CVE-2026-81726 — nltk

Summary

Several model-artifact APIs still treat caller-controlled model paths as ordinary filenames even when NLTK path security is enforced. The same outside-root paths are rejected by guarded helpers, but these public read and write flows still use raw file APIs.

Details

• Vulnerability type: File sandbox bypass
• Affected component: TransitionParser.train, TransitionParser.parse, AveragedPerceptron.save, AveragedPerceptron.load, PerceptronTagger.save_to_json, save_maxent_params
• Affected versions: Published 3.9.4 and current source v3.10.0-rc2 both reproduced.
• Patched versions: Not yet patched
• Root cause: Model import and export helpers use built-in open() on caller-controlled paths instead of pathsec-aware helpers.

TransitionParser.train() writes outside allowed roots, TransitionParser.parse() reads outside allowed roots, AveragedPerceptron bypasses the sandbox in both directions, and adjacent read-side helpers in the same family already show the intended guarded behavior. I confirmed outside-root reads and writes while pathsec.open() or the guarded sibling helpers rejected the same paths.

PoC

Preconditions

• The application enables pathsec enforcement and lets untrusted workflows choose model import or export paths.

Steps

1. Enable pathsec.ENFORCE=True and restrict allowed roots to a dedicated sandbox directory.
2. Use public model import or export APIs with paths that point outside that root.
3. Observe the same paths are rejected by negative-control guarded helpers such as pathsec.open(), PerceptronTagger.load_from_json(), or load_maxent_params().
4. Observe the vulnerable APIs still read or write outside-root files successfully.

Minimal reproducible excerpt

transition_train_exists True
transition_parse_loader_read_bytes 13
averaged_load_keys ['bias']
maxent_save wrote ['alwayson.tab', 'labels.txt']

Impact

Consumers that rely on pathsec for local containment can be tricked into reading or overwriting files outside approved roots through normal model persistence and loading APIs.

Remediation

Route all model-path file access through nltk.pathsec.open() or existing pathsec-aware helpers, and add regression tests that pair each vulnerable API with a negative control on the same path.