VulnerabilityPublished 20 Mar 2026Updated 25 Mar 2026

CVE-2026-33036: NaturalIntelligence fast-xml-parser · XML Entity Expansion

Affects NaturalIntelligence fast-xml-parserSource CVE Database V5

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Mentioned vulnerabilities

CVE-2026-26278

Not known to be exploited · CVSS 7.5 High : upgrade to 5.5.6 soon.

Data as of 1 Nov 2026, 22:58 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.5% chance of exploitation in 30 days – higher than 48% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 5.5.6

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 20 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 1 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade NaturalIntelligence fast-xml-parser to 5.5.6.

Affected software

Product	Affected	Fixed in
NaturalIntelligencefast-xml-parser	Affected >=4.0.0-beta.3 <5.5.6	Fixed in 5.5.6

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AI-suggested

To mitigate CVE-2026-33036, organizations should immediately upgrade fast-xml-parser to version 5.5.6 or later, where the vulnerability is fixed. If upgrading is not immediately feasible, implement strict input validation and sanitization to reject XML inputs containing excessive numeric character references or suspicious entity expansions. Employ XML parsing configurations that disable or limit entity expansion features where possible. Monitor application logs and resource usage for unusual spikes in memory or CPU consumption that may indicate exploitation attempts. Consider deploying Web Application Firewalls (WAFs) with rules to detect and block XML entity expansion attacks. Additionally, conduct code reviews and dependency audits to identify and remediate usage of vulnerable fast-xml-parser versions across all projects. Educate developers and DevOps teams about secure XML parsing practices and the risks of entity expansion vulnerabilities. Finally, maintain up-to-date threat intelligence feeds to stay informed about any emerging exploits targeting this vulnerability.

Description

fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. Versions 4.0.0-beta.3 through 5.5.5 contain a bypass vulnerability where numeric character references (&#NNN;, &#xHH;) and standard XML entities completely evade the entity expansion limits (e.g., maxTotalExpansions, maxExpandedLength) added to fix CVE-2026-26278, enabling XML entity expansion Denial of Service. The root cause is that replaceEntitiesValue() in OrderedObjParser.js only enforces expansion counting on DOCTYPE-defined entities while the lastEntities loop handling numeric/standard entities performs no counting at all. An attacker supplying 1M numeric entity references like A can force ~147MB of memory allocation and heavy CPU usage, potentially crashing the process—even when developers have configured strict limits. This issue has been fixed in version 5.5.6.

Impact

The primary impact of CVE-2026-33036 is a denial of service (DoS) condition caused by excessive memory allocation and CPU consumption when processing malicious XML inputs. This can lead to application crashes, service outages, or degraded performance, affecting availability. Since fast-xml-parser is commonly used in web applications, APIs, and backend services that handle XML data, exploitation could disrupt critical business operations, especially in environments processing untrusted XML data. The vulnerability does not affect confidentiality or integrity directly but can indirectly impact system reliability and availability. Organizations relying on fast-xml-parser in production environments may face service interruptions, increased operational costs due to downtime, and potential cascading failures in dependent systems. The ease of exploitation (no authentication or user interaction required) increases the risk of automated attacks or exploitation by remote adversaries. Although no known exploits are reported in the wild yet, the vulnerability's high CVSS score (7.5) and straightforward exploitation method make it a significant threat.

Summary

CVE-2026-33036 is a vulnerability classified under CWE-776 (Improper Restriction of Recursive Entity References in DTDs) affecting the fast-xml-parser library developed by NaturalIntelligence. This library is widely used in JavaScript environments to parse XML data without relying on C/C++ libraries or callbacks. The vulnerability exists in versions from 4.0.0-beta.3 up to 5.5.5, where numeric character references (e.g., &#NNN;, &#xHH;) and standard XML entities are not properly accounted for in the entity expansion limits designed to prevent XML entity expansion attacks (a type of denial of service). The parser's function replaceEntitiesValue() enforces expansion limits only on entities defined in the DOCTYPE section, while numeric and standard entities processed in the lastEntities loop bypass these checks entirely. An attacker can exploit this by supplying XML containing a large number (e.g., one million) of numeric character references such as A, which expands to the character 'A'. This causes the parser to allocate approximately 147MB of memory and consume significant CPU resources, potentially crashing the application or service using the parser. The vulnerability does not require any privileges or user interaction to exploit and can be triggered remotely by processing malicious XML input. The issue was addressed and fixed in version 5.5.6 of fast-xml-parser.

CWE-776Improper Restriction of Recursive Entity References in DTDs (Billion Laughs Attack) (opens in a new tab)
Base weaknessLikelihood of exploit: MediumOWASP Top 10 A02:2025: Security Misconfiguration (opens in a new tab)
The product uses XML documents and allows their structure to be defined with a Document Type Definition (DTD), but it does not properly control the number of recursive definitions of entities.
Operation
If possible, prohibit the use of DTDs or use an XML parser that limits the expansion of recursive DTD entities.
Implementation
Before parsing XML files with associated DTDs, scan for recursive entity declarations and do not continue parsing potentially explosive content.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
Exponential Data Expansion (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Mar 2026, 22:37 UTC

WeaknessesCWE-776

Technical details

References

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