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Medium

In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. T

  • Disclosed: 03.08.2026
  • Updated: 16.10.2026 13:17

In nltk version 3.9.4, the nltk.downloader.Downloader._download_package() function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. This allows an attacker to tamper with the package response body for info.url through a compromised mirror, malicious proxy, or other source-substitution condition, leading to the installation of attacker-controlled package bytes. The vulnerability can result in malicious corpus or model content being trusted by downstream users or applications.

Affected software

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PlatformPackageVulnerable versionsFixed in
Python (PyPI) nltk <= 3.9.4 no fix

References

CVSS score 6.5/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
Weakness (CWE)
CWE-494
Platform
Python (PyPI)
Package
nltk
Disclosed
03.08.2026
Updated
16.10.2026 13:17

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2026-3729 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-5wp5-5229-5g6q - CC BY 4.0

OSV / GitHub Advisory Database - GHSA-gf32-cmjh-8m9v - CC BY 4.0

OSV / GitHub Advisory Database - PYSEC-2026-4318 - PyPA Advisory Database; CC BY 4.0

Vulnerability history: nltk

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (52) in the archive
Total entries
52
First → latest
08.2019 09.2026
Without a fix
6
Exploited in attacks
0

Severity breakdown

  • Critical 5
  • High 32
  • Medium 13
  • Low 2

Disclosures per year

Severity:
  1. NLTK: Uncontrolled search path when invoking the Graphviz 'dot' binary
    High · 7.8 CVE-2026-78680 < 3.10.3 3.10.3
  2. NLTK: Model-artifact APIs bypass pathsec and touch files outside allowed roots
    High · 7.0 CVE-2026-81726 <= 3.10.3 no fix
  3. NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions
    High CVE-2026-80206 < 3.10.3 3.10.3
  4. NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions
    High · 7.5 CVE-2026-80205 < 3.10.0 3.10.0
  5. NLTK: JVM argument injection bypass via per-call options in the NLTK Stanford wrappers (incomplete fix of CVE-2026-12841)
    Critical · 9.8 CVE-2026-79675 < 3.10.3 3.10.3
  6. NLTK: Corpus readers follow symlinks outside trusted roots despite pathsec enforcement
    High CVE-2026-79676 < 3.10.3 3.10.3
  7. NLTK: Corpus Reader Sandbox Bypass
    High CVE-2026-79674 < 3.10.3 3.10.3
  8. NLTK: Allowlisted pickle loaders still permit code execution in current source
    Critical CVE-2026-79657 < 3.10.3 3.10.3
  9. NLTK: Unsafe Pickle Deserialization in TransitionParser Allows Remote Code Execution
    Critical CVE-2026-78683 < 3.10.0 3.10.0
  10. NLTK: Entity-expansion DoS (billion laughs) via remaining raw ElementTree parses
    High CVE-2026-78681 < 3.10.3 3.10.3
  11. NLTK: pathsec SSRF protection can be bypassed when a proxy is configured
    High CVE-2026-78682 < 3.10.3 3.10.3
  12. NLTK AllowlistUnpickler dotted-name validation bypass allows remote code execution
    High · 8.8 CVE-2026-71513 >= 3.10.0, < 3.10.3 3.10.3
  13. NLTK: Symlink escape in CorpusReader allows arbitrary local file read outside the corpus root
    High · 6.2 CVE-2026-70626 < 3.9.4 3.9.4
  14. NLTK: StreamBackedCorpusView Bypasses pathsec.ENFORCE - Arbitrary Local File Read
    High CVE-2026-63312 < 3.10.0 3.10.0
  15. NLTK: Symlink-based sandbox bypass in FramenetCorpusReader (bypasses the fix for CVE-2026-54292)
    High · 7.5 CVE-2026-62384 >= 3.10.0, < 3.10.2 3.10.2
  16. NLTK: Stable FrameNet and NKJP readers parse outside-root XML
    High · 5.9 CVE-2026-62385 < 3.10.0 3.10.0
