CVE-2021-39144
CVE-2021-39144 is a security flaw in XStream. Attackers can run their own code on XStream. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 10 Mar 2023.[1]
XStream is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. XStream has published a fix or advice.
Description
CISA calls it a remote code execution in XStream XStream. In its words: “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.”[1]
The CVE record adds: “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.”[2]
The underlying weakness is code injection (CWE-94), deserialization of untrusted data (CWE-502), in MITRE’s list of common software weaknesses.
It scores 8.5 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; XStream’s advisory has them[3]. CISA names the product as XStream XStream.
Most families won’t have XStream; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 10 Mar 2023, 564 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 31 Mar 2023 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 98%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
XStream has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.XStream is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed, or not in use. CISA gave US government agencies until 31 Mar 2023 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 8.5.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 98%.[4] |
See also
Other code injection flaws
- CVE-2026-0322, Palo Alto Networks PAN-OS: code injection ·
- CVE-2026-65660, Microsoft SharePoint: code injection ·
- CVE-2026-60004, Gitea: code injection ·
- CVE-2026-72530, TrueConf Server: code injection ·
- CVE-2025-62593, Ray-Project Ray: code injection ·
Added the same day
- CVE-2020-5741, Plex Media Server: remote code execution ·
References
- ^ Cybersecurity and Infrastructure Security Agency. “XStream Remote Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 10 Mar 2023.
- ^ National Institute of Standards and Technology. “CVE-2021-39144 Detail”. National Vulnerability Database, published 23 Aug 2021.
- ^ XStream. Advisories and fixes: vmware.com, x-stream.github.io, github.com, oracle.com, oracle.com, and 1 more on NVD’s record.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.981, percentile 99.9, 31 Oct 2026.
- CVE Program. “CVE-2021-39144”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.