NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of… (CVE-2026-66393)
NLTK versions before 3.9.4 have an unbounded recursion vulnerability in the JSONTaggedDecoder.decode_obj() function. This flaw allows attackers to cause a denial of service by providing deeply nested JSON inputs that exceed the recursion limit, triggering a RecursionError and crashing the Python process. The vulnerability is identified as CVE-2026-66393 and has a high severity rating.
AI Analysis
Technical Summary
The vulnerability in NLTK prior to version 3.9.4 is due to unbounded recursion in the JSONTaggedDecoder.decode_obj() method. An attacker can supply maliciously crafted deeply nested JSON structures that cause the function to recurse beyond the Python interpreter's recursion limit. This results in an unhandled RecursionError, leading to a denial of service by crashing the process running the vulnerable code. The CVSS v3.1 base score is 7.5, reflecting network attack vector, low attack complexity, no privileges or user interaction required, and impact limited to availability.
Potential Impact
Successful exploitation causes denial of service by crashing the Python process using the vulnerable NLTK version. There is no impact on confidentiality or integrity. The service or application relying on NLTK may become unavailable until restarted.
Mitigation Recommendations
Upgrade NLTK to version 3.9.4 or later where this vulnerability is fixed. Since no official patch link is provided, users should verify the upgrade from the official NLTK release notes or repository. Avoid processing untrusted deeply nested JSON inputs with affected versions until patched.
NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of… (CVE-2026-66393)
Description
NLTK versions before 3.9.4 have an unbounded recursion vulnerability in the JSONTaggedDecoder.decode_obj() function. This flaw allows attackers to cause a denial of service by providing deeply nested JSON inputs that exceed the recursion limit, triggering a RecursionError and crashing the Python process. The vulnerability is identified as CVE-2026-66393 and has a high severity rating.
CVSS v3.1
Score 7.5high
Affected software
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Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability in NLTK prior to version 3.9.4 is due to unbounded recursion in the JSONTaggedDecoder.decode_obj() method. An attacker can supply maliciously crafted deeply nested JSON structures that cause the function to recurse beyond the Python interpreter's recursion limit. This results in an unhandled RecursionError, leading to a denial of service by crashing the process running the vulnerable code. The CVSS v3.1 base score is 7.5, reflecting network attack vector, low attack complexity, no privileges or user interaction required, and impact limited to availability.
Potential Impact
Successful exploitation causes denial of service by crashing the Python process using the vulnerable NLTK version. There is no impact on confidentiality or integrity. The service or application relying on NLTK may become unavailable until restarted.
Mitigation Recommendations
Upgrade NLTK to version 3.9.4 or later where this vulnerability is fixed. Since no official patch link is provided, users should verify the upgrade from the official NLTK release notes or repository. Avoid processing untrusted deeply nested JSON inputs with affected versions until patched.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- GHSA-cv2g-m8rr-888c
- Osv Schema Version
- 1.4.0
- Aliases
- ["CVE-2026-66393"]
- Ecosystems
- []
- Database Specific Severity
- HIGH
- Cvss Version
- 3.1
Threat ID: 6a8a27f7acd9273b499bc998
Added to database: 08/22/2026, 22:51:35 UTC
Last enriched: 08/22/2026, 22:52:22 UTC
Last updated: 08/22/2026, 23:21:35 UTC
Views: 3
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