VulnerabilityPublished 24 Mar 2026

CVE-2026-33349: NaturalIntelligence fast-xml-parser · Improper Validation of Specified Quantity in Input

Affects NaturalIntelligence fast-xml-parserSource CVE Database V5

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Not known to be exploited · CVSS 5.9 Medium : upgrade to 5.5.7 when convenient.

Data as of 30 Oct 2026, 11:52 UTC

CVSS 3.1 base score

5.9Medium

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

EPSS: 0.4% chance of exploitation in 30 days – higher than 40% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 5.5.7

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

Attack Vector: Network
Attack Complexity: High
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: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
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 24 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 30 Oct 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 5.5.7.

Affected software

Product	Affected	Fixed in
NaturalIntelligencefast-xml-parser	Affected >=4.0.0-beta.3 <5.5.7	Fixed in 5.5.7

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

The primary mitigation is to upgrade fast-xml-parser to version 5.5.7 or later, where the issue is fixed by proper validation of maxEntityCount and maxEntitySize parameters without relying on truthy/falsy checks. Until upgrading is possible, developers should avoid setting maxEntityCount or maxEntitySize to zero; instead, set them to a positive integer that enforces strict limits on entity expansion. Additionally, application-level input validation can be implemented to detect and reject XML inputs containing excessive or recursive entity definitions before parsing. Employing runtime resource limits (e.g., memory quotas, process timeouts) on XML parsing operations can help mitigate the impact of potential DoS attempts. Monitoring and logging XML parsing errors and resource usage spikes can provide early detection of exploitation attempts. Finally, developers should review XML parsing configurations and ensure that entity expansion features are disabled or tightly controlled according to the application's security requirements.

Description

CVE-2026-33349 is a medium severity vulnerability in the fast-xml-parser library versions from 4.0.0-beta.3 up to before 5.5.7. The issue arises because the DocTypeReader component uses JavaScript truthy checks on maxEntityCount and maxEntitySize configuration parameters. When these limits are explicitly set to zero, intending to disallow XML entities or restrict their size, the falsy evaluation causes the limits to be bypassed. This allows an attacker supplying crafted XML input to trigger unbounded entity expansion, resulting in memory exhaustion and denial of service (DoS). No authentication or user interaction is required, and the vulnerability can be exploited remotely over the network.

Impact

This vulnerability can cause denial of service conditions in applications using affected versions of fast-xml-parser by allowing attackers to trigger unbounded XML entity expansion. The memory exhaustion can crash or severely degrade the performance of services processing XML input, potentially leading to downtime or degraded user experience. Since fast-xml-parser is widely used in JavaScript environments for XML processing without native dependencies, many web applications, APIs, and backend services could be impacted. The lack of authentication or user interaction requirements means that any attacker able to send XML input to a vulnerable service can exploit this issue remotely. This could be leveraged as part of a larger attack chain to disrupt critical services, especially those relying on XML data interchange. However, the impact is limited to availability; confidentiality and integrity are not directly affected. The medium severity rating reflects the moderate complexity of exploitation and the limited scope of impact to denial of service only.

CWE-1284Improper Validation of Specified Quantity in Input (opens in a new tab)
Base weakness
The product receives input that is expected to specify a quantity (such as size or length), but it does not validate or incorrectly validates that the quantity has the required properties.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 31 Mar 2026, 20:17 UTC

WeaknessesCWE-1284

Technical details

References

Community reviews

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