CVE-2026-27942: fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C+

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HIGH 7.5REMOTEUPGRADE

CVE-2026-27942 is a high-severity flaw (CVSS 7.5) affecting fast-xml-parser. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C++ based libraries and no callback. Prior to version 5.3.8, the application crashes with stack overflow when user use XML builder with `preserveOrder:true`. Version 5.3.8 fixes the issue. As a workaround, use XML builder with `preserveOrder:false` or check the input data before passing to builder.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 55%)
CISA exploited list	No
Weakness	CWE-120
Published	2026-02-26
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: naturalintelligence

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-02-26 02:16 UTCPublished CVE record published
2026-06-17 10:27 UTCUpdated Record last modified

naturalintelligence fast-xml-parser UPGRADE

Affected: < 4.5.4 · >= 5.0.0 and < 5.3.8

Fixed in: 4.5.4, 5.3.8

Safe to downgrade to: a release before 5.0.0

1. Check installed version

apt-cache policy fast-xml-parser || rpm -q fast-xml-parser || fast-xml-parser --version

2. UPGRADE to 4.5.4 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade fast-xml-parser

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade fast-xml-parser

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 5.0.0 (Debian / Ubuntu)

apt-cache madison fast-xml-parser sudo apt install fast-xml-parser=<VERSION_BEFORE_5.0.0> sudo apt-mark hold fast-xml-parser

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list fast-xml-parser sudo dnf downgrade fast-xml-parser-<VERSION_BEFORE_5.0.0> sudo dnf versionlock add fast-xml-parser

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-27942?

fast-xml-parser: upgrade to 4.5.4, 5.3.8. Exact commands for each OS are on this page.

Is CVE-2026-27942 being exploited?

CVE-2026-27942 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-27942 affect?

CVE-2026-27942 affects fast-xml-parser. Severity is high (CVSS 7.5). fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C++ based libraries and no callback. Prior to version 5.3.8, the application crashes with stack overflow when user use XML builder w

How urgent is CVE-2026-27942?

Priority 43/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-41650 MEDIUM 6.1 UPGRADE fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks
CVE-2026-25896 CRITICAL 9.3 UPGRADE fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C+
CVE-2026-33349 MEDIUM 5.9 UPGRADE fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks
CVE-2026-33036 HIGH 7.5 UPGRADE fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks

References

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