Refuse local-use NAT64 addresses, and IPv4-compatible ones in blocked_networks (closes #299)

The check that keeps pixa from fetching internal addresses let through
64:ff9b:1::/48, the local-use NAT64 range, and matched an address in the
IPv4-compatible form (::/96) against blocked_networks only as IPv6, so
::203.0.113.5 passed with 203.0.113.0/24 listed. The local-use NAT64
range is now blocked next to 64:ff9b::/96, and an address in ::/96 is
refused when blocked_networks holds it as written or the IPv4 address it
holds. The mapped form stays matched as IPv4 only, as Go keeps plain
IPv4 addresses in that form; README.md says so.

Model: opus-5-5

README.md

@@ -694,7 +694,10 @@ Key settings in more detail:
   and generator pages. Default: empty
 - `blocked_networks` — list of CIDR ranges to refuse for SSRF protection, added
   to the always-enforced built-in ranges (loopback, private, link-local, CGNAT,
-  benchmark, NAT64, and the like); an invalid CIDR aborts startup
+  benchmark, NAT64, and the like); an invalid CIDR aborts startup. An IPv6
+  address in the IPv4-mapped form (`::ffff:a.b.c.d`) is matched only as the IPv4
+  address it holds, so an IPv6 range such as `::/0` refuses neither it nor a
+  plain IPv4 address
 - `trusted_proxies` — list of CIDR ranges of the reverse proxies in front of
   pixa. `X-Forwarded-For` is believed only when the direct peer falls inside one
   of these ranges; the logged and login-recorded client address is then the
@@ -725,11 +728,14 @@ Key settings in more detail:
 - `signing_key` — HMAC secret for URL signatures
 - `db_url` — the SQLite database to open; omitted, it is
   `file:<state_dir>/state.sqlite3?_pragma=journal_mode(WAL)`, which keeps the
-  database in WAL mode. pixa opens one connection to it, so its own reads and
-  writes run one at a time. pixa adds `_pragma=busy_timeout(5000)` to any
-  `db_url`, so a write that finds another program writing to the file waits up
-  to five seconds for it instead of failing. WAL mode comes only from the URL:
-  keep `_pragma=journal_mode(WAL)` in one you set
+  database in WAL mode. `state_dir` is percent-encoded in it, so a `%`, `?` or
+  `#` in the name is written as `%25`, `%3F` or `%23`. For a `state_dir` of `/`
+  or one starting with `//` it starts `file:////`, so SQLite does not read the
+  first name in the path as a host. pixa opens one connection to it, so its own
+  reads and writes run one at a time. pixa adds `_pragma=busy_timeout(5000)` to
+  any `db_url`, so a write that finds another program writing to the file waits
+  up to five seconds for it instead of failing. WAL mode comes only from the
+  URL: keep `_pragma=journal_mode(WAL)` in one you set
 - `cache_max_bytes` — disk cache size limit in bytes; `0` disables the disk
   cache entirely; omitted defaults to 75% of the sum of the free space on the
   filesystem containing `<state_dir>/cache/` and the bytes of source and

TODO.md

@@ -30,6 +30,40 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 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 refuses an IPv6 address in the IPv4-compatible
+  form (`::/96`) 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`; 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. The mapped form
+  (`::ffff:a.b.c.d`) is still matched only as the IPv4 address it holds: Go
+  keeps a plain IPv4 address in that form, and an IPv6 entry such as `::/0` must
+  not refuse every IPv4 address.
+- 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
+  URL kept the brackets when no port followed them, could not look the host up,
+  and the request answered 500. It now checks the address without brackets, so a
+  private one is refused with 403, as with a port.
+- 2026-10-16 a `state_dir` whose name holds `%`, `?` or `#` keeps its database
+  at `state.sqlite3` in that directory (closes #290): the `db_url` derived from
+  it put the name into a `file:` URL as it was, so SQLite read `%41` as `A`, a
+  `?` or `#` ended the path early, and the journal mode parameter was lost. The
+  derived `db_url` is now built with Go's `net/url`, which writes those
+  characters as `%25`, `%3F` and `%23`. For a `state_dir` of `/` or one starting
+  with `//` it starts `file:////`, so SQLite no longer reads the first name in
+  the path as a host.
 - 2026-10-16 a size with one side `0` keeps the side computed from the source's
   proportions at 1 or more (closes #283): it was rounded down, so for a thin
   source it came out at 0, and libvips then made the size square. An 8192x1

internal/config/config.go

@@ -338,8 +338,7 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 				settingName(keyDBURL), errValueEmpty)
 		}
 
