Medium [CVE-2026-63311] Server-Side Request Forgery bypass via DNS resolution failures
This medium-severity Red Hat Linux advisory covers CVE-2026-63311 affecting Exploit Intelligence, Lightspeed Core, OpenShift Lightspeed.
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Summary
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). A flaw was found in NLTK.
This server-side request forgery (SSRF) vulnerability allows an attacker to bypass network access restrictions. By triggering DNS resolution failures or using DNS rebinding, an attacker can cause the `validate_network_url()` function to incorrectly validate URLs.
This flaw in NLTK allows a server-side request forgery (SSRF) bypass when DNS resolution fails or through DNS rebinding. This could enable an attacker to access restricted internal network resources, including cloud metadata endpoints, within Red Hat environments utilizing affected NLTK versions.
Red Hat severity: Moderate — CVSS 4.2 (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N). Weakness: CWE-918.
- < 3.10.0
- < 3.9.4
Official advisory · high-confidence parse· fetched 13 days ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 13 days ago·verify at source
Mitigation checklist
- Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Official advisory · high-confidence parse· fetched 13 days ago·verify at source
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