CVE-2026-78683
NLTK before 3.10.0 (affected versions <=3.9.4) contains an unsafe pickle deserialization vulnerability in the TransitionParser.parse() method (nltk/parse/transitionparser.py).
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.29%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
Description
NLTK before 3.10.0 (affected versions <=3.9.4) contains an unsafe pickle deserialization vulnerability in the TransitionParser.parse() method (nltk/parse/transitionparser.py). The method calls pickle_load() with the default restricted=False, routing deserialization through WarningUnpickler, which does not override find_class() and therefore permits arbitrary class resolution. When an application loads an attacker-crafted model file, embedded pickle gadget chains execute arbitrary Python code with the privileges of the user running the application. NLTK provides a RestrictedUnpickler for safe deserialization, but it is not used by production code paths. Fixed in 3.10.0.
- CVSS 4.0
- 9.4 CRITICALCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M
- EPSS
- 0.29% probability · 22th percentile
- CISA KEV
- Not listed
- Weakness
- CWE-502
- Affected
- nltk/nltk
- Source
- disclosure@vulncheck.com
References
- https://github.com/nltk/nltk/security/advisories/GHSA-rhp5-r9x4-f5g2Exploit, Vendor Advisory
- https://www.vulncheck.com/advisories/nltk-before-remote-code-execution-via-unsafe-pickle-deserializationThird Party Advisory
- https://github.com/nltk/nltk/security/advisories/GHSA-rhp5-r9x4-f5g2Exploit, Vendor Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.