CVE-2021-21317Network attack vector

Service interruption via overlapping regex capture groups

Published Feb 16, 2021 · Updated Aug 3, 2024

Regular expression denial of service in ua-parser uap-core before 0.11.0 allows remote attackers to interrupt services via crafted User-Agent headers. The regexes.yaml dataset contains expressions with overlapping capture groups that perform excessive backtracking on maliciously long input. No authentication or user interaction is required; impact is limited to CPU exhaustion and resulting slowdown or interruption in consuming parsers.

CVSS severity5.3
Medium
EPSS probability2.52%
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 ua-parser uap-core before 0.11.0 allows remote attackers to interrupt services via crafted User-Agent headers. The regexes.yaml dataset contains expressions with overlapping capture groups that perform excessive backtracking on maliciously long input. No authentication or user interaction is required; impact is limited to CPU exhaustion and resulting slowdown or interruption in consuming parsers.

The record

CVE
CVE-2021-21317
Published
Feb 16, 2021
Updated
Aug 3, 2024
Vendor
ua-parser
Product
uap-core
Classifications
CWE-400, CWE-1333, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 16, 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=< 0.11.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ua-parser · uap-coreversion=< 0.11.0

Published CVSS scores

5.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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 exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
CWE-1333Inefficient Regular Expression Complexity
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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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