Threats / XStream / CVE-2021-39144
CVE-2021-39144
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-07
XStream vulnerability
XStream deserializes untrusted data, allowing remote code execution through object injection. Attackers can manipulate input streams to execute arbitrary commands on affected servers.
Verdict
Today item — known-exploited.
An attacker exploits unsafe deserialization in XStream to inject malicious objects into the processing stream, achieving remote code execution and command execution on the target system. This vulnerability is actively exploited in the wild.
CISA KEV Yes · 2023-03-105EPSS 0.98124 (verify live)6CVSS 8.53CISA SSVC Exploitation: active · Automatable: no · Impact: total4Exploit Weaponized · public exploit · Metasploit module · nuclei template14
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
395 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2023-03-10).
Probability (EPSS)
EPSS 0.98124 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 8.5 (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H) · Affected: XStream, XStream. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-94 Code Injection, CWE-502 Deserialization of Untrusted Data — weakness family: Injection.CWE — CNA-assigned (CVE.org); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: no · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 564 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
Public exploit available
Public exploit tooling is catalogued for this CVE (a functional/weaponized public exploit). Catalog entries: Metasploit module, nuclei template. We link the catalog page(s) only — no payload or code is hosted or restated here.Link, never host — the catalog page is the source; basicsecurity.net does not redistribute payloads.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft a malicious serialized object and send it to an application using XStream for deserialization.
Business
The organization's server executes arbitrary commands under the application's privileges, compromising system integrity.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I manipulate the input stream to replace legitimate objects with gadget chains that trigger code execution.
Business
Attackers gain persistent access to infrastructure, enabling data theft, lateral movement, and operational disruption.
3
Lateral reach — past segmentation narrative 3
Attacker
I leverage the remote code execution to establish a foothold and escalate privileges within the target environment.
Business
The organization faces potential complete system compromise, regulatory violations, and reputational damage from security breach.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05
Coverage & confidence
— what we know, and what we don’tEstablished (cited)
Coverage gaps — stated, not hidden
Disclosure & credit2
Catalogued by GitHub_MCNA
Credited with finding itNo finder named in the public CVE record — the work behind this find is unattributed.
Found an error? Propose a correction.
A correction is a cited counter-claim — it must cite a source,
gets reviewed by a second human, and is never auto-applied. See the
correction convention and our identity
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