NLTK: Quadratic-time CPU denial of service in ConllCorpusReader SRL instance builder via nested span rescan

Exploitation

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

CVSS

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

EPSS

• CVE-2026-126297: 0.58% estimated exploitation activity in the next 30 days; percentile 48.33 (FIRST EPSS, 2026-11-13). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary `ConllCorpusReader.srl_instances()` builds semantic-role-labeling instances from a CoNLL corpus block. For a sentence block with P predicates and P span columns, the instance builder rescans every span column (and every span within it) once per predicate while searching for each predicate's head, giving O(P^2 * R) work. A single well-formed block that places the head span in the last column forces the full scan on every predicate, so an application that reads an attacker-supplied CoNLL SRL corpus can be driven into tens of seconds of single-threaded CPU. No authentication or user interaction is required.

Root Cause `_get_srl_instances` iterates over the P predicates of a block; for each predicate it scans every span list and every span within it, using an inner `break` that stops only when a span tagged `V` or `C-V` containing the predicate word is found. When the sole head span is placed in the last span column, no earlier column matches, so the `for ... else:…

Published by the source

14 Nov 2026, 00:05 UTC

Last modified by the source

14 Nov 2026, 00:15 UTC

Source last checked

14 Nov 2026, 07:50 UTC

1. Revision 2 · amended · 14 Nov 2026

   Amended from osv record GHSA-v8f3-6phw-6mh9

2. Revision 1 · created · 14 Nov 2026

   Published from ghsa record GHSA-v8f3-6phw-6mh9

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# Patch Next advisory brief: CVE-2026-126297

Advisory: NLTK: Quadratic-time CPU denial of service in ConllCorpusReader SRL instance builder via nested span rescan
IDs: CVE-2026-126297, GHSA-v8f3-6phw-6mh9
Record: https://patchnext.com/advisories/cve-2026-126297 (revision 2, source checked 14 Nov 2026, 07:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected < 3.10.4; fixed in 3.10.4

## 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-v8f3-6phw-6mh9 (GitHub-reviewed advisory GHSA-v8f3-6phw-6mh9 lists affected ranges: PyPI nltk >=0 <=3.10.3)
- Verified: https://osv.dev/vulnerability/GHSA-v8f3-6phw-6mh9 (OSV record GHSA-v8f3-6phw-6mh9 lists affected ranges: PyPI nltk >=0 <3.10.4 (fixed))

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