# NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions

Advisory record · `GHSA-w3v8-gmh9-3wv7`

- **Identifiers:** `GHSA-w3v8-gmh9-3wv7` · `CVE-2026-80206` · `PYSEC-2026-3751`
- **State:** Confirmed
- **Severity:** CVSS 8.2 High
- **Critical:** No
- **Weakness (CWE):** [CWE-1333](https://cwe.mitre.org/data/definitions/1333.html)
- **Tags:** [Denial of service](https://agentprime.io/advisories/tag/dos.md)
- **Published:** 8 Sept 2026
- **Updated:** 8 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 8 Sept 2026, 20:28 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `nltk` (PyPI): affected `<3.10.3`; fixed in `3.10.3`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> NLTK versions before 3.10.3 are vulnerable to regular expression denial of service through the nltk.tgrep module, which passes user-supplied regular expressions to Python's re engine without a timeout or validation. An attacker who can supply the pattern to tgrep_positions() or tgrep_compile(), such as through a web API, can send a single crafted request that causes catastrophic backtracking and blocks the process indefinitely. Upgrade to NLTK 3.10.3 or later, which contains the fix; alternatively, do not pass untrusted input to tgrep patterns.

Written 5 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 8.2 High, CVSS 4.0: `CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`, from CNA (VulnCheck)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0044, the 38th percentile, on 14 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:38 UTC: First seen by agentprime
- 31 Oct 2026, 14:38 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-w3v8-gmh9-3wv7)

## Sources

- [GitHub Advisory Database: GHSA-w3v8-gmh9-3wv7](https://github.com/advisories/GHSA-w3v8-gmh9-3wv7)
- [OSV: GHSA-w3v8-gmh9-3wv7](https://osv.dev/vulnerability/GHSA-w3v8-gmh9-3wv7)
- <https://github.com/nltk/nltk/security/advisories/GHSA-w3v8-gmh9-3wv7>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-80206>
- <https://github.com/nltk/nltk/commit/0072ea2fb8be22e038a36e887b7061bb6b9339d9>
- <https://github.com/nltk/nltk>
- <https://github.com/nltk/nltk/releases/tag/v3.10.3>
- <https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3751.yaml>
- <https://www.vulncheck.com/advisories/nltk-3.10.2-regular-expression-denial-of-service-via-tgrep>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-w3v8-gmh9-3wv7>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-w3v8-gmh9-3wv7/>. Everything for agents: <https://agentprime.io/llms.txt>.