CVE-2021-21342Public exploitNetwork attack vector

SSRF via type-driven object injection during unmarshalling

Published Mar 22, 2021 · Updated Aug 3, 2024

SSRF in XStream before 1.4.16 allows remote attackers to access arbitrary intranet or localhost URLs through crafted input. During unmarshalling, attacker-controlled type metadata instantiates a PriorityQueue gadget chain whose URLDataSource retrieves the supplied URL. Exposure requires an application to unmarshal attacker-controlled data while relying on the default blocklist; an allowlist restricted to required types prevents the request.

CVSS severity5.3
Medium
EPSS probability49.99%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SSRF in XStream before 1.4.16 allows remote attackers to access arbitrary intranet or localhost URLs through crafted input. During unmarshalling, attacker-controlled type metadata instantiates a PriorityQueue gadget chain whose URLDataSource retrieves the supplied URL. Exposure requires an application to unmarshal attacker-controlled data while relying on the default blocklist; an allowlist restricted to required types prevents the request.

The record

CVE
CVE-2021-21342
Published
Mar 22, 2021
Updated
Aug 3, 2024
Vendor
XStream
Product
XStream
Classifications
CWE-918, CWE-502
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 22, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.4.16

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

XStream · XStreamversion=< 1.4.16

Published CVSS scores

5.3GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-918Server-Side Request Forgery (SSRF)
CWE-502Deserialization of Untrusted Data

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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