Bug 2521348 (CVE-2026-63311)

Summary:	CVE-2026-63311 nltk: NLTK: Server-Side Request Forgery bypass via DNS resolution failures
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	anpicker, bparees, dschmidt, ebourniv, hasun, jfula, jlanda, jowilson, kshier, nyancey, ometelka, ptisnovs, sbunciak, simaishi, stcannon, syedriko, teagle, xdharmai, yguenane
Target Milestone:	---	Keywords:	Security
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Hardware:	All
OS:	Linux
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Doc Text:	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 enables access to restricted internal network resources, such as cloud metadata endpoints.	Story Points:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
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Description OSIDB Bzimport 2026-08-22 14:32:35 UTC

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).