Nltk Nltk
CVE-2026-66393
NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.
What this means for your business
- It affects Nltk. It matters if your company, or a supplier that handles your data, runs it.
- Code showing how to exploit it has been published, but real attacks have not been confirmed.
- An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
- FIRST's prediction model gives it a 0.4% chance of attack attempts being seen in the next 30 days, ranking above 29% of all known flaws.
What to do
- 1Check whether your company or your suppliers run Nltk, and which version. The affected versions are listed further down this page.
- 2If you do, apply the vendor's fix. A patch or vendor advisory has been published.
Fastnexa security experts
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Scoring
- CVSS
- 7.5 (v3.1)
- Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- CVSS v4
- 8.7
- Weakness
- CWE-674
- Assigned by
- disclosure@vulncheck.com
Dates
- Published
- 2026-08-22
- Last modified
- 2026-08-27
- Sources
- NVD
Affected products
- Nltk Nltk- 3.9.4
As listed in the NVD configuration data. Not a statement about your estate.