flat: Denial of service via unflatten() with numeric array index keys
Summary
The JavaScript npm library flat through version 6.0.1 allows attackers to create huge sparse arrays through unbounded numeric keys in unflatten(). Attackers can supply flattened keys such as a.4294967294, which causes iteration with forEach, spread or JSON.stringify to block the Node.js event loop. The attack is only relevant if an application passes attacker-controlled keys into unflatten(), and the impact is limited to availability.
Affected
- flat up to and including 6.0.1 (CNA range: 0 to <= 6.0.1)
- Older branches: no vendor statement found; not confirmed as affected
Fixed in
Unknown.
Workaround
- Do not pass untrusted keys into unflatten().
- Where needed, validate key structure first and reject numeric segments above a sensible limit.
- Place Node.js services behind a reverse proxy with request size and timeout limits, and monitor event-loop lag.
Exploitation
- VulDB states that remote exploitation is possible; no exploit is available.
- A public GitHub issue describes the mechanism and claims a single small request can stall the process; this claim has not been verified.
- No confirmed in-the-wild exploitation found.
- EPSS: 0.32 % (percentile 25).
Our recommendation
- Check whether any Node.js application uses flat, directly or transitively (for example with npm ls flat), and whether it runs in production.
- Review whether untrusted input reaches unflatten(); if so, apply the workarounds above with priority.
- Monitor the upstream project and the VulnCheck advisory for a fixed release, and update or replace the library once available.
- Do not treat the score alone as decisive: severity ratings differ between sources (CVSS 4.0: 8.2; CVSS 3.1: 7.5; CVSS 5.9).
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- VulnCheck advisory www.vulncheck.com
- hughsk/flat on GitHub github.com
- flat index.js (vulnerable code, commit 1d68e1d) github.com
- flat issue 195 (reporter description) github.com
- VulDB entry 428405 vuldb.com
- CVE record mirror (threatint) cve.threatint.com
- Strix CVE mirror www.strix.ai
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 14:42.
Original description (NVD)
flat through 6.0.1 contains an uncontrolled resource allocation vulnerability in unflatten() that allows attackers to create huge sparse arrays via unbounded numeric keys. Attackers can supply flattened keys such as a.4294967294, causing iteration with forEach, spread or JSON.stringify to block the Node.js event loop.
NVD references (5)
- https://github.com/hughsk/flat
- https://github.com/hughsk/flat/blob/1d68e1d06c0210cff31e5c61ac177f3e2d3fa564/index.js#L137-L142
- https://github.com/hughsk/flat/blob/1d68e1d06c0210cff31e5c61ac177f3e2d3fa564/index.js#L65-L75
- https://github.com/hughsk/flat/issues/195
- https://www.vulncheck.com/advisories/flat-through-6.0.1-denial-of-service-via-unflatten-numeric-array-index-k…
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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