CVE-2021-21366Network attack vector

XML integrity loss via lossy DOCTYPE serialization

Published Mar 12, 2021 · Updated Aug 3, 2024

Input misinterpretation in xmldom 0.4.0 and earlier allows attackers to alter XML syntax through repeated parsing and serialization. The parser discards quote context from DOCTYPE public and system identifiers, while XMLSerializer reconstructs those identifiers with fixed quoting. Exploitation requires a downstream application to accept crafted XML and parse and serialize it repeatedly, producing altered document structure or meaning.

CVSS severity4.3
Medium
EPSS probability1.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Input misinterpretation in xmldom 0.4.0 and earlier allows attackers to alter XML syntax through repeated parsing and serialization. The parser discards quote context from DOCTYPE public and system identifiers, while XMLSerializer reconstructs those identifiers with fixed quoting. Exploitation requires a downstream application to accept crafted XML and parse and serialize it repeatedly, producing altered document structure or meaning.

The record

CVE
CVE-2021-21366
Published
Mar 12, 2021
Updated
Aug 3, 2024
Vendor
xmldom
Product
xmldom
Classifications
CWE-436, CWE-115
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 12, 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=< 0.5.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

xmldom · xmldomversion=< 0.5.0

Published CVSS scores

4.3NIST National Vulnerability DatabaseAV:N/AC:M/Au:N/C:N/I:P/A:N
4.3NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
4.3GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

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-436Interpretation Conflict
CWE-115Misinterpretation of Input

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

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo