KEV Intelligence

Vulnerability report

Exploited in the wild Confirmed confidence In CISA KEV

CVE-2021-39144

XStream is vulnerable to a Remote Command Execution attack

x-stream / xstream · affected before 1.4.18

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Severity
CVSS 8.5 · High
Confidence
Confirmed
Exploit status
Exploited in the wild
EPSS
98.1%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-39144 is XStream is vulnerable to a Remote Command Execution attack. XStream is a simple library to serialize objects to XML and back again. In affected versions this vulnerability may allow a remote attacker...

Is it exploited?

Yes. KEV Intelligence tracks this CVE as a known exploited vulnerability. Confidence is confirmed.

Who is affected?

x-stream / xstream affected before 1.4.18.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

XStream is a simple library to serialize objects to XML and back again

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.

Vendor Product Affected Status
x-stream xstream Before 1.4.18 Affected
View vendor advisory (opens in new tab)
Published
23 Aug 2021
Exploitation Reported
10 Mar 2023
Attack vector
Remote
Complexity
High
Privileges
Low
User interaction
None

Tags

cisa metasploit nuclei_scanner

CVE References

Exploitation evidence

Why KEV Intelligence marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

CISA

Recorded 10 Mar 2023

Independent exploitation attestation added to the KEV Intelligence record.

Proof of concept available

Nuclei Templates

Recorded 28 Apr 2025

Public scanner or PoC coverage increases practical exploitability.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the KEV Intelligence Pro API.

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Source Added
CISA First 2023-03-10 00:00 UTC
The Shadowserver 2026-07-04 00:00 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets
0
User-Agents
0

Raw values available in Pro and Enterprise.

Virtual patch status

No KEV Intelligence virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

Scanner Reference Detected
Metasploit https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/linux/http/vmware_nsxmgr_xstream_rce_cve_2021_39144.rb 28 Apr 2025
Nuclei https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-39144.yaml 25 Apr 2025

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

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

EPSS

98.1%

Weaknesses

Missing Authentication for Critical Function CWE-306 Deserialization of Untrusted Data CWE-502 Improper Control of Generation of Code ('Code Injection') CWE-94

All CVSS Scores

CVSS v3.1 8.5 High

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

CVSS v2.0 6.0 Medium

AV:N/AC:M/Au:S/C:P/I:P/A:P

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

XStream 1.4.18 - Remote Code Execution

nuclei · Created Unknown

vmware_nsxmgr_xstream_rce_cve_2021_39144

metasploit · Created Unknown

Metasploit module for CVE-2021-39144

Timeline

From disclosure to observed exploitation

  1. KEV confirmed by The Shadowserver

    Exploitation attested by an external source

  2. Metasploit module available

    Exploit module available

  3. Public PoC available

    Public proof-of-concept code published

  4. Nuclei template available

    Scanner coverage available

  5. Added to CISA KEV

    Listed in the CISA Known Exploited Vulnerabilities catalog

  6. CVE published

    Vulnerability disclosed publicly

  7. CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-39144

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2021-39144",
  "confidence": "Confirmed",
  "cvss_score": 8.5,
  "cvss_estimated": false,
  "epss_score": 0.98124,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}

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