-		// The driver sets the journal mode only through a _pragma parameter.
-		c.DBURL = fmt.Sprintf("file:%s/state.sqlite3?_pragma=journal_mode(WAL)", c.StateDir)
+		c.DBURL = defaultDBURL(c.StateDir)
 	}
 
 	if loader.err != nil {
@@ -354,6 +353,30 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 	return c, nil
 }
 
+// defaultDBURL returns the db_url used when db_url is omitted: the file
+// state.sqlite3 in stateDir, in WAL mode, which the driver sets only
+// through a _pragma parameter. String() percent-encodes any %, ? or # in
+// the directory name, which SQLite decodes back, so the URL names exactly
+// this file. OmitHost writes file:/var/lib/pixa/... instead of
+// file:///var/lib/pixa/..., and keeps a relative stateDir from being read
+// as a host name. The path written into the URL is stateDir followed by
+// /state.sqlite3; when it starts with //, as it does for a stateDir of /
+// or one starting with //, the empty host is kept, so the URL starts
+// file://// and SQLite does not read the first name in the path as a
+// host.
+func defaultDBURL(stateDir string) string {
+	dbPath := stateDir + "/state.sqlite3"
+
+	dbURL := url.URL{
+		Scheme:   "file",
+		OmitHost: !strings.HasPrefix(dbPath, "//"),
+		Path:     dbPath,
+		RawQuery: "_pragma=journal_mode(WAL)",
+	}
+
+	return dbURL.String()
+}
+
 // validateKnownKeys rejects configuration files containing keys the
 // application does not understand, so typos fail at startup instead of
 // being silently ignored, and rejects keys that are explicitly set to

internal/config/config_validation_internal_test.go

@@ -135,6 +135,104 @@ func TestDefaultDBURLOpensTheDatabaseInWALMode(t *testing.T) {
 	}
 }
 
