fast-xml-parser: Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation
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.
Noroxi analysis
No Noroxi analysis for this record yet
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We use this product, ask for help- Published
- Mar 24, 2026
- Updated
- Jun 17, 2026
- EPSS
- 0.5% · 39th percentile
- CWE
- CWE-1284
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Report tools
Action score
23
Monitor
Low priority for now.
- CVSS
- 23 / 40 · 5.9 / 10
- CISA KEV
- 0 / 30 · Not listed
- EPSS
- 0 / 30 · 0.5%
CISA SSVC decision
- Exploitation
- proof of concept
- Automatable
- no
- Technical impact
- partial
Vulnrichment: CISA's decision-tree inputs.
CNA vs NVD score
- NVD
- —
- CNA · GitHub_M
- 5.9
- NVD has not scored this yet; the score shown is the CNA’s.
The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.
Affected systems
| Vendor | Product | CPE |
|---|---|---|
| naturalintelligence | fast-xml-parser | cpe:2.3:a:naturalintelligence:fast-xml-parser |
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- naturalintelligence fast-xml-parser4.0.1 and later · before 4.5.5
- naturalintelligence fast-xml-parser5.0.0 and later · before 5.5.7
- naturalintelligence fast-xml-parser4.0.0
Versions reported by the vendor
Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.
NaturalIntelligence fast-xml-parser
- >= 4.0.0-beta.3, < 5.5.7affected
Package-level exposure
OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.
| Ecosystem | Package | Affected range | Fix |
|---|---|---|---|
| Debian:12 | node-webfont | all versions | — |
| npm | fast-xml-parser | from 4.0.0-beta.3 · before 4.5.5 | 4.5.5 |
| npm | fast-xml-parser | from 5.0.0 · before 5.5.7 | 5.5.7 |
Same product
naturalintelligence: all recordsOther highest-scoring records for the same primary product.
- CVE-2026-25896fast-xml-parser has an entity encoding bypass via regex injection in DOCTYPE entity names37Monitor
- CVE-2026-73569fast-xml-parser: Repeated DOCTYPE declarations reset entity expansion limits34Monitor
- CVE-2026-33036fast-xml-parser affected by numeric entity expansion bypassing all entity expansion limits (incomplete fix for CVE-2026-26278)30Monitor
- CVE-2026-26278fast-xml-parser affected by DoS through entity expansion in DOCTYPE (no expansion limit)30Monitor
- CVE-2026-25128fast-xml-parser has RangeError DoS Numeric Entities Bug30Monitor
- CVE-2024-41818ReDOS at currency parsing fast-xml-parser30Monitor
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
| Product / package | Fixed version | Source |
|---|---|---|
| NaturalIntelligence fast-xml-parser | 5.5.7 · description | Vendor (CNA) |
| npm:fast-xml-parser | 4.5.5 | Package registry (OSV) |
| npm:fast-xml-parser | 5.5.7 | Package registry (OSV) |
Exploit status
No known public exploit
No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
No dated events beyond publication.
FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).
Patch and commit links
Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.
Credits
All researchersFinders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.
No credits in the CNA record.
Variant candidates
Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.
No nightly-computed relations.
Chain candidates
An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.
—
Bug bounty scope
No known public program.
Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).
Technical details
Attack conditions
- Anyone who can reach it over the internet can trigger it.
- No account or password is required.
- No user action is required.
- Special conditions such as timing or configuration are required.
If successful
- Confidentiality
- none
- Integrity
- none
- Availability
- high · the service can be disrupted
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- None
- Integrity impact
- None
- Availability impact
- High
Weakness class (CWE)
CWE-1284 · Improper Validation of Specified Quantity in InputCVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
nvd-secondary
Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
MITRE has no CAPEC/ATT&CK mapping for this CWE.
Change log
- Fix✗ → ✓
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References
Vendor advisories and official records. Exploit/PoC links are deliberately left out.