CVE-2026-33349 | fast-xml-parser: Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation

Exp

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 JavaScript causes the guard conditions to short-circuit, completely bypassing the limits. An attacker who can supply XML input to such an application can trigger unbounded entity expansion, leading to memory exhaustion and denial of service. This issue has been patched in version 5.5.7.

Published: 2026-03-24 Last update: 2026-06-17 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-33349 is rated Exploit Available (51.5/100): CVSS Medium severity, with low exploitation likelihood (EPSS 0.45%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2026-33349

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2026-33349

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.01% 0.45% +0.43%
2 2026-03-25 — 0.01% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-33349

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
5.9 3.1 MEDIUM
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 2.2 3.6 [email protected]

Weakness enumeration for CVE-2026-33349

GitHub Security Advisory for CVE-2026-33349

GHSA-jp2q-39xq-3w4g · Severity: medium · Ecosystem: npm — Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation in fast-xml-parser

OS Trackers for CVE-2026-33349

vendor priority summary link
debian not yet assigned CVE-2026-33349 not yet assigned priority: Debian including 1 source packages (node-webfont), 4 status rows across 4 suites (bookworm, forky, sid, trixie): undetermined 4. https://security-tracker.debian.org/tracker/CVE-2026-33349
redhat medium — https://access.redhat.com/security/cve/CVE-2026-33349

Affected software / configurations for CVE-2026-33349

Vendor Product Version Raw CPE
naturalintelligence fast-xml-parser >= 4.0.1, < 4.5.5 cpe:2.3:a:naturalintelligence:fast-xml-parser:*:*:*:*:*:*:*:*
naturalintelligence fast-xml-parser >= 5.0.0, < 5.5.7 cpe:2.3:a:naturalintelligence:fast-xml-parser:*:*:*:*:*:*:*:*
naturalintelligence fast-xml-parser 4.0.0 cpe:2.3:a:naturalintelligence:fast-xml-parser:4.0.0:-:*:*:*:*:*:*
naturalintelligence fast-xml-parser 4.0.0 cpe:2.3:a:naturalintelligence:fast-xml-parser:4.0.0:beta3:*:*:*:*:*:*
naturalintelligence fast-xml-parser 4.0.0 cpe:2.3:a:naturalintelligence:fast-xml-parser:4.0.0:beta4:*:*:*:*:*:*
naturalintelligence fast-xml-parser 4.0.0 cpe:2.3:a:naturalintelligence:fast-xml-parser:4.0.0:beta5:*:*:*:*:*:*
naturalintelligence fast-xml-parser 4.0.0 cpe:2.3:a:naturalintelligence:fast-xml-parser:4.0.0:beta6:*:*:*:*:*:*
naturalintelligence fast-xml-parser 4.0.0 cpe:2.3:a:naturalintelligence:fast-xml-parser:4.0.0:beta7:*:*:*:*:*:*
naturalintelligence fast-xml-parser 4.0.0 cpe:2.3:a:naturalintelligence:fast-xml-parser:4.0.0:beta8:*:*:*:*:*:*

References for CVE-2026-33349

cvelogic Threat Intelligence