+// TestDerivedDBURLOpensStateDirHoldingURLCharacters opens the db_url
+// derived from a state_dir whose name holds %41, ? and #, and checks that
+// the database is state.sqlite3 in that directory, in WAL mode. Put into
+// a file: URL as it is, the name would lose its meaning: %41 is read as
+// A, and the path ends at the ?, which also loses the journal mode.
+func TestDerivedDBURLOpensStateDirHoldingURLCharacters(t *testing.T) {
+	t.Parallel()
+
+	stateDir := filepath.Join(t.TempDir(), "a%41b?c#d")
+
+	err := os.Mkdir(stateDir, 0o750)
+	if err != nil {
+		t.Fatalf("failed to create %q: %v", stateDir, err)
+	}
+
+	// Quoted so YAML takes the whole name as it is.
+	c, err := configFromYAML(t, signingKeyLine+"state_dir: '"+stateDir+"'\n")
+	if err != nil {
+		t.Fatalf("config with state_dir %q should be valid, got: %v", stateDir, err)
+	}
+
+	t.Logf("state_dir %q gives db_url %q", stateDir, c.DBURL)
+
+	checkDBURLOpensFileInWALMode(t, c.DBURL, filepath.Join(stateDir, "state.sqlite3"))
+}
+
+// TestDerivedDBURLOpensStateDirStartingWithTwoSlashes opens the db_url
+// derived from a state_dir that starts with //, and checks that the
+// database is state.sqlite3 in that directory, in WAL mode. Written right
+// after file:, such a name gives a URL starting file://, and SQLite reads
+// the first directory name as a host and refuses to open it.
+func TestDerivedDBURLOpensStateDirStartingWithTwoSlashes(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	stateDir := "/" + dir
+
+	c, err := configFromYAML(t, signingKeyLine+"state_dir: "+stateDir+"\n")
+	if err != nil {
+		t.Fatalf("config with state_dir %q should be valid, got: %v", stateDir, err)
+	}
+
+	t.Logf("state_dir %q gives db_url %q", stateDir, c.DBURL)
+
+	checkDBURLOpensFileInWALMode(t, c.DBURL, filepath.Join(dir, "state.sqlite3"))
+}
+
+// TestDerivedDBURLForRootStateDir checks the db_url derived from a
+// state_dir of /. The path written into the URL is then
+// //state.sqlite3, and written right after file: it gives a URL starting
+// file://, where SQLite reads state.sqlite3 as a host and refuses to open
+// it. The test only checks the URL, since it cannot write to /.
+func TestDerivedDBURLForRootStateDir(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine+"state_dir: /\n")
+	if err != nil {
+		t.Fatalf("config with state_dir / should be valid, got: %v", err)
+	}
+
+	t.Logf("state_dir / gives db_url %q", c.DBURL)
+
+	wantDBURL := "file:////state.sqlite3?_pragma=journal_mode(WAL)"
+	if c.DBURL != wantDBURL {
+		t.Errorf("DBURL = %q, want %q", c.DBURL, wantDBURL)
+	}
+}
+
+// checkDBURLOpensFileInWALMode opens dbURL with the SQLite driver pixad
+// uses, and checks that the database is in WAL mode and is the file
+// dbFile.
+func checkDBURLOpensFileInWALMode(t *testing.T, dbURL, dbFile string) {
+	t.Helper()
+
+	db, err := sql.Open("sqlite", dbURL)
+	if err != nil {
+		t.Fatalf("failed to open %q: %v", dbURL, err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	var journalMode string
+
+	err = db.QueryRowContext(t.Context(), "PRAGMA journal_mode").Scan(&journalMode)
+	if err != nil {
+		t.Fatalf("failed to read the journal mode of %q: %v", dbURL, err)
+	}
+
+	if journalMode != "wal" {
+		t.Errorf("journal mode of %q = %q, want wal", dbURL, journalMode)
+	}
+
+	_, err = os.Stat(dbFile)
+	if err != nil {
+		t.Errorf("opening %q did not create %q: %v", dbURL, dbFile, err)
+	}
+}
+
 func TestExplicitValidValuesAreUsed(t *testing.T) {
 	t.Parallel()
 

internal/httpfetcher/httpfetcher.go

@@ -59,15 +59,17 @@ const (
 // builtinBlockedPrefixes are internal or special-use ranges that Go's
 // net.IP predicates (IsPrivate, IsLinkLocalUnicast, and the like) do not
 // already cover. They are always blocked, in addition to any
-// operator-supplied networks. IPv4-mapped IPv6 addresses are unmapped
-// before matching, so these IPv4 ranges are caught in both forms.
+// operator-supplied networks. IPv4-mapped and IPv4-compatible IPv6
+// addresses are read as the IPv4 address they hold before matching, so
+// these IPv4 ranges are caught in all three forms.
 //
 //nolint:gochecknoglobals // immutable built-in blocklist
 var builtinBlockedPrefixes = []netip.Prefix{
-	netip.MustParsePrefix("100.64.0.0/10"), // RFC 6598 CGNAT / carrier-grade NAT
-	netip.MustParsePrefix("192.0.0.0/24"),  // RFC 6890 IETF protocol assignments
-	netip.MustParsePrefix("198.18.0.0/15"), // RFC 2544 benchmarking range
-	netip.MustParsePrefix("64:ff9b::/96"),  // RFC 6052 NAT64 (maps onto IPv4)
+	netip.MustParsePrefix("100.64.0.0/10"),  // RFC 6598 CGNAT / carrier-grade NAT
+	netip.MustParsePrefix("192.0.0.0/24"),   // RFC 6890 IETF protocol assignments
+	netip.MustParsePrefix("198.18.0.0/15"),  // RFC 2544 benchmarