PyPI · nltk
NLTK: pathsec SSRF protection can be bypassed when a proxy is configured
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.
nltk.pathsec.urlopen, nltk.data.load, nltk.downloader.Downloader.index, nltk.downloader.Downloader.downloadv3.10.0-rc2; published 3.9.4 was a negative control and did not reproduce._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().
Preconditions
pathsec to keep network fetches SSRF-safe.Steps
pathsec.urlopen() or nltk.data.load() on the public URL and observe the internal response is returned.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'}
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.
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.
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.
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:
| Scenario | Result |
|---|---|
| proxied (env) + ENFORCE | PermissionError — blocked |
| proxied + opt-in | returns secret — escape hatch works |
explicit ProxyHandler (not env) + ENFORCE |
PermissionError — blocked (whole class) |
| no proxy (direct) | internal IP still refused — pinning intact |
proxied + ENFORCE=False |
returns secret + warns |
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.
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.
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Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.