CVE-2026-66393 NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder
Attackers can crash the application by sending deeply nested JSON data to NLTK's decoder, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-66393?
CVE-2026-66393 is a high-severity denial of service vulnerability in nltk, published on 22 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.
Weakness: CWE-674
Which versions are affected by CVE-2026-66393?
3 open source packages ship an affected version, including nltk. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
nltk PyPI | All versions | 3.9.4 |
nltk PyPI | All versions | 3.9.4 |
nltk PyPI | All versions | 3.9.4 |
Products include Nltk Nltk.
How do I fix CVE-2026-66393?
Upgrade nltk to 3.9.4 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-66393 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66393 exists.
Other vulnerabilities in nltk
- CVE-2026-0848 Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading
- CVE-2025-14009 Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution
- CVE-2026-78683 NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization
- CVE-2026-79657 NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization
- CVE-2026-79675 NLTK before 3.10.3 JVM Argument Injection via Per-Call Options
Sources
Also known as DEBIAN-CVE-2026-66393, ECHO-1e3a-1e96-134c, GHSA-rf74-v2fm-23pw, MINI-9v5c-fgp7-c634, PYSEC-2026-3724, UBUNTU-CVE-2026-66393.