Parser stall via quadratic TEI regex rescans
Published Aug 27, 2026 · Updated Aug 27, 2026
Regular expression denial of service in NLTK before 3.10.3 allows remote attackers to stall corpus parsing with malformed TEI blocks. In TEICorpusView.read_block, lazy whole-block expressions repeatedly rescan attacker-controlled text from each unmatched opening tag, producing near-quadratic CPU growth. Exploitation requires an application to pass attacker-influenced PL196X or TEI-like files to words() or tagged_words(), resulting in heavy CPU use and parser-thread stalling.
Summary
What happened
Regular expression denial of service in NLTK before 3.10.3 allows remote attackers to stall corpus parsing with malformed TEI blocks. In TEICorpusView.read_block, lazy whole-block expressions repeatedly rescan attacker-controlled text from each unmatched opening tag, producing near-quadratic CPU growth. Exploitation requires an application to pass attacker-influenced PL196X or TEI-like files to words() or tagged_words(), resulting in heavy CPU use and parser-thread stalling.
The record
- CVE
- CVE-2026-81725
- Published
- Aug 27, 2026
- Updated
- Aug 27, 2026
- Vendor
- NLTK Project
- Product
- NLTK
- Classifications
- CWE-400, CWE-1333, T1499
- 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
Compare your product and version with the public record. A matching version still requires validation against your environment.
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?
Published CVSS scores
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.
Weakness, pattern, technique
Public exploit references
- GHSA malformed TEI block reproduction procedureproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo