NLTK: Quadratic-time CPU denial of service in CCG lexicon parser (fromstring) on wide bracket bodies and operator chains

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-2026-126299: 0.58% estimated exploitation activity in the next 30 days; percentile 48.34 (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 The public CCG lexicon parser `nltk.ccg.lexicon.fromstring` parses caller-influenced lexicon text with two pure-Python loops whose cost is quadratic in the input width. A service that parses an untrusted CCG lexicon string can be driven into tens of seconds of single-threaded CPU by a few-megabyte input, starving the request worker. No authentication or user interaction is required.

Root Cause `matchBrackets` consumes a bracket body one character at a time with `inside = inside + rest[0]; rest = rest[1:]`. The `rest = rest[1:]` tail re-slice copies the entire remaining string on every iteration, so a flat bracket body costs O(n^2). `augParseCategory` drives a `while rest != "":` operator loop that re-copies the regular-expression match tail (`rest = app[3]`) once per operator, which is also O(n^2) over a long operator chain. The parser carries a recursion-depth guard (`MAX_PARSE_DEPTH = 500`) that raises on deeply nested categories, but both quadratic loops run on flat…

Published by the source

14 Nov 2026, 00:08 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-89p3-fcch-88ph

2. Revision 1 · created · 14 Nov 2026

   Published from ghsa record GHSA-89p3-fcch-88ph

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# Patch Next advisory brief: CVE-2026-126299

Advisory: NLTK: Quadratic-time CPU denial of service in CCG lexicon parser (fromstring) on wide bracket bodies and operator chains
IDs: CVE-2026-126299, GHSA-89p3-fcch-88ph
Record: https://patchnext.com/advisories/cve-2026-126299 (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-89p3-fcch-88ph (GitHub-reviewed advisory GHSA-89p3-fcch-88ph lists affected ranges: PyPI nltk >=0 <=3.10.3)
- Verified: https://osv.dev/vulnerability/GHSA-89p3-fcch-88ph (OSV record GHSA-89p3-fcch-88ph lists affected ranges: PyPI nltk >=0 <3.10.4 (fixed))

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