Service interruption via unbounded feature-structure recursion
Published Aug 27, 2026 · Updated Aug 27, 2026
Uncontrolled recursion in NLTK before 3.10.3 allows remote attackers to interrupt applications by submitting nested feature structures. FeatStructReader cycles through read_fstruct_value and read_partial without a depth check, so nested brackets raise an uncaught RecursionError. Exposure requires an application to pass externally supplied feature structures or feature grammars into the parser; web frameworks often limit the result to one failed request, while unisolated workers can terminate.
Summary
What happened
Uncontrolled recursion in NLTK before 3.10.3 allows remote attackers to interrupt applications by submitting nested feature structures. FeatStructReader cycles through read_fstruct_value and read_partial without a depth check, so nested brackets raise an uncaught RecursionError. Exposure requires an application to pass externally supplied feature structures or feature grammars into the parser; web frameworks often limit the result to one failed request, while unisolated workers can terminate.
The record
- CVE
- CVE-2026-81724
- Published
- Aug 27, 2026
- Updated
- Aug 27, 2026
- Vendor
- NLTK Project
- Product
- NLTK
- Classifications
- CWE-674, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 27, 2026CVE publishedPublication date reported by the CVE source.
- Aug 27, 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.
- Affected versionversion=0 <3.10.3
What conditions does exploitation require?
What is affected?
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.
Weakness, pattern, technique
Public exploit references
- GHSA-cw6x-m8jw-qmrh minimal reproductionproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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