Guard UnifiedPush endpoints against SSRF

[juanfradb]

Summary

• validate URL-shaped Android/UnifiedPush tokens as public HTTPS endpoints before storing them
• revalidate UnifiedPush endpoints before dispatch and add a DNS resolver that filters connection-time private/internal DNS results
• add optional UNIFIEDPUSH_ALLOWED_HOSTS_REGEX host allowlist and document the policy

Fixes #4.
Also covers the registration-side validation requested in #13.

Validation

• cargo fmt
• cargo test unifiedpush -- --nocapture
• cargo test rejects_invalid_unifiedpush_allowed_hosts_regex -- --nocapture
• cargo test register_rejects -- --nocapture
• cargo test register_accepts_public_https_unifiedpush_url -- --nocapture
• cargo test
• cargo clippy --all-targets -- -D warnings
• git diff --check

Note: cargo test --doc reports no library targets for this binary crate.

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📒 Files selected for processing (12)

• .env.example
• Cargo.toml
• config.toml.example
• docs/api.md
• docs/configuration.md
• docs/unifiedpush.md
• src/api/routes.rs
• src/api/test_support.rs
• src/config.rs
• src/main.rs
• src/push/mod.rs
• src/push/unifiedpush.rs

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[chatgpt-codex-connector]

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Reviewed commit: 9b9a7574a8

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[chatgpt-codex-connector]

Revalidate or disable UnifiedPush redirects

When a registered public HTTPS endpoint returns a redirect, this request uses reqwest's default redirect policy, which follows redirects automatically, but only the original endpoint_url was validated. In that scenario a valid distributor can redirect the server to an unvalidated scheme or host (for example outside UNIFIEDPUSH_ALLOWED_HOSTS_REGEX, or a private IP literal that does not require DNS resolution), bypassing the SSRF/TLS policy this change is adding. Disable redirects for UnifiedPush or install a redirect policy that validates every Location before following it.

Useful? React with 👍 / 👎.

[AndreaDiazCorreia]

Thanks for tackling this. A few things need to change before merge:

Blocking

1. Redirects bypass the SSRF guard. The shared reqwest::Client uses the default redirect policy, so a distributor can respond 302 Location: https://10.0.0.1/... (literal IP skips PublicDnsResolver) or redirect to a public host outside
2. UNIFIEDPUSH_ALLOWED_HOSTS_REGEX. Same concern Codex raised. Please disable redirects on the UnifiedPush dispatch path or install a policy that re-runs validate_endpoint_url on every Location. Note http_client is shared with FCM — a separate client for UnifiedPush is probably cleaner.
3. New direct deps. regex and hyper are added to Cargo.toml. Per CLAUDE.md constraint 6, that needs explicit maintainer sign-off. hyper is unavoidable for reqwest::dns::Resolve; regex could be swapped for suffix matching if we want to avoid it.
4. /api/register fixture regression. URL-shaped Android tokens that previously got 200 OK now return 400 Invalid UnifiedPush endpoint URL, breaking the byte-identity invariant in CLAUDE.md constraint 3. Either document a new exception in CLAUDE.md (like the whitelist 403) or move the rejection to dispatch-only.

The IPv4 classifier, double-validation pattern, fail-fast on bad regex, and test coverage on the acceptance criteria all look great. Once these three are addressed I think this is good to merge.