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bdu:2026-12703

BREW-acronym-CVE-2026-78682 — NLTK: pathsec SSRF protection can be bypassed when a proxy is configured

Affected: Homebrew: acronym

### 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…

fkie_cve-2026-78682

NLTK before 3.10.3 contains a server-side request forgery vulnerability in nltk.pathsec.urlopen (and callers nltk.data.load, nltk.downloader.Downloader.index/download) when an HTTP proxy is configured. pathsec.urlopen validates the requested hostname locally, but proxy-handler inheritance disables the safe HTTP/HTTPS handlers so the actual fetch is performed by the proxy against a destination that is never re-validated. An attacker can supply a validated public URL that the proxy forwards to an internal loopback-only service, allowing disclosure of internal HTTP resources, loading of forged downloader indexes, and installation of attacker-chosen package content.

GHSA-6ww7-3frv-cqxh — NLTK: pathsec SSRF protection can be bypassed when a proxy is configured