CVE-2026-63311
NLTK before 3.10.0 SSRF via DNS Resolution Failure
Description
NLTK before 3.10.0 (affected versions <= 3.9.4) contains a server-side request forgery (SSRF) vulnerability in the validate_network_url() function in nltk/pathsec.py. The _resolve_hostname() helper catches OSError and ValueError during socket.getaddrinfo() and returns an empty list; when DNS resolution fails, the validation loop executes no IP checks and the function fails open, allowing urlopen() to proceed without validation. An attacker who can trigger DNS resolution failures or use DNS rebinding can bypass SSRF protections and reach restricted network resources, including cloud metadata endpoints (e.g., 169.254.169.254).
INFO
Published Date :
Aug. 22, 2026, 3:16 p.m.
Last Modified :
Oct. 14, 2026, 1:10 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 3.1 | MEDIUM | MITRE-CVE | ||||
| CVSS 4.0 | MEDIUM | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 4.0 | MEDIUM | [email protected] |
Solution
- Update NLTK to version 3.10.0 or later.
- Review network access controls.
- Monitor for suspicious network activity.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-63311.
| URL | Resource |
|---|---|
| https://github.com/nltk/nltk/security/advisories/GHSA-3gqm-fcw5-w839 | Exploit Mitigation Vendor Advisory |
| https://www.vulncheck.com/advisories/nltk-before-ssrf-via-dns-resolution-failure | Third Party Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-63311 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-63311
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-63311 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-63311 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Oct. 14, 2026
Action Type Old Value New Value Added Translation Title: nltk, Description: NLTK anterior a 3.10.0 (versiones afectadas menor o igual que 3.9.4) contiene una vulnerabilidad de falsificación de petición del lado del servidor (SSRF) en la función validate_network_url() en nltk/pathsec.py. La función auxiliar _resolve_hostname() captura OSError y ValueError durante socket.getaddrinfo() y devuelve una lista vacía; cuando la resolución DNS falla, el bucle de validación no ejecuta comprobaciones de IP y la función falla abierta, permitiendo que urlopen() proceda sin validación. Un atacante que puede desencadenar fallos de resolución DNS o usar el reenlace DNS puede eludir las protecciones SSRF y alcanzar recursos de red restringidos, incluyendo puntos finales de metadatos en la nube (p. ej., 169.254.169.254). -
Initial Analysis by [email protected]
Aug. 27, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:nltk:nltk:*:*:*:*:*:*:*:* versions up to (excluding) 3.10.0 Added Reference Type VulnCheck: https://github.com/nltk/nltk/security/advisories/GHSA-3gqm-fcw5-w839 Types: Exploit, Mitigation, Vendor Advisory Added Reference Type VulnCheck: https://www.vulncheck.com/advisories/nltk-before-ssrf-via-dns-resolution-failure Types: Third Party Advisory -
CVE Modified by [email protected]
Aug. 26, 2026
Action Type Old Value New Value Added Reference https://github.com/nltk/nltk/security/advisories/GHSA-3gqm-fcw5-w839 Added Reference https://www.vulncheck.com/advisories/nltk-before-ssrf-via-dns-resolution-failure Removed Reference https://github.com/nltk/nltk/security/advisories/GHSA-3gqm-fcw5-w839 Removed Reference https://www.vulncheck.com/advisories/nltk-before-ssrf-via-dns-resolution-failure -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Aug. 26, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-63311', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-08-26T15:58:19.898915Z'} -
New CVE Received by [email protected]
Aug. 22, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'nltk', 'product': 'nltk', 'versions': [{'status': 'affected', 'version': '0', 'lessThan': '3.10.0', 'versionType': 'semver'}, {'status': 'unaffected', 'version': '3.10.0', 'versionType': 'semver'}], 'packageURL': 'pkg:pypi/nltk', 'defaultStatus': 'unaffected'}] Added Description NLTK before 3.10.0 (affected versions <= 3.9.4) contains a server-side request forgery (SSRF) vulnerability in the validate_network_url() function in nltk/pathsec.py. The _resolve_hostname() helper catches OSError and ValueError during socket.getaddrinfo() and returns an empty list; when DNS resolution fails, the validation loop executes no IP checks and the function fails open, allowing urlopen() to proceed without validation. An attacker who can trigger DNS resolution failures or use DNS rebinding can bypass SSRF protections and reach restricted network resources, including cloud metadata endpoints (e.g., 169.254.169.254). Added CVSS V4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:L/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 Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N Added CWE CWE-918 Added Reference https://github.com/nltk/nltk/security/advisories/GHSA-3gqm-fcw5-w839 Added Reference https://www.vulncheck.com/advisories/nltk-before-ssrf-via-dns-resolution-failure