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** ```text framenet_frame_definition FRAME_LEAK framenet_doc_text DOC_LEAK framenet_lu_text LU_LEAK nkjp_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

CVSS 3.1

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

Severity from

GitHub (reviewed advisory)

Weakness

CWE-22, CWE-706, CWE-73

Also known as

CVE-2026-62385, PYSEC-2026-3728

• github.com/nltk/nltk/security/advisories/GHSA-568f-pv23-39p4
• nvd.nist.gov/vuln/detail/CVE-2026-62385
• github.com/nltk/nltk/pull/3579
• github.com/nltk/nltk/pull/3581
• github.com/nltk/nltk/commit/7d1389d0789c1eca56bd0ed444089e0a3972e3ed
• github.com/nltk/nltk/commit/bf3bf32786791394a1008258b4917a7f2d4dbcda
• github.com/nltk/nltk
• github.com/nltk/nltk/releases/tag/v3.10.0
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3728.yaml
• www.vulncheck.com/advisories/nltk-path-traversal-via-framenet-and-nkjp-readers