CVE-2021-21349Public exploitNetwork attack vector

Server-side request forgery via unsafe type instantiation

Published Mar 22, 2021 · Updated Aug 3, 2024

Server-side request forgery in XStream before 1.4.16 allows remote attackers to request internal resources through crafted serialized input. During unmarshalling, XStream trusts attacker-controlled type information, instantiates a gadget chain, and opens an arbitrary URL whose response data is collected. Exposure requires an application to unmarshal untrusted input under the default blacklist; the request can reach intranet or localhost resources from the host's network position.

CVSS severity8.6
High
EPSS probability46.83%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Server-side request forgery in XStream before 1.4.16 allows remote attackers to request internal resources through crafted serialized input. During unmarshalling, XStream trusts attacker-controlled type information, instantiates a gadget chain, and opens an arbitrary URL whose response data is collected. Exposure requires an application to unmarshal untrusted input under the default blacklist; the request can reach intranet or localhost resources from the host's network position.

The record

CVE
CVE-2021-21349
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

6.1GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
8.6NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
5.0NIST NVDAV:N/AC:L/Au:N/C:P/I:N/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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