CVE-2021-3517Network attack vector

Application crash via truncated UTF-8 entity encoding

Published May 19, 2021 · Updated Dec 2, 2025

Heap buffer overflow in XML entity encoding in XMLSoft libxml2 before 2.9.11 allows remote attackers to crash linked applications. xmlEncodeEntitiesInternal assumes valid UTF-8 input and reads beyond the allocation when a truncated multibyte sequence ends the buffer. Remote attackers must cause an application using the affected encoder to process crafted XML; memory disclosure or corruption requires further exploitation.

CVSS severity8.6
High
EPSS probability8.28%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap buffer overflow in XML entity encoding in XMLSoft libxml2 before 2.9.11 allows remote attackers to crash linked applications. xmlEncodeEntitiesInternal assumes valid UTF-8 input and reads beyond the allocation when a truncated multibyte sequence ends the buffer. Remote attackers must cause an application using the affected encoder to process crafted XML; memory disclosure or corruption requires further exploitation.

The record

CVE
CVE-2021-3517
Published
May 19, 2021
Updated
Dec 2, 2025
Vendor
XMLSoft
Product
libxml2
Classifications
CWE-787, CWE-125, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 19, 2021CVE publishedPublication date reported by the CVE source.
  2. Dec 2, 2025Record 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

8.6Red HatCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-787Out-of-bounds Write
CWE-125Out-of-bounds Read
T1203Exploitation for Client Execution

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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