CVE-2021-3537Network attack vector

Application crash via unpropagated mixed-content errors

Published May 14, 2021 · Updated Aug 3, 2024

NULL pointer dereference in XML mixed-content parsing in XMLSoft libxml2 before 2.9.11 allows remote attackers to crash applications. The mixed-content parser fails to propagate parse errors, leaves an invalid document structure, and post-validation dereferences a NULL pointer. Exploitation requires an untrusted XML document to be parsed in recovery mode and then post-validated; the bounded consequence is an application crash.

CVSS severity5.9
Medium
EPSS probability3.50%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

NULL pointer dereference in XML mixed-content parsing in XMLSoft libxml2 before 2.9.11 allows remote attackers to crash applications. The mixed-content parser fails to propagate parse errors, leaves an invalid document structure, and post-validation dereferences a NULL pointer. Exploitation requires an untrusted XML document to be parsed in recovery mode and then post-validated; the bounded consequence is an application crash.

The record

CVE
CVE-2021-3537
Published
May 14, 2021
Updated
Aug 3, 2024
Vendor
XMLSoft
Product
libxml2
Classifications
CWE-476, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 14, 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=libxml2 2.9.11

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

XMLSoft · libxml2version=libxml2 2.9.11

Published CVSS scores

5.9NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
4.3NIST National Vulnerability DatabaseAV:N/AC:M/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.

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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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-476NULL Pointer Dereference
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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