diff --git a/TODO.md b/TODO.md
index 58098f7..5296f5e 100644
--- a/TODO.md
+++ b/TODO.md
@@ -32,13 +32,17 @@ P2: security: per-origin rate limiting
 
 - 2026-10-16 P1 security bug: the check that keeps pixa from fetching internal
   addresses refuses the local-use NAT64 range (`64:ff9b:1::/48`, RFC 8215) as it
-  refuses `64:ff9b::/96`, and matches an IPv6 address in the IPv4-compatible
-  form (`::/96`) against `blocked_networks` by the IPv4 address it holds, as it
-  already matched the mapped form (`::ffff:a.b.c.d`) (closes #299). A source
-  host such as `[64:ff9b:1::a00:1]` was fetched, which on a network that runs
-  NAT64 on that range reaches the IPv4 address it holds, here `10.0.0.1`; and
-  with `203.0.113.0/24` in `blocked_networks`, `[::ffff:203.0.113.5]` was
-  refused but `[::203.0.113.5]` was fetched. Both now answer 403.
+  refuses `64:ff9b::/96`, and refuses an IPv6 address that holds an IPv4
+  address, in the IPv4-compatible form (`::/96`) or the mapped form
+  (`::ffff:a.b.c.d`), when `blocked_networks` contains either the address as
+  written or the IPv4 address it holds (closes #299). A source host such as
+  `[64:ff9b:1::a00:1]` was fetched, which on a network that runs NAT64 on that
+  range reaches the IPv4 address it holds, here `10.0.0.1`; with
+  `203.0.113.0/24` in `blocked_networks`, `[::ffff:203.0.113.5]` was refused but
+  `[::203.0.113.5]` was fetched; and with `::ffff:0:0/96` listed,
+  `[::ffff:203.0.113.5]` was fetched. All now answer 403. Go keeps a plain IPv4
+  address in the mapped form, so an IPv6 entry covering `::ffff:0:0/96`, such as
+  `::/0`, now refuses every IPv4 address too.
 - 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
diff --git a/internal/httpfetcher/httpfetcher.go b/internal/httpfetcher/httpfetcher.go
index 97db365..0ebb436 100644
--- a/internal/httpfetcher/httpfetcher.go
+++ b/internal/httpfetcher/httpfetcher.go
@@ -599,26 +599,41 @@ func isPrivateIP(ip net.IP) bool {
 }
 
 // isBlockedIP reports whether ip is refused, either by the built-in
-// internal-range check or by one of the operator-supplied prefixes.
+// internal-range check or by one of the operator-supplied prefixes. An IPv6
+// address that holds an IPv4 address is refused when a prefix contains the
+// address as written or the IPv4 address it holds.
 func isBlockedIP(ip net.IP, blocked []netip.Prefix) bool {
 	if isPrivateIP(ip) {
 		return true
 	}
 
-	// An IPv4-compatible or IPv4-mapped IPv6 address is matched as the IPv4
-	// address it holds
-	addr, ok := netip.AddrFromSlice(ipv4CompatibleToIPv4(ip))
-	if !ok {
+	asIPv6, asIPv4 := ipv6AndIPv4Forms(ip)
+	if !asIPv6.IsValid() {
 		return true
 	}
 
-	addr = addr.Unmap()
-
 	return slices.ContainsFunc(blocked, func(prefix netip.Prefix) bool {
-		return prefix.Contains(addr)
+		return prefix.Contains(asIPv6) || prefix.Contains(asIPv4)
 	})
 }
 
+// ipv6AndIPv4Forms returns ip as an IPv6 address and, when it holds an IPv4
+// address in the IPv4-mapped form (::ffff:0:0/96) or the IPv4-compatible form
+// (::/96), that IPv4 address. Otherwise the IPv4 form is the zero netip.Addr,
+// which no prefix contains. Go keeps a plain IPv4 address in the mapped form,
+// so the IPv6 form of 203.0.113.5 is ::ffff:203.0.113.5. An ip that is not 4
+// or 16 bytes long gives two zero addresses.
+func ipv6AndIPv4Forms(ip net.IP) (netip.Addr, netip.Addr) {
+	asIPv6, ok := netip.AddrFromSlice(ip.To16())
+	if !ok {
+		return netip.Addr{}, netip.Addr{}
+	}
+
+	asIPv4, _ := netip.AddrFromSlice(ipv4CompatibleToIPv4(ip).To4())
+
+	return asIPv6, asIPv4
+}
+
 // ipv4CompatibleToIPv4 returns the IPv4 address that an IPv4-compatible IPv6
 // address (in ::/96, such as ::127.0.0.1) holds in its last four bytes, and
 // any other address unchanged.