NLTK: SSRF Fail-Open in validate_network_url() via DNS Resolution Failure
There is an SSRF vulnerability in NLTK 3.9.4's network URL validation. The validate_network_url() function in nltk/pathsec.py fails open when DNS resolution returns an error.
The _resolve_hostname() helper at lines 193-234 catches OSError and ValueError during socket.getaddrinfo() and returns an empty list []. When this happens, the validation loop in validate_network_url() iterates over nothing (for addr in resolved: ... never executes), with no else/fallback check. The function returns normally, and urlopen() proceeds to make the request without any IP validation.
This means:
- If DNS is temporarily unavailable, ALL SSRF protections are disabled
- DNS rebinding attacks bypass the check after the LRU cache entry expires
- In environments with unreliable resolvers, the protection is permanently bypassed
PoC:
import nltk.pathsec
import unittest.mock
# Simulate DNS failure
with unittest.mock.patch('socket.getaddrinfo', side_effect=OSError('DNS unavailable')):
# This SHOULD raise but doesn't -- fails open
nltk.pathsec.validate_network_url('http://169.254.169.254/latest/meta-data/')
# Returns normally, allowing SSRF to cloud metadataThe correct behavior is fail-closed: if DNS resolution fails, the URL should be REJECTED (not allowed). The function should raise an exception or return a failure status when _resolve_hostname() returns an empty list.
This is distinct from CVE-2024-39705 (which addressed pickle deserialization) and CVE-2026-33236 (which addressed XML path traversal). This finding targets the newly-added pathsec.py security layer introduced to fix those earlier issues.
Suggested fix: Add an explicit check after _resolve_hostname() returns: if the result is empty, raise a SecurityError. Never allow a URL request to proceed when IP validation was impossible.
CVSS Note: The CVSS use SC:H (High subsequent confidentiality) because the advisory explicitly identifies cloud metadata endpoints (169.254.169.254) as an attack target. Access to AWS IMDS or GCP metadata exposes credentials or service account tokens, which constitutes High-impact disclosure on downstream systems. This justifies SC:H over NVD's SC:L.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | nltk | < 3.10.0 | 3.10.0 |
References
- https://www.vulncheck.com/advisories/nltk-before-ssrf-via-dns-resolution-failure
- https://github.com/nltk/nltk/security/advisories/GHSA-3gqm-fcw5-w839
- https://nvd.nist.gov/vuln/detail/CVE-2026-63311
- https://github.com/nltk/nltk/pull/3582
- https://github.com/nltk/nltk/commit/4a820afa58810cd05049b6c6eae306694d6cfe65
- https://github.com/nltk/nltk
- https://github.com/nltk/nltk/releases/tag/v3.10.0
- https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3723.yaml
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:L- Weakness (CWE)
CWE-918- Platform
- Python (PyPI)
- Package
nltk- Disclosed
- 22.08.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-3723 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-3gqm-fcw5-w839 - CC BY 4.0
Vulnerability history: nltk
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 52
- First → latest
- 08.2019 09.2026
- Without a fix
- 6
- Exploited in attacks
- 0
Severity breakdown
- Critical 5
- High 32
- Medium 13
- Low 2
Disclosures per year
- 2019: 1
- 2020: 0
- 2021: 2
- 2022: 1
- 2023: 0
- 2024: 1
- 2025: 0
- 2026: 47
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NLTK: SSRF Fail-Open in validate_network_url() via DNS Resolution Failure this entry
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