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

Summary

Current NLTK source reopens SSRF in proxied environments. pathsec.urlopen() validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path.

Details

• Vulnerability type: Server-side request forgery
• Affected component: nltk.pathsec.urlopen, nltk.data.load, nltk.downloader.Downloader.index, nltk.downloader.Downloader.download
• Affected versions: Current source v3.10.0-rc2; published 3.9.4 was a negative control and did not reproduce.
• Patched versions: Not yet patched
• Root cause: Proxy-handler inheritance disables _SafeHTTPHandler and _SafeHTTPSHandler, so the validated hostname no longer matches the actual egress destination.

The hardened direct path pins the validated numeric destination IP before opening the socket. The proxied branch instead copies ProxyHandler instances from the global opener, marks the request as proxied, and skips the pinned handlers. I confirmed that a validated public URL can be fetched from a loopback-only internal service through the proxy path via pathsec.urlopen(), nltk.data.load(), Downloader.index(), and Downloader.download().

PoC

Preconditions

• The runtime has an HTTP proxy configured and the caller relies on pathsec to keep network fetches SSRF-safe.

Steps

1. Start a loopback-only HTTP server that serves secret text, a valid downloader index, and a ZIP payload.
2. Configure a proxy that forwards a validated public URL to that internal loopback service.
3. Call pathsec.urlopen() or nltk.data.load() on the public URL and observe the internal response is returned.
4. Instantiate Downloader(server_index_url=...), call index() and download(), and observe internal-only content is parsed and installed.

Minimal reproducible excerpt

{'urlopen': 'PROXY_TEXT_SECRET', 'data_load': 'PROXY_TEXT_SECRET', 'downloaded_file': 'INTERNAL_ZIP_SECRET'}

Impact

Consumers that trust pathsec as an SSRF barrier in proxied environments can be made to read internal-only HTTP resources, load forged downloader indexes, and install attacker-chosen package content fetched from the proxy's network view.

Remediation

Preserve destination validation for the actual proxy egress target or fail closed when the request would otherwise downgrade into an unpinned proxied path. Add regression tests across pathsec.urlopen, nltk.data.load, and downloader fetches with a configured proxy.

References

• https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L468-L518
• https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/data.py#L1247-L1283
• https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L875-L889
• https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/downloader.py#L1220-L1226
• https://github.com/nltk/nltk/blob/3.9.4/nltk/pathsec.py#L245-L250

Fix + attack demonstration (verified)

NLTK cannot pin the egress through a proxy, so it stops pretending to: under ENFORCE a proxied fetch is refused rather than performed unvalidated. Operators who trust their proxy opt back in with NLTK_ALLOW_PROXIED_URLOPEN=1 or nltk.pathsec.ALLOW_PROXIED_FETCH=True; under ENFORCE=False the refusal degrades to a warning. This closes the whole class (environment proxies and explicit ProxyHandler alike), because NLTK declines any fetch whose egress it cannot validate.

Attack demonstration (reproduced; captured output)

A loopback HTTP server stands in for the internal target; http_proxy points at it; NLTK is asked for a public IP URL.

Before the fix — the internal secret is exfiltrated through the proxy:

validate_network_url(public): PASSED
*** BYPASS: pathsec.urlopen returned INTERNAL content via proxy: 'INTERNAL_ONLY_SECRET'

After the fix — five scenarios, isolated subprocesses:

Tests

nltk/test/unit/test_pathsec.py: 64 passed. Added an end-to-end regression (test_proxied_fetch_does_not_reach_internal_target) plus test_env_proxy_fails_closed_under_enforce; the prior test_env_proxy_skips_pinning_handlers (which encoded the vulnerable path) is re-expressed as the opt-in case. Existing direct-path DNS-rebinding and IP-policy tests unchanged and passing. pre-commit (isort/black/ruff) clean.

Note

The upfront validate_network_url() and the direct-path IP pinning (from the earlier DNS-rebinding fixes, CVE-2026-54296 / GHSA-qvv7) are unchanged — this only closes the proxied downgrade they didn't cover.