CVE-2026-80206 NLTK 3.10.2 Regular Expression Denial of Service via tgrep
Attackers can cause CPU exhaustion and service disruption in NLTK by supplying malicious regex patterns to the tgrep module.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-80206?
CVE-2026-80206 is a high-severity denial of service vulnerability in nltk, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.
Weakness: CWE-1333
Which versions are affected by CVE-2026-80206?
3 open source packages ship an affected version, including nltk. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
nltk PyPI | All versions | 3.10.3 |
nltk PyPI | All versions | 3.10.3 |
nltk PyPI | All versions | 3.10.3 |
Products include Nltk Nltk.
How do I fix CVE-2026-80206?
Upgrade nltk to 3.10.3 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-2026-80206 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-80206 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-2026-80206, ECHO-45e2-48ef-7336, GHSA-w3v8-gmh9-3wv7, MINI-mvr5-3rph-8wf8, PYSEC-2026-3751, UBUNTU-CVE-2026-80206.