# CVE-2026-81723: NLTK XMLCorpusView quadratic CPU exhaustion

Source: https://junglewise.ai/threats/cve-2026-81723-nltk-xmlcorpusview-quadratic-cpu-exhaustion  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-27. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-81723 · Severity: low · CVSS 3.7 · Published 2026-08-27

NLTK is a Python natural language processing library widely used for corpus analysis and text processing. The XMLCorpusView component, which reads XML corpus files used for NLP tasks, has a performance flaw that allows an attacker with write access to a corpus file path to cause severe CPU exhaustion through a specially crafted malformed XML file. Processing an 8 MiB malformed file can consume ~48 CPU-seconds, potentially denying service to applications that rely on automated corpus processing.

Technologies: [nltk (PyPI)](https://junglewise.ai/threats/technologies/nltk). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-81723 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-81723 being exploited in the wild.

**How severe is CVE-2026-81723?** Low severity, with a CVSS score of 3.7, published 27 August 2026.

**Which products and versions are affected?** nltk (PyPI) <= 3.10.2, ECOSYSTEM:introduced=0,fixed=3.10.3.

**Is there a fix for CVE-2026-81723?** Yes. nltk (PyPI): upgrade to 3.10.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-81723 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-81723?** Check whether you run nltk (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- nltk (PyPI): upgrade to 3.10.3 or later

  ```sh
  pip install "nltk>=3.10.3"
  ```


## Technical details

The vulnerability is an algorithmic inefficiency in XMLCorpusView._read_xml_fragment() (nltk/corpus/reader/xmldocs.py, lines 261–308). The function reads XML corpus files in 1 KiB blocks, appending each block to a `fragment` string, then calls `_VALID_XML_RE.match(fragment)` to validate the accumulated buffer on every iteration. Because the regex match rescans the entire accumulated fragment from the beginning each iteration, total work grows O(n²) rather than O(n). For a payload of a single `<` followed by (N-1) `a` bytes, the unterminated tag prevents regex match success and `fragment.rfind("<")` returns 0, bypassing the backtrack optimization. The only exit is EOF after consuming all N bytes. This affects all readers derived from XMLCorpusView (BNCCorpusReader, NPSChatCorpusReader, SemcorCorpusReader, MTECorpusReader, NKJPCorpusReader, FrameNetCorpusReader, VerbNetCorpusReader) but not XMLCorpusReader.xml() which uses defusedxml.safe_parse(). Exploitation requires only write access to a corpus file path the reader will open; no authentication or special privileges are needed. A patch (NLTK 3.10.3+) is available that eliminates quadratic rescanning via incremental parsing or bounded accumulation.

## Affected products

- NLTK Project NLTK 3.10.2 and earlier; including 3.9.4, 3.10.0; potentially all versions since 2007 introduction of XMLCorpusView

## Identifiers

- CVE-2026-81723
- GHSA-vp2x-qp44-57v7
- PYSEC-2026-3871

## Timeline

- 2026-09-02: disclosed: Advisory published to GitHub Advisory Database and CVE
- 2026-09-02: patched: NLTK 3.10.3 released with fix (commit 7808692)

## References

- [https://api.github.com/users/ibfavas](https://api.github.com/users/ibfavas)
- [https://github.com/ibfavas](https://github.com/ibfavas)
- [https://api.github.com/users/ibfavas/gists%7B/gist_id%7D](https://api.github.com/users/ibfavas/gists%7B/gist_id%7D)
- [https://api.github.com/users/ibfavas/repos](https://api.github.com/users/ibfavas/repos)
- [https://avatars.githubusercontent.com/u/188737388?v=4](https://avatars.githubusercontent.com/u/188737388?v=4)
- [https://api.github.com/users/ibfavas/events%7B/privacy%7D](https://api.github.com/users/ibfavas/events%7B/privacy%7D)

## Related threats

- [NLTK WekaClassifier path traversal in model validation](https://junglewise.ai/threats/cve-2026-109956-nltk-through-3-10-3-contains-a-path-traversal-vulnerability-in)
- [NLTK RecursiveDescentParser uncontrolled recursion denial of service](https://junglewise.ai/threats/cve-2026-12876-nltk-recursivedescentparser-uncontrolled-recursion-denial-of)
- [NLTK tgrep regular expression denial of service](https://junglewise.ai/threats/nltk-tgrep-regular-expression-denial-of-service-02fca1b2)
- [NLTK Downloader hardlink filesystem containment bypass](https://junglewise.ai/threats/nltk-downloader-hardlink-filesystem-containment-bypass-522cb97d)
- [NLTK Pl196xCorpusReader quadratic ReDoS on malformed TEI blocks](https://junglewise.ai/threats/nltk-pl196xcorpusreader-quadratic-redos-on-malformed-tei-blocks-8473e6ef)