How severe is CVE-2026-12876?

No CVSS score has been assigned to CVE-2026-12876 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-12876?

CVE-2026-12876 affects the following packages: nltk (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-12876?

Update nltk to 3.10.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-12876 is resolved across your whole dependency graph.

How do I detect CVE-2026-12876 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nltk. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2026-12876 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2026-12876?

O3 pinpoints whether CVE-2026-12876 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-12876 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-12876 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was CVE-2026-12876 published, and has it been updated?

CVE-2026-12876 was published on September 2, 2026 and was last updated on September 2, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

Not in CISA KEV

CVE-2026-12876

Fix: nltk/nltk#3649

CVE-2026-12876 is a security vulnerability in nltk. O3 Security confirms whether CVE-2026-12876 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars

Published

Sep 2, 2026

Updated

Sep 2, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Sep 2, 2026 · OSV.dev, FIRST.org (EPSS)

Real-World Exposure

1 pkg affected

🐍nltk

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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:

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`nltk`	all versions	3.10.3

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nltk. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update nltk to 3.10.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-12876 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-12876 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-12876. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

`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 =

O3 Security · Impact-Aware SCA

Is CVE-2026-12876 in your dependencies?

O3 detects CVE-2026-12876 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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