GHSA-gx65-c5hj-vpv5

Duplicate Advisory: [CWE-502] Unsafe Pickle Deserialization in TransitionParser Allows Remote Code Execution

Critical Withdrawn Sep 2, 2026 PyPI

Severity: 0.0 / 10; Critical · CVSS 4.0
Exploit chance (EPSS): —; No score published
Known exploited: No; Not in CISA's catalog
Weakness: CWE-502 Deserialization of Untrusted Data
Fix: None yet; No fixed version published

Description

Duplicate Advisory

This advisory has been withdrawn because it is a duplicate of GHSA-rhp5-r9x4-f5g2. This link is maintained to preserve external references.

Original 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.

Affected versions

PyPI

Package	Vulnerable	Fixed in
nltk	`<= 3.9.4`	No fix yet

How to fix

No fixed version has been published yet. Until one is, remove or replace the package, or turn off or isolate the affected feature. This page updates when a fix ships.

Severity breakdown

CVSS 4.0 0.0

CVSS: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/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Attack vector: Network
Attack complexity: Low
Attack requirements: None
Privileges required: None
User interaction: Passive
Confidentiality (vulnerable system): High
Integrity (vulnerable system): High
Availability (vulnerable system): High
Confidentiality (subsequent systems): High
Integrity (subsequent systems): High
Availability (subsequent systems): High
Exploit maturity: Not defined
Scope: X

CVSS 3.1 9.6

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: Required
Scope: Changed
Confidentiality: High
Integrity: High
Availability: High

Weakness

CWE-502: Deserialization of Untrusted Data

The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.

How to prevent it

Architecture and Design, Implementation: If available, use the signing/sealing features of the programming language to assure that deserialized data has not been tainted. For example, a hash-based message authentication code (HMAC) could be used to ensure that data has not been modified.
Implementation: When deserializing data, populate a new object rather than just deserializing. The result is that the data flows through safe input validation and that the functions are safe.
Implementation: Explicitly define a final object() to prevent deserialization.
Architecture and Design, Implementation: Make fields transient to protect them from deserialization. An attempt to serialize and then deserialize a class containing transient fields will result in NULLs where the transient data should be. This is an excellent way to prevent time, environment-based, or sensitive variables from being carried over and used improperly.
Implementation: Avoid having unnecessary types or gadgets (a sequence of instances and method invocations that can self-execute during the deserialization process, often found in libraries) available that can be leveraged for malicious ends. This limits the potential for unintended or unauthorized types and gadgets to be leveraged by the attacker. Add only acceptable classes to an allowlist. Note: new gadgets are constantly being discovered, so this alone is not a sufficient mitigation.
Architecture and Design, Implementation: Employ cryptography of the data or code for protection. However, it's important to note that it would still be client-side security. This is risky because if the client is compromised then the security implemented on the client (the cryptography) can be bypassed.
Operation: Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth [REF-1481].

References

Advisories

nvd.nist.gov/vuln/detail/CVE-2026-78683

Timeline

Aug 25, 2026 Published
Sep 2, 2026 Withdrawn
Sep 2, 2026 Last updated by the source

Advisory	Severity	Why it's related
GHSA-j456-xh4h-cpf2 NLTK: WekaClassifier passes an unvalidated caller-controlled model path to the weka JVM, bypassing nltk.pathsec containment	High	Also affects nltk
GHSA-ff5c-cp5c-9wjf NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars	Medium	Also affects nltk
GHSA-f794-5jv7-7672 NLTK: Downloader.download follows hardlinks and overwrites outside-root files	High	Also affects nltk
GHSA-8mgp-746c-j5xp NLTK: Model-artifact APIs bypass pathsec and touch files outside allowed roots	High	Also affects nltk
GHSA-8mpw-7fpc-4gqj NLTK: Pl196xCorpusReader has quadratic ReDoS on malformed TEI blocks	Low	Also affects nltk
GHSA-cw6x-m8jw-qmrh NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input	Medium	Also affects nltk
GHSA-vp2x-qp44-57v7 NLTK: Quadratic CPU Exhaustion in `XMLCorpusView._read_xml_fragment()`	Low	Also affects nltk
GHSA-ww6m-cw3f-q94g NLTK: Quadratic-time DoS in PorterStemmer via long runs of 'y'	High	Also affects nltk

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