NLTK: Model-artifact APIs bypass pathsec and touch files outside allowed roots

### 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** ```text 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. ### References - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/parse/transitionparser.py#L551-L571 - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/tag/perceptron.py#L106-L113 - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/tag/perceptron.py#L262-L273 - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/classify/maxent.py#L1583-L1602

CVSS 3.1

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

Severity from

GitHub (reviewed advisory)

Weakness

CWE-22, CWE-59, CWE-73

Also known as

CVE-2026-81726, PYSEC-2026-3740

• github.com/nltk/nltk/security/advisories/GHSA-8mgp-746c-j5xp
• nvd.nist.gov/vuln/detail/CVE-2026-81726
• github.com/nltk/nltk/pull/3757
• github.com/nltk/nltk/pull/3759
• github.com/nltk/nltk/pull/3813
• github.com/nltk/nltk/commit/2a92b71827d754ae8920261e7ed0c4bb283ab2d7
• github.com/nltk/nltk/commit/a44a7af69bca87e92d9c4a701fcbbe4512e8d450
• github.com/nltk/nltk/commit/cbc98458b43de5f792f0382583c16df39e5c5117
• github.com/nltk/nltk
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3740.yaml
• www.vulncheck.com/advisories/nltk-through-3.10.3-path-traversal-via-model-artifact-apis