CVE-2021-39144: XStream Remote Code Execution Vulnerability
- Is it exploited?
Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 10 Mar 2023, 564 days after the CVE record was published.
- How likely is exploitation in the next 30 days?
- 98.1%
EPSS score, percentile 99.9 of all scored records
- How severe?
- 8.5 High
CVSS v3.1 base score, given by the CNA
- What should I do?
Required action (CISA), due 31 Mar 2023: Apply updates per vendor instructions.
- Which versions are affected?
Affected: < 1.4.18
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.
What to do
Required action, set by CISA
Apply updates per vendor instructions.
CISA due date for this action: 31 March 2023.
Advisories and patches
Linked from CISA's catalog entry
- https://www.vmware.com/security/advisories/VMSA-2022-0027.html
- https://x-stream.github.io/CVE-2021-39144.html
- https://nvd.nist.gov/vuln/detail/CVE-2021-39144
Affected products and versions
As listed in the CVE record.
| Status | Versions |
|---|---|
| Affected | < 1.4.18 |
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.
About this vulnerability
XStream contains a remote code execution vulnerability that allows an attacker to manipulate the processed input stream and replace or inject objects that result in the execution of a local command on the server. This vulnerability can affect multiple products, including but not limited to VMware Cloud Foundation.
Summary: CISA's KEV catalog, as of 28 Oct 2026.
- CISA added CVE-2021-39144 to its Known Exploited Vulnerabilities (KEV) catalog on 10 Mar 2023.
- CISA's ransomware flag for this entry is "Unknown": no evidence of ransomware use in the catalog, which is not proof of none.
- The due date CISA sets for the required action is 31 Mar 2023.
- The CVE record was published on 23 Aug 2021, 564 days before it was added to KEV.
- The CVE record carries a CVSS v3.1 base score of 8.5 (High), given by its CNA.
- FIRST's EPSS model puts the probability of exploitation in the next 30 days at 98.1% (percentile 99.9), as of 28 Oct 2026.
- CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact total.
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.
Description
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 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: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.
Weakness
- CWE-94: Improper Control of Generation of Code ('Code Injection')
- CWE-502: Deserialization of Untrusted Data
CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.
Timeline
| Date | Event |
|---|---|
| 23 Aug 2021 | CVE record published |
| 10 Mar 2023 | Added to CISA KEV |
| 31 Mar 2023 | CISA due date for the required action |
| 21 Oct 2025 | CVE record last updated |