Vulnerability · Published Aug 22, 2026 · Assigned by VulnCheck

CVE-2026-63311NLTK before 3.10.0 SSRF via DNS Resolution Failure

Medium 5.3

In plain English

A medium-severity server-side request forgery in nltk from nltk. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 27% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.3Medium

Exploit odds (30 days)0.3%top 73.1% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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).

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	nltk	no fix listed	DEBIAN-CVE-2026-63311
Debian:13	nltk	no fix listed	DEBIAN-CVE-2026-63311
Debian:14	nltk	3.10.0-1	DEBIAN-CVE-2026-63311
PyPI	nltk	3.10.0	GHSA-3gqm-fcw5-w839
PyPI	nltk	3.10.0	PYSEC-2026-3723
Ubuntu:26.10	nltk	no fix listed	UBUNTU-CVE-2026-63311

Affected products (as reported)

Vendor	Product	Versions
nltk	nltk	0 – before 3.10.0

Weakness: CWE-918 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

References

GitHub Security Advisory (GHSA-3gqm-fcw5-w839) vendor-advisory
VulnCheck Advisory: NLTK before 3.10.0 SSRF via DNS Resolution Failure third-party-advisory