NLTK: pathsec SSRF protection can be bypassed when a proxy is configured

Published September 8, 2026 CVE-2026-78682

NLTK apps using an HTTP proxy can expose private internal web content. Attackers can also make them install attacker-chosen package content.

Severity
HighCVSS 3.1 · 7.5
Fix
Fixed in 3.10.3
Affected versions
before 3.10.3
Weakness
CWE-918Server-Side Request Forgery (SSRF)
Exploit likelihood
0.25% in 30 daysEPSS, higher than 17% of known flaws
Affects
NLTK

How it works

  • The affected application must use NLTK through an HTTP proxy.
  • An attacker supplies a public web address that passes NLTK's initial safety check.
  • The proxy then connects to a different destination without another check.
  • This can expose internal web resources or deliver attacker-chosen package content.

What to do

Run python3 -m pip show nltk in every Python environment used by the application. A version of 3.10.2 or earlier requires action. Confirm whether the application sends NLTK downloads through an HTTP proxy. Finding an affected version does not prove anyone exploited it.

Run this in the application environment you want to check:

python3 -m pip show nltk

Update each affected environment to NLTK 3.10.3 or later using its normal dependency process. Run the version check again and confirm it reports 3.10.3 or later. The fixed release refuses proxied downloads by default unless an operator explicitly trusts the proxy.

Technical details

CVE-2026-78682 affects NLTK versions through 3.10.2 when an HTTP proxy is configured. NLTK approves the requested address locally, but the proxy makes the real connection without validating its destination. Version 3.10.3 blocks these proxied requests by default. A published proof-of-concept demonstrated access to internal content, but the reviewed evidence does not establish malicious exploitation.

Severity breakdown

  • Attack vector Network Requires network access to the vulnerable service
  • Attack complexity Low Low complexity under the assessed conditions
  • Privileges required None Attacker needs no account or login
  • User interaction None No victim action needed
  • Scope Unchanged Impact stays within the same security authority
  • Confidentiality impact High Sensitive data can be exposed with serious impact
  • Integrity impact None No data tampering
  • Availability impact None No availability impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N Open in FIRST.org calculator

References

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