CVE-2026-12876: NLTK Remote Code Execution Vulnerability

MODERATE
Advisory Identifier: GHSA-ff5c-cp5c-9wjf
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CWE-674 CWE-407
Published
Advisory Summary

CVE-2026-12876 is a MODERATE remote code execution vulnerability (CWE-674: Uncontrolled Recursion) affecting NLTK libraries in the PyPI ecosystem. The issue involves uncontrolled resource consumption in recursivedescentparser via ambiguous or left-recursive grammars. Remediation is available by upgrading affected packages to version 3.10.3.

CVE-2026-12876 at a Glance

CVE Identifier CVE-2026-12876
GHSA Identifier GHSA-ff5c-cp5c-9wjf
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-674: Uncontrolled Recursion
Affected Product NLTK
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix CVE-2026-12876

To remediate this vulnerability, upgrade all affected NLTK dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

nltk fixed: 3.10.3 
pip install nltk>=3.10.3
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show nltk

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-12876, inspect your installed packages using pip:

pip show nltk

Analyze the terminal output to locate any version of NLTK in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.10.3), your application is affected.

Note: The pip show nltk command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-674: Uncontrolled Recursion, where the product does not properly control the amount of recursion that takes place, consuming excessive resources, such as allocated memory or the program stack. In NLTK, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

`nltk.parse.RecursiveDescentParser` (and `SteppingRecursiveDescentParser`) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely — a denial of service. ## Proof of concept Both of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop: ```python from nltk import CFG from nltk.parse import RecursiveDescentParser # (a) left recursion -> unbounded recursion g = CFG.fromstring("S -> S S | 'a'") list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs # (b) ambiguous grammar -> exponential number of parses g = CFG.fromstring("S -> 'a' S | 'a' S S | 'a'") list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs ``` ## Impact An application that runs `RecursiveDescentParser` on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only. ## Sibling The RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.

CVE-2026-12876 Affected NLTK Versions

1 component(s)
Fixed Versions:
3.10.3
Product / Package Ecosystem Affected Versions Fixed Version
nltk
pypi/nltk
PyPI
< 3.10.3
3.10.3

References

GitHub Advisory / Wiki ADVISORY
https://github.com/nltk/nltk/security/advisories/GHSA-ff5c-cp5c-9wjf
NLTK Source Repository REPOSITORY
https://github.com/nltk/nltk/pull/3649
Security Fix Commit FIX COMMIT
https://github.com/nltk/nltk/commit/43aaca1b9024138421c97f970bf13ee19ac8129d
NLTK Source Repository REPOSITORY
https://github.com/nltk/nltk
NLTK Release RELEASE
https://github.com/nltk/nltk/releases/tag/v3.10.3

Frequently Asked Questions

What is CVE-2026-12876?
CVE-2026-12876 is a security vulnerability in NLTK involving Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars under CWE-674: Uncontrolled Recursion.
Which NLTK versions are affected?
Affected versions include releases prior to 3.10.3 across affected packages: nltk.
What version fixes CVE-2026-12876?
Upgrading to 3.10.3 (or the latest patched release for your respective branch) resolves the vulnerability.
Is nltk affected?
Yes, nltk is affected in releases prior to 3.10.3. Upgrade your dependency to 3.10.3 or later.
What CWE is associated with CVE-2026-12876?
CVE-2026-12876 is classified under CWE-674: Uncontrolled Recursion (The product does not properly control the amount of recursion that takes place, consuming excessive resources, such as allocated memory or the program stack.).
How can I check PyPI dependencies for CVE-2026-12876?
Run the dependency tree command: 'pip show nltk'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-ff5c-cp5c-9wjf",
  "modified" : "2026-09-02T14:33:39Z",
  "published" : "2026-09-02T14:33:38Z",
  "aliases" : [ "CVE-2026-12876" ],
  "summary" : "NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars",
  "details" : "`nltk.parse.RecursiveDescentParser` (and `SteppingRecursiveDescentParser`) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely — a denial of service.\n\n## Proof of concept\n\nBoth of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:\n\n```python\nfrom nltk import CFG\nfrom nltk.parse import RecursiveDescentParser\n\n# (a) left recursion -> unbounded recursion\ng = CFG.fromstring(\"S -> S S | 'a'\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24))   # hangs\n\n# (b) ambiguous grammar -> exponential number of parses\ng = CFG.fromstring(\"S -> 'a' S | 'a' S S | 'a'\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24))   # hangs\n```\n\n## Impact\n\nAn application that runs `RecursiveDescentParser` on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.\n\n## Sibling\n\nThe RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.10.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.10.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/security/advisories/GHSA-ff5c-cp5c-9wjf"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/pull/3649"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/43aaca1b9024138421c97f970bf13ee19ac8129d"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/releases/tag/v3.10.3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-407", "CWE-674" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T14:33:38Z",
    "nvd_published_at" : null
  }
}