CVE-2026-33349: fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks

This week

MEDIUM 5.9REMOTEUPGRADE

CVE-2026-33349 is a medium-severity flaw (CVSS 5.9) 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 process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, the DocTypeReader in fast-xml-parser uses JavaScript truthy checks to evaluate maxEntityCount and maxEntitySize configuration limits. When a developer explicitly sets either limit to 0 — intending to disallow all entities or restrict entity size to zero bytes — the falsy nature of 0 in JavaScrip

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 60%)
CISA exploited list	No
Weakness	CWE-1284
Published	2026-03-24
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

2026-03-24 20:16 UTCPublished CVE record published
2026-06-17 10:37 UTCUpdated Record last modified

naturalintelligence fast-xml-parser UPGRADE

Affected: >= 4.0.1 and < 4.5.5 · >= 5.0.0 and < 5.5.7 · 4.0.0 · 4.0.0 beta3 · 4.0.0 beta4 · 4.0.0 beta5 · 4.0.0 beta6 · 4.0.0 beta7

Fixed in: 4.5.5, 5.5.7

Safe to downgrade to: a release before 4.0.0

1. Check installed version

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

2. UPGRADE to 4.5.5 (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 4.0.0 (Debian / Ubuntu)

apt-cache madison fast-xml-parser sudo apt install fast-xml-parser=<VERSION_BEFORE_4.0.0> 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_4.0.0> 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-2026-33349?

fast-xml-parser: upgrade to 4.5.5, 5.5.7. Exact commands for each OS are on this page.

Is CVE-2026-33349 being exploited?

CVE-2026-33349 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-2026-33349 affect?

CVE-2026-33349 affects fast-xml-parser. Severity is medium (CVSS 5.9). fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, the DocTypeReader in fast-xml-parser uses JavaScript truthy checks to evaluate maxEnti

How urgent is CVE-2026-33349?

Priority 41/100. Public exploit reference; 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-2026-33036 HIGH 7.5 UPGRADE fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks
CVE-2026-27942 HIGH 7.5 UPGRADE fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C+

References

← Back to latest CVEs