Gptline: NLTK: pathsec SSRF protection can be bypassed when a proxy is configured (CVE-2026-78682)

The vulnerability (CVE-2026-78682) in NLTK's pathsec.urlopen and related functions allows SSRF bypass when an HTTP proxy is configured. The root cause is that proxy-handler inheritance disables the _SafeHTTPHandler and _SafeHTTPSHandler, so the validated hostname no longer matches the actual egress destination. This enables an attacker to fetch internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content by exploiting the proxy's network view. The vulnerability affects NLTK version 3.10.0-rc2 and is not present in 3.9.4. The remediation involves refusing proxied fetches whose egress cannot be validated unless explicitly allowed by environment variables or flags. This fix closes the entire class of proxied SSRF bypasses.

Consumers relying on pathsec as an SSRF protection mechanism in proxied environments can be tricked into accessing internal-only HTTP services and loading malicious content. This can lead to unauthorized disclosure of internal secrets and installation of attacker-controlled packages. The vulnerability undermines the trust boundary of network fetches in NLTK when proxies are used, potentially exposing sensitive internal resources to untrusted code.

A patch is available that enforces refusal of proxied fetches when the egress destination cannot be validated, effectively blocking this SSRF bypass. Operators who trust their proxy can opt back in with environment variables (NLTK_ALLOW_PROXIED_URLOPEN=1) or flags, but the default behavior under ENFORCE mode is to block. Users should upgrade to a patched version (>=3.10.3) or apply the fix that disables unvalidated proxied fetches. Regression tests have been added to ensure this protection. Until patched, avoid relying on pathsec for SSRF protection in proxied environments or disable proxy usage.