CVE-2021-3541Public exploitNetwork attack vector

Service interruption via unchecked parameter-entity expansion

Published Jul 9, 2021 · Updated Aug 3, 2024

XML entity expansion in XMLSoft libxml2 2.9.11 allows attackers to interrupt services by supplying crafted XML documents. The parser.c xmlParsePEReference path expands delayed DTD parameter entities without applying the entity-density check, enabling exponential growth. The attack requires an application to parse attacker-controlled DTD parameter entities and exhausts CPU and memory until processing hangs.

CVSS severity6.5
Medium
EPSS probability1.96%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

XML entity expansion in XMLSoft libxml2 2.9.11 allows attackers to interrupt services by supplying crafted XML documents. The parser.c xmlParsePEReference path expands delayed DTD parameter entities without applying the entity-density check, enabling exponential growth. The attack requires an application to parse attacker-controlled DTD parameter entities and exhausts CPU and memory until processing hangs.

The record

CVE
CVE-2021-3541
Published
Jul 9, 2021
Updated
Aug 3, 2024
Vendor
XMLSoft
Product
libxml2
Classifications
CWE-776, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jul 9, 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=2.9.11

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

XMLSoft · libxml2version=2.9.11

Published CVSS scores

4.0NIST NVDAV:N/AC:L/Au:S/C:N/I:N/A:P
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-776Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')
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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