TODO.md

@@ -30,6 +30,14 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-16 P1 security bug: the check that keeps pixa from fetching internal
+  addresses judges an IPv6 address in the IPv4-compatible form (`::/96`), such
+  as `::127.0.0.1` or `::10.0.0.1`, by the IPv4 address in its last four bytes
+  (closes #297). It read only the mapped form (`::ffff:a.b.c.d`) as IPv4, so a
+  source host such as `[::127.0.0.1]` was checked as a public IPv6 address and
+  fetched, which on a network that routes it reaches the internal IPv4 host; it
+  is now refused with 403. The NAT64 form (`64:ff9b::/96`) was already refused
+  as a whole, as one of the built-in blocked ranges, and still is.
 - 2026-10-16 a source host that is an IPv6 address in brackets with no port,
   such as `[2001:db8::a]`, is fetched like one with a port, through `/v1/image/`
   and through an encrypted URL (closes #295). The fetcher's check of the source

internal/httpfetcher/httpfetcher.go

@@ -543,6 +543,13 @@ func isPrivateIP(ip net.IP) bool {
 		return true
 	}
 
+	// An IPv4-compatible IPv6 address (in ::/96, such as ::127.0.0.1) holds
+	// an IPv4 address in its last four bytes and is judged as that address
+	if addr, ok := netip.AddrFromSlice(ip); ok &&
+		netip.MustParsePrefix("::/96").Contains(addr) {
+		return isPrivateIP(ip[12:])
+	}
+
 	// Check for loopback
 	if ip.IsLoopback() {
 		return true