CVE-2021-21348Public exploitNetwork attack vector

Thread hang via malicious regex deserialization

Published Mar 22, 2021 · Updated Aug 3, 2024

Regular expression denial of service in XStream before 1.4.16 allows remote attackers to hang a processing thread through crafted input. During unmarshalling, attacker-controlled type information instantiates a gadget chain that evaluates a malicious regular expression with catastrophic backtracking. The thread consumes maximum CPU time and never returns when the default blacklist is used; a minimal type whitelist blocks the chain.

CVSS severity5.3
Medium
EPSS probability13.83%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in XStream before 1.4.16 allows remote attackers to hang a processing thread through crafted input. During unmarshalling, attacker-controlled type information instantiates a gadget chain that evaluates a malicious regular expression with catastrophic backtracking. The thread consumes maximum CPU time and never returns when the default blacklist is used; a minimal type whitelist blocks the chain.

The record

CVE
CVE-2021-21348
Published
Mar 22, 2021
Updated
Aug 3, 2024
Vendor
XStream
Product
XStream
Classifications
CWE-400, CWE-502, T1499.004
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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.8NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:C
5.3GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H

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-400Uncontrolled Resource Consumption
CWE-502Deserialization of Untrusted Data
T1499.004Application or System Exploitation

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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