NLTK: SSRF Fail-Open in validate_network_url() via DNS Resolution Failure

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.9 · 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 · score source

EPSS

• CVE-2026-63311: 0.33% estimated exploitation activity in the next 30 days; percentile 26.87 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

There is an SSRF vulnerability in NLTK 3.9.4's network URL validation. The validatenetworkurl() function in nltk/pathsec.py fails open when DNS resolution returns an error. The resolvehostname() helper at lines 193-234 catches OSError and ValueError during socket.getaddrinfo() and returns an empty list []. When this happens, the validation loop in validatenetworkurl() 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: 1. If DNS is temporarily unavailable, ALL SSRF protections are disabled 2. DNS rebinding attacks bypass the check after the LRU cache entry expires 3. In environments with unreliable resolvers, the protection is permanently bypassed PoC: The 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…

Published by the source

2 Sep 2026, 15:39 UTC

Last modified by the source

27 Oct 2026, 07:58 UTC

Source last checked

27 Oct 2026, 08:10 UTC

1. Revision 2 · amended · 27 Oct 2026

   Amended from ghsa record GHSA-3gqm-fcw5-w839

2. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-3gqm-fcw5-w839

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# Patch Next advisory brief: CVE-2026-63311

Advisory: NLTK: SSRF Fail-Open in validate_network_url() via DNS Resolution Failure
IDs: CVE-2026-63311, GHSA-3gqm-fcw5-w839
Record: https://patchnext.com/advisories/cve-2026-63311 (revision 2, source checked 27 Oct 2026, 08:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.9.4; fixed in 3.10.0

## Suggested inspection (read-only)
- `python -m pip show nltk`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nltk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-3gqm-fcw5-w839 (GitHub-reviewed advisory GHSA-3gqm-fcw5-w839 lists affected ranges: PyPI nltk >=0 <=3.9.4)

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