  17. NLTK: Default ENFORCE=False Disables All pathsec Security Controls
    High CVE-2026-62388 < 3.10.0 3.10.0
  18. NLTK TweetTokenizer vulnerable to denial of service through catastrophic regex backtracking
    High · 7.5 CVE-2026-72818 < 3.10.1 3.10.1
  19. In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. T this entry
    Medium · 6.5 CVE-2026-12259 <= 3.9.4 no fix
  20. Natural Language Toolkit (NLTK): DNS-rebinding SSRF filter bypass in nltk.pathsec.urlopen (nltk.download / nltk.data.load) defeats ENFORCE mode
    High · 8.6 CVE-2026-12075 < 3.10.0 3.10.0
  21. Natural Language Toolkit (NLTK): ReDoS in NLTK ReviewsCorpusReader FEATURES regex
    High · 7.5 CVE-2026-12061 < 3.10.0 3.10.0
  22. Natural Language Toolkit (NLTK): Path Traversal in NKJPCorpusReader leads to Arbitrary File Read and bypasses the nltk.pathsec sandbox (ENFORCE=True)
    High · 7.5 CVE-2026-12072 < 3.10.0 3.10.0
  23. Natural Language Toolkit (NLTK) has path traversal in FramenetCorpusReader.frame() that allows arbitrary XML file read, bypassing the nltk.pathsec sandbox (ENFORCE=True)
    High · 7.5 CVE-2026-12074 < 3.10.0 3.10.0
  24. NLTK (Natural Language Toolkit) before version 3.9.3 contains an eval injection vulnerability in the nltk.collocations module that allows an attacker who controls command-line arguments to execute arb
    High CVE-2025-71408 < 3.9.3 3.9.3
  25. In nltk/nltk versions 3.9.3 and earlier, five Stanford interface classes (StanfordPOSTagger, StanfordNERTagger, StanfordParser, StanfordDependencyParser, and StanfordNeuralDependencyParser) are vulner
    High · 7.8 CVE-2026-12252 < 3.9.4 3.9.4
  26. NLTK version 3.9.4 is vulnerable to a path traversal attack due to an incomplete fix for GitHub Issue #3504. The `_UNSAFE_NO_PROTOCOL_RE` regex in `nltk/data.py` checks for literal `../` sequences but
    High · 7.5 CVE-2026-12243 <= 3.9.4 no fix
  27. Natural Language Toolkit (NLTK): URL-Encoded Path Traversal in nltk.data.load() Allows Arbitrary Local File Read
    High · 7.5 CVE-2026-54293 < 3.10.0 3.10.0
  28. NLTK has a Downloader Path Traversal Vulnerability (AFO) - Arbitrary File Overwrite
    High · 8.1 CVE-2026-33236 <= 3.9.2 no fix
  29. Unauthenticated remote shutdown in nltk.app.wordnet_app
    High · 7.5 CVE-2026-33231 < 3.9.4 3.9.4
  30. A vulnerability in the `filestring()` function of the `nltk.util` module in nltk version 3.9.2 allows arbitrary file read due to improper validation of input paths. The function directly opens files s
    High · 7.5 CVE-2026-0846 <= 3.9.2 no fix
  31. NLTK versions <=3.9.2 are vulnerable to arbitrary code execution due to improper input validation in the StanfordSegmenter module. The module dynamically loads external Java .jar files without verific
    Critical · 10.0 CVE-2026-0848 < 3.9.3 3.9.3
  32. A vulnerability in NLTK versions up to and including 3.9.2 allows arbitrary file read via path traversal in multiple CorpusReader classes, including WordListCorpusReader, TaggedCorpusReader, and Brack
    High · 7.5 CVE-2026-0847 < 3.9.3 3.9.3
  33. NLTK has a Zip Slip Vulnerability
    Critical · 10.0 CVE-2025-14009 < 3.9.3 3.9.3
  34. ntlk unsafe deserialization vulnerability
    High · 7.5 CVE-2024-39705 < 3.9 3.9
  35. NLTK Vulnerable to REDoS
    High · 7.5 CVE-2021-3842 < 3.6.6 3.6.6
  36. Inefficient Regular Expression Complexity in nltk (word_tokenize, sent_tokenize)
    High · 7.5 CVE-2021-43854 < 3.6.6 3.6.6
  37. NLTK Vulnerable to REDoS
    High · 7.5 CVE-2021-3828 < 3.6.4 3.6.4
  38. NLTK Vulnerable To Path Traversal
    High · 7.5 CVE-2019-14751 < 3.4.5 3.4.5

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