EUVD-2026-66397Severity by source
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Network vector applies when tgrep API is exposed; AC:H captures the non-default exposed-input prerequisite; PR:N because no authentication is implied; A:H for indefinite CPU saturation; no C or I impact.
Primary rating from Vendor (VulnCheck).
CVSS VectorVendor: VulnCheck
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Lifecycle Timeline
3DescriptionCVE.org
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.
AnalysisAI
ReDoS in NLTK's tgrep module exposes applications to indefinite CPU saturation when user-controlled tgrep patterns containing /regex/ nodes reach the _tgrep_node_action function, which compiles and executes them via Python's re.search without validation or timeout. All NLTK versions before 3.10.3 are affected when tgrep_positions() or tgrep_compile() accept external input. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires the application to pass attacker-controlled strings as the tgrep pattern argument to tgrep_positions() or tgrep_compile() - specifically, the attacker must control the pattern itself (not merely the tree being searched). … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 4.0 score of 8.2 reflects High availability impact (VA:H) with zero confidentiality or integrity exposure. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker identifies a web API endpoint that accepts tgrep search patterns and passes them without sanitization to NLTK's tgrep_positions() function. The attacker submits a crafted tgrep pattern containing a /regex/ node with a polynomial backtracking expression (e.g., a nested quantifier pattern like /(a+)+$/) as the node label matcher. … |
| Remediation | Upgrade NLTK to version 3.10.3 or later, which addresses the unsafe regex handling in the tgrep module. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours, identify all systems running NLTK and determine if tgrep_positions() or tgrep_compile() accept external input; document exposure scope. …
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External POC / Exploit Code
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