VulnerabilityPublished 10 Nov 2026

CVE-2026-126066: fast-xml-parser · Quadratic CPU consumption scanning unterminated stop-node tags

Affects fast-xml-parserSource GCVE Database

Compare

Authoritative record:CVE-2026-126066· Low 3.7

Not known to be exploited · CVSS 3.7 Low : upgrade to 5.12.0 when convenient.

Data as of 10 Nov 2026, 22:34 UTC

CVSS 3.1 base score

3.7Low

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

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	3.7

Patch: Available – fixed in 5.12.0

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

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

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: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 10 Nov 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 10 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 fast-xml-parser to 5.12.0.

Affected software

Product	Affected	Fixed in
npmfast-xml-parser	Affected <5.12.0	Fixed in 5.12.0

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

Description

### Summary With the non-default stopNodes option, a stop node's body with many "<a " fragments and no closing ">" anywhere after makes readStopNodeData() rescan the entire remaining input from every "<". The failed lookahead is neither cached nor bounded, so n fragments cost O(n²) before the parser throws. Availability-only (CPU exhaustion), needs the non-default stopNodes option — not benchmarked yet, see Proof of Concept. ### Details **CWE-407** — Inefficient Algorithmic Complexity **Code** - `OrderedObjParser.js:520` — `parseXml()` calls `readStopNodeData()` after a matching stop node opens - `OrderedObjParser.js:810` — calls `readTagExp()` for every non-special `<` - `OrderedObjParser.js:688-722` — `tagExpWithClosingIndex()` scans to end-of-input and returns `undefined` when no `>` exists - `OrderedObjParser.js:812-818` — on failure, `i` isn't advanced, so the next `<` repeats the same full scan - `OrderedObjParser.js:423` — the main parse loop throws immediately on the same failure; only the stop-node path retries **Root cause:** a failed closing-`>` lookahead is neither cached nor bounded — `n` copies of `<a ` with no later `>` means `n` scans over the remaining input, O(n²) work, before it finally throws. **Fix** 1. Throw immediately in `readStopNodeData()` when `readTagExp()` returns `undefined` 2. Or break the loop on the first failed scan 3. Or find the next `>` once and reuse/bound that result 4. Add a test asserting near-linear scaling when payload size doubles ### Proof of concept Not benchmarked — static-analysis finding, confidence 0.5. To confirm: configure `stopNodes` matching a root element. Build input as `'<stop>' + '<a '.repeat(n)`. Time `parse()` until it throws `Unexpected end of <tag>`. Start around n=10,000 and double it — quadratic should show ~4x time per doubling, linear ~2x. Control: same input with a `>` present, or `stopNodes` unset. ### Impact CPU-exhaustion DoS in Node.js services parsing untrusted XML with `stopNodes` enabled. A ~100KB payload is estimated at ~10⁹ character operations (roughly a second or more of blocked event loop) — analytical estimate, not a benchmark. No confidentiality/integrity impact. CVSS 3.1: `AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L` (3.7, Low).

CWE-407Inefficient Algorithmic Complexity (opens in a new tab)
Class weaknessLikelihood of exploit: Low
An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-407

Technical details

References

Community reviews

Related threats