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fix(readurls): block CGNAT/shared-address and multicast ranges in SSRF guard - #336
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codelion merged 1 commit intoOct 10, 2026
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- Switch the guard from a private-range deny-list to a global-routability allow-list (not is_global), which additionally blocks CGNAT/shared space (100.64.0.0/10, e.g. Tailscale) that is neither private nor global - Keep an explicit multicast check (ipaddress marks globally-routable multicast ranges as is_global) - Fail closed when resolution returns no addresses - Document the residual DNS rebinding (TOCTOU) limitation in code and README - Add 3 tests: CGNAT blocked, multicast blocked, empty resolution blocked
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Summary
Fills two gaps in #334 (merged in 4088efc) in the readurls SSRF guard. The hardened guard is a strict superset of the previous blocklist (verified), so no previously-blocked address becomes reachable.
Gap 1 — CGNAT / shared address space was reachable.
100.64.0.0/10(RFC 6598, including Tailscale's100.100.100.100and other CGNAT gateway services) was not on the blocklist and passed the old checks. The guard now rejects anything that is notip.is_globalor is multicast, instead of enumerating specific ranges — which also future-proofs it against other special-purpose ranges (RFC 1918/4193, loopback, link-local, documentation ranges, etc. remain covered viais_global).Gap 2 — empty
getaddrinforesults now fail closed. Previously, a resolver returning zero addresses could fall through and be treated as allowed; an empty result now raises and the request is refused.Known limitation
DNS rebinding remains a TOCTOU risk: the host is checked via
getaddrinfobefore the fetch, but the underlying HTTP client performs its own resolution and may observe a different answer. This limitation is now documented in the README, which recommends egress filtering for high-security deployments. A complete fix requires a pinned-IP transport layer; happy to follow up if maintainers want it.Testing
tests/test_readurls_ssrf.py: 22 passed (3 new cases covering CGNAT/shared ranges, multicast, and empty-resolution fail-closed behavior)tests/test_ssl_config.py+tests/test_plugins.py: 40 passed (regression)Refs #323.
Collaboration disclosure: This change was developed with AI assistance (human-reviewed).