EUVD-2026-64310
GHSA-cv2g-m8rr-888c
Severity by source
CVSS:4.0/AV:N/AC:L/AT:N/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-reachable, no auth or complexity required; impact is solely process crash (A:H), with no confidentiality or integrity effect and no scope change.
Primary rating from Vendor (VulnCheck).
CVSS VectorVendor: VulnCheck
CVSS:4.0/AV:N/AC:L/AT:N/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
4Blast Radius
ecosystem impact- 7 pypi packages depend on nltk (7 direct, 0 indirect)
Ecosystem-wide dependent count for version 3.9.3.
DescriptionCVE.org
NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.
AnalysisAI
Unbounded recursion in NLTK's JSONTaggedDecoder.decode_obj() (nltk/jsontags.py) allows unauthenticated remote attackers to crash the Python process by submitting a deeply nested JSON payload exceeding Python's default recursion limit of 1000. All NLTK releases before 3.9.4 are affected; exploitation depends on whether the calling application exposes JSONTaggedDecoder to externally-supplied JSON input. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires that the target application passes externally-supplied or attacker-controlled JSON data to nltk.jsontags.JSONTaggedDecoder - either directly or through a higher-level NLTK API such as those in nltk/data.py. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 4.0 score of 8.7 (AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H) accurately reflects the accessibility of this flaw - no authentication, no complexity, network-reachable - but the real-world exploitability is conditional on the application directly passing attacker-controlled JSON to JSONTaggedDecoder. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker targeting a web service that uses NLTK to deserialize user-supplied JSON (such as a data annotation tool or NLP API) crafts a payload consisting of 1050 levels of nested JSON objects - trivially generated with a one-liner as shown in the published POC. Submitting this payload to the endpoint causes JSONTaggedDecoder.decode_obj() to recurse past Python's default limit of 1000 frames, raising an unhandled RecursionError that terminates the entire Python worker process, resulting in service downtime. … |
| Remediation | The primary fix is to upgrade NLTK to version 3.9.4 or later, which is implied as the patched release by the affected-version range 'nltk 0 <3.9.4' and the vendor advisory at https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw; however, the package metadata field 'fixed in' is listed as None, meaning a tagged PyPI release at 3.9.4 is not independently confirmed at time of writing - verify availability via pip install --upgrade nltk and check PyPI before deploying. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours, audit all systems and applications to identify those running NLTK versions before 3.9.4 and determine which expose JSONTaggedDecoder to untrusted or external JSON sources. …
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Same weakness CWE-674 – Uncontrolled Recursion
View allSame technique Denial Of Service
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External POC / Exploit Code
Leaving vuln.today