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.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.36%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
Description
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.
- CVSS 4.0
- 8.7 HIGHCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M
- EPSS
- 0.36% probability · 30th percentile
- CISA KEV
- Not listed
- Weakness
- CWE-674
- Affected
- nltk/nltk
- Source
- disclosure@vulncheck.com
References
- https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pwExploit, Mitigation, Vendor Advisory
- https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoderThird Party Advisory
- https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pwExploit, Mitigation, Vendor Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.