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Fencer Intel
high 7.5

GHSA-m42h-3232-vpv3 json

nltk: Arbitrary File Read via Path Traversal in nltk.data.load() through Percent-Encoded Sequences

published 4mo ago modified 2mo ago also CVE-2026-12243 , PYSEC-2026-597

cvss
7.5
version 3.1, GitHub
cisa kev
Not listed
not in CISA's catalog
epss
—
no score, EPSS covers CVEs only
public exploits
0
none known

Summary

# Summary nltk.data.load() and nltk.data.find() resolve user-supplied resource names to filesystem paths using url2pathname(), which decodes percent-encoded sequences (e.g. %2e%2e to ..). Path safety checks are performed on the raw, still-encoded string before decoding occurs. An attacker supplying %2e%2e instead of .. bypasses all path validation and reads arbitrary files outside the NLTK data directory.

# Vulnerable Code nltk/data.py - find() function: url2pathname() decodes %2e%2e -> .. AFTER any safety check p = os.path.join(path_, url2pathname(resource_name)) if os.path.exists(p): return FileSystemPathPointer(p)

# Proof of Concept import nltk.data nltk.data.path = ["/home/user/nltk_data"] %2e%2e decodes to .. via url2pathname(), escaping the data dir data = nltk.data.load("%2e%2e/SECRET_credentials.txt", format="raw") print(data) b'AWS_SECRET_KEY=AKIAIOSFODNN7EXAMPLE\nDATABASE_PASS=hunter2\n' All of these bypass path checks and decode identically:

# Payload After url2pathname() %2e%2e/secret ../secret .%2e/secret ../secret %2e./secret ../secret %2E%2E/secret ../secret Root Cause url2pathname() is called after path safety checks, not before. Encoding .. as %2e%2e passes every check, then decodes to a traversal sequence at filesystem access time.

# Fix Decode before checking:

from urllib.parse import unquote resource_name = unquote(resource_name) # decode first, then validate

# Impact An attacker who controls the resource name passed to nltk.data.load() can read any file the process has permission to access - credentials, environment files, SSH private keys, /etc/passwd, /proc/self/environ, application config files, etc. This affects any application that passes user-controlled input to nltk.data.load() or nltk.data.find().

Patch / fix

  • pypi nltk 3.10.0

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CVSS scores

assigned by version score vector
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
OSV 3.0 7.5 CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Affected packages

ecosystem package affected fixed in
pypi nltk
<3.10.0
<=3.9.4
3.10.0

Classification

Weakness
  • CWE-22 Improper Limitation of a Pathname to a Restricted Directory

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

The maintainers of nltk/nltk published their own advisory on GitHub. Reviewed sources take precedence over it.