible, restrict the SDDC Manager UI (TCP/443) via firewall to admin jump hosts only.
      • Apply OS package updates: Debian 9/10/11 via apt upgrade libxstream-java, Fedora 33/34/35 via dnf update xstream, NetApp SnapManager per advisory NTAP-20211118.
      • If patching is blocked: switch XStream programmatically to whitelist mode (XStream.setupDefaultSecurity(xstream) plus explicit xstream.allowTypes(...)) and redeploy.
      • Rotate service-account credentials of affected applications if exploit attempts show in logs — the RCE runs under the JVM process user.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: pull XStream-using services (SDDC Manager, internal Java web apps) out of the DMZ; limit inbound access to deserialisation endpoints (REST/SOAP accepting application/xml, text/xml) to the management VLAN.
      • WAF / reverse-proxy rule: block XML request bodies carrying classic XStream gadget markers — <dynamic-proxy>, <java.lang.ProcessBuilder>, <jdk.nashorn.internal.objects.NativeString>, <javax.imageio.ImageIO$ContainsFilter>, <handler class="java.beans.EventHandler">.
      • JVM hardening: egress firewall rule on application servers that denies outbound DNS, HTTP, LDAP, and RMI to the internet — payloads frequently rely on out-of-band callbacks for validation.
      • Least privilege: run the JVM as a dedicated non-root user with noexec-mounted /tmp and an SELinux/AppArmor profile that blocks spawning /bin/sh, curl, wget.
      • Dependency pinning: produce an SBOM and surface transitively pulled XStream < 1.4.18; wire OWASP Dependency-Check or Snyk into the build pipeline with fail-on-build.

    3. Runbook step 3

      Detection rules

      • Web access logs: POST requests with Content-Type: application/xml or text/xml whose body contains dynamic-proxy, ProcessBuilder, EventHandler, or javax.naming.InitialContext — Sigma shape: selection: cs-method: POST AND cs-body|contains_any: ['dynamic-proxy','ProcessBuilder','EventHandler'].
      • Sysmon EID 1 / auditd execve: Java process (java, catalina, sddc-manager) as parent of /bin/sh, /bin/bash, cmd.exe, powershell.exe, curl, wget, nc — the canonical JVM spawn chain.
      • EDR process tree: java.exe → cmd.exe /c or java → bash -c with base64-decoded commands, especially under Tomcat/Jetty/SDDC services.
      • Network telemetry (Zeek/Suricata): outbound DNS queries from application servers to *.oast.*, *.interactsh.com, *.burpcollaborator.net, *.oastify.com — attacker payload-validation infrastructure.
      • Application logs: XStream exceptions such as com.thoughtworks.xstream.security.ForbiddenClassException or stack traces containing ConversionException, indicating either blocked or in-progress exploit attempts.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    XStream is a simple library to serialize objects to XML and back again. In affected versions this vulnerability may allow a remote attacker has sufficient rights to execute commands of the 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. XStream 1.4.18 uses no longer a blacklist by default, since it cannot be secured for general purpose.

    Source: CSA/SingCERT, Debian Security Advisories, NVD (NIST), OSV (Open Source Vulnerabilities), cvelistv5, CISA KEV Catalog, redhat_csaf, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    x-streamxstream
    • < 1.4.18
    • 1.4.18
    netappsnapmanager
    • —
    • —
    oraclebusiness_activity_monitoring
    • 12.2.1.4.0
    • —
    oraclecommerce_guided_search
    • 11.3.2
    • —
    oraclecommunications_billing_and_revenue_management_elastic_charging_engine
    • 11.3
    • 12.0
    • —
    • —
    oraclecommunications_cloud_native_core_automated_test_suite
    • 1.9.0
    • —
    oraclecommunications_cloud_native_core_binding_support_function
    • 1.10.0
    • —
    oraclecommunications_cloud_native_core_policy
    • 1.14.0
    • —
    oraclecommunications_unified_inventory_management
    • 7.4.0
    • 7.4.1
    • 7.3.5
    • 7.3.4
    • 7.4.2
    • —
    • —
    • —
    • —
    • —
    oracleretail_xstore_point_of_service
    • 17.0.4
    • 19.0.2
    • 18.0.3
    • 16.0.6
    • 20.0.1
    • —
    • —
    • —
    • —
    • —
    oracleutilities_framework
    • 4.3.0.6.0
    • 4.2.0.2.0
    • 4.2.0.3.0
    • 4.3.0.1.0
    • 4.4.0.3.0
    • 4.4.0.2.0
    • 4.4.0.0.0
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    oracleutilities_testing_accelerator
    • 6.0.0.1.1
    • —
    oraclewebcenter_portal
    • 12.2.1.3.0
    • 12.2.1.4.0
    • —
    • —
    xstreamxstream
    • < 1.4.18
    • 1.4.18
    mavencom.thoughtworks.xstream:xstream
    • 1.2.2
    • 1.4.13-java7
    • 1.4.12-java7
    • 1.0
    • 1.4.8
    • 1.4.5
    • 0.1
    • 1.4.7
    • 1.4.4
    • 1.1
    • 1.2.1
    • 1.4.1
    • 1.4.11-java7
    • 1.4.16
    • 1.4.10
    • 1.4.13
    • 1.4.6
    • 1.4.3
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebiandebian_linux10.0
    Linuxdebiandebian_linux11.0
    Linuxdebiandebian_linux9.0
    Linuxdebianlibxstream-javabullseye
    Linuxdebianlibxstream-javabuster
    Otherfedoraprojectfedora33
    Otherfedoraprojectfedora34
    Otherfedoraprojectfedora35

    Weakness classes (CWE)

    • CWE-94Improper Control of Generation of Code ('Code Injection')

      Base

      The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.

      Entry at MITRE CWE-94

    • CWE-502Deserialization of Untrusted Data

      Base

      The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.

      Entry at MITRE CWE-502

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    CSA/SingCERTCritical Vulnerability in VMware Cloud Foundation (opens in a new tab)CSA/SingCERTactively exploited
    Packet StormVMware NSX Manager XStream Unauthenticated Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • github.comx-stream/xstream/security/advisories/GHSA-j9h8-phrw-h4fhweb, Vendor advisory
    • NVD (NIST)vuln/detail/CVE-2021-39144advisory
    • x-stream.github.ioCVE-2021-39144.htmlweb, Exploit, Vendor advisory
    • oracle.comsecurity-alerts/cpujul2022.htmlweb, Patch, Third-party advisory
    • oracle.comsecurity-alerts/cpujan2022.htmlweb, Patch, Third-party advisory
    • oracle.comsecurity-alerts/cpuapr2022.htmlweb, Patch, Third-party advisory
    • debian.orgDSA-5004web, Vendor advisory, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-39144web, government-resource, US government resource
    • security.netapp.comadvisory/ntap-20210923-0003web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.debian.org[debian-lts-announce] 20210929 [SECURITY] [DLA 2769-1] libxstream-java security updateweb, Mailing list, Mailing list, Third-party advisory
    • github.comx-stream/xstreampackage
    • packetstormsecurity.comfiles/169859/VMware-NSX-Manager-XStream-Unauthenticated-Remote-…web, Exploit, Third-party advisory, Vulnerability database
    • lists.fedoraproject.orgFEDORA-2021-fbad11014aVendor advisory, Broken link, Mailing list, Release notes
    • lists.fedoraproject.orgFEDORA-2021-d894ca87dcVendor advisory, Broken link, Mailing list, Release notes
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