CVE-2026-80206Public exploitNetwork attack vector

Process hang via unbounded tgrep regex backtracking

Published Aug 26, 2026 · Updated Aug 26, 2026

Regular expression denial of service in NLTK before 3.10.3 allows remote attackers to block a Python process via a crafted tgrep pattern. The _tgrep_node_action function compiles a caller-controlled /regex/ node and runs re.search against tree labels without validation or a timeout, enabling catastrophic backtracking. Exposure requires an application to pass external input to tgrep_compile() or tgrep_positions(); a single request can saturate CPU and hang that process.

CVSS severity8.2
High
EPSS probability0.26%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in NLTK before 3.10.3 allows remote attackers to block a Python process via a crafted tgrep pattern. The _tgrep_node_action function compiles a caller-controlled /regex/ node and runs re.search against tree labels without validation or a timeout, enabling catastrophic backtracking. Exposure requires an application to pass external input to tgrep_compile() or tgrep_positions(); a single request can saturate CPU and hang that process.

The record

CVE
CVE-2026-80206
Published
Aug 26, 2026
Updated
Aug 26, 2026
Vendor
NLTK Project
Product
NLTK
Classifications
CWE-1333, CAPEC-492, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 26, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 26, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <3.10.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

NLTK Project · NLTKversion=0 <3.10.3

Published CVSS scores

8.2GitHubCVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
5.9Red HatCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
CAPEC-492Regular Expression Exponential Blowup
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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