NLTK: Quadratic CPU Exhaustion in XMLCorpusView._read_xml_fragment()

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2026-81723: 0.28% estimated exploitation activity in the next 30 days; percentile 20.63 (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 XMLCorpusView.readxmlfragment() reads a corpus file in 1 KiB blocks, appending each block to a growing fragment string, then calls VALIDXMLRE.match(fragment) on the full accumulated buffer every iteration. Because each iteration rescans the entire accumulated fragment, the total amount of work grows quadratically with input size. Commit c9c332284 (CWE-1333) made each match() call linear. The quadratic behavior is separate: the loop calls match() once per 1 KiB block, each time on a longer buffer. On the test system, an 8 MiB malformed XML file consumed approximately 48 CPU-seconds through the public BNCCorpusReader.words() API with no source modification. Absolute timings vary by hardware. readxmlfragment() imposes no limit on fragment size or iteration count. Details File: nltk/corpus/reader/xmldocs.py Function: XMLCorpusView.readxmlfragment(), lines 261–308 The relevant loop: For a payload of b'<' + b'a' (N-1): - For this malformed input, VALIDXMLRE.match(fragment) does not…

Published by the source

2 Sep 2026, 14:34 UTC

Last modified by the source

2 Sep 2026, 14:34 UTC

Source last checked

26 Oct 2026, 13:51 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-vp2x-qp44-57v7

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-81723

Advisory: NLTK: Quadratic CPU Exhaustion in `XMLCorpusView._read_xml_fragment()`
IDs: CVE-2026-81723, GHSA-vp2x-qp44-57v7
Record: https://patchnext.com/advisories/cve-2026-81723 (revision 1, 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: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.10.2; fixed in 3.10.3

## 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-vp2x-qp44-57v7 (GitHub-reviewed advisory GHSA-vp2x-qp44-57v7 lists affected ranges: PyPI nltk >=0 <=3.10.2)

Commands are suggestions: review each one before running it.