NLTK: Stable FrameNet and NKJP readers parse outside-root XML

Summary

Published nltk==3.9.4 still contains several XML-reader entrypoints that build parser paths from caller-controlled selectors or trusted-looking index state without preserving the corpus-root boundary.

Details

• Vulnerability type: Path traversal and trusted-root bypass
• Affected component: FramenetCorpusReader.frame_by_name, FramenetCorpusReader.doc, FramenetCorpusReader.lu, NKJPCorpusReader.header
• Affected versions: Published 3.9.4 reproduced. Current source v3.10.0-rc2 acted as a negative control and blocked the same payloads.
• Patched versions: Patched in version 3.10.0, which includes the path-safety rejections seen in the release candidate.
• Root cause: Stable reader paths still construct raw XML filenames from unsafe selectors, poisoned index state, or unsafe file identifiers.

I confirmed four public stable entrypoints return parsed outside-root content: a parent-segment traversal frame name, a poisoned fulltext index filename, a poisoned LU id, and an unsafe NKJP header file identifier. Current source rejects the same payloads with explicit path-safety errors, which shows the bug is real but version-scoped to the published stable package.

PoC

Preconditions

• The application exposes FrameNet or NKJP reader APIs while trusting NLTK to keep XML parsing inside a corpus root.

Steps

1. Create a minimal FrameNet or NKJP corpus root and place attacker-chosen XML files outside that root.
2. Feed unsafe selectors or poisoned index state into the relevant public stable 3.9.4 APIs.
3. Observe frame_by_name, doc, lu(...).exemplars, or header return parsed outside-root values.
4. Run the same payloads against current source and observe explicit path-safety rejections.

Minimal reproducible excerpt

 1framenet_frame_definition FRAME_LEAK
 2framenet_doc_text DOC_LEAK
 3framenet_lu_text LU_LEAK
 4nkjp_header_title HEADER_LEAK

Impact

Applications that process attacker-influenced FrameNet or NKJP corpus selectors or state can be made to parse XML outside the trusted corpus root through normal public reader responses.

Remediation

Keep these reader paths on the same root-confinement model as CorpusReader.open() and nltk.pathsec. Reject unsafe path components before constructing filenames from frame names, document filenames, LU ids, or NKJP file identifiers.

Resources

• https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/framenet.py#L1366-L1369
• https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/framenet.py#L1456-L1460
• https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/framenet.py#L1803-L1810
• https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/nkjp.py#L96-L103
• https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/nkjp.py#L251-L256
• https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/corpus/reader/framenet.py#L1388-L1399
• https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/corpus/reader/nkjp.py#L96-L128