NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-80205: 0.65% estimated exploitation activity in the next 30 days; percentile 51.25 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary NLTK's Text.findall() and TokenSearcher.findall() methods accept user-supplied regular expressions and pass them to the Python re engine without timeout or validation, enabling catastrophic backtracking (ReDoS). This issue is isolated to the nltk.text module and was resolved in a prior commit. Affected Code nltk/text.py — TokenSearcher.findall() (line 255) / Text.findall() (line 620) TokenSearcher.init builds an internal string by wrapping each token in angle brackets. The findall() method preprocesses the caller-supplied regexp and runs it directly against this string with no timeout: The preprocessing transforms < and angle-bracket syntax but does not inspect or reject catastrophically backtracking patterns. Proof of Concept Impact Applications that expose Text.findall() to external input are vulnerable to a denial of service. An unauthenticated attacker can cause indefinite CPU saturation with one request, denying service to all other users of the Python process.…

Published by the source

8 Sep 2026, 20:28 UTC

Last modified by the source

8 Sep 2026, 20:45 UTC

Source last checked

26 Oct 2026, 13:51 UTC

1. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-rrv8-h7p8-rx55

2. Revision 3 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-rrv8-h7p8-rx55

4. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-rrv8-h7p8-rx55

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# Patch Next advisory brief: CVE-2026-80205

Advisory: NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions
IDs: CVE-2026-80205, GHSA-rrv8-h7p8-rx55
Record: https://patchnext.com/advisories/cve-2026-80205 (revision 4, source checked 26 Oct 2026, 13:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.9.4; fixed in 3.10.0

## Suggested inspection (read-only)
- `python -m pip show nltk`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nltk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rrv8-h7p8-rx55 (GitHub-reviewed advisory GHSA-rrv8-h7p8-rx55 lists affected ranges: PyPI nltk >=0 <=3.9.4)
- Verified: https://osv.dev/vulnerability/GHSA-rrv8-h7p8-rx55 (OSV record GHSA-rrv8-h7p8-rx55 lists affected ranges: PyPI nltk >=0 <3.10.0 (fixed))

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