Matrix outbound image downloads validated only the final URL after
following redirects, so a public URL that 302-redirects to loopback /
private-network / cloud-metadata endpoints had already connected to the
unsafe hop before the check ran.
Re-validate every redirect hop before following it:
- aiohttp path resolves redirects manually with allow_redirects=False,
validating each Location via is_safe_url (aiohttp can't use the httpx
response event hook).
- httpx fallback installs the shared _ssrf_redirect_guard event hook.
Regression tests cover per-hop blocking of an unsafe redirect, following
a safe redirect chain, and httpx guard wiring.
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