fast-toml: Prototype pollution via __proto__ keys in parsed TOML
Summary
The fast-toml JavaScript TOML parser through version 0.5.4 allows prototype pollution. An attacker who can supply TOML to parse() can use __proto__, constructor or prototype keys to modify Object.prototype, which affects all objects in the Node.js process. Possible results are denial of service or authorization bypass. The issue is relevant only if an application parses attacker-controlled TOML with this library.
Affected
- fast-toml 0 to 0.5.4 (per the CVE record, CNA VulnCheck)
Fixed in
Unknown. A third-party aggregator reports 0.5.5, but the vendor has not confirmed this.
Workaround
- No vendor workaround found.
- Do not parse TOML from untrusted sources until the library is updated or replaced.
Exploitation
- CISA SSVC: exploitation none, automatable yes, technical impact partial.
- Not listed as known exploited.
- The public GitHub issue #8 describes reproduction steps. No public exploit has been confirmed.
Our recommendation
- Check lockfiles and dependency trees (including transitive dependencies) for fast-toml.
- Where it is present, upgrade to a version confirmed as fixed by the maintainer, or replace it with a maintained TOML parser.
- Prioritise internet-facing applications that accept user-supplied TOML.
- Monitor the upstream repository for an official fix.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- fast-toml GitHub repository github.com
- GitHub issue #8: Prototype Pollution via TOML table/key parsing github.com
- GitHub pull request #3 github.com
- VulnCheck advisory www.vulncheck.com
- Tenable CVE-2021-48012 www.tenable.com
- OpenCVE CVE-2021-48012 app.opencve.io
- Strix CVE-2021-48012 www.strix.ai
- HOL Guard CVE-2021-48012 hol.org
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 31 Oct 2026, 20:23.
Original description (NVD)
fast-toml through 0.5.4 contains a prototype pollution vulnerability that allows attackers to modify Object.prototype by supplying TOML containing __proto__, constructor or prototype key segments. Attackers can pass [__proto__] or [constructor.prototype] table headers or __proto__.x dotted keys to parse() to pollute all objects, causing denial of service or authorization bypass.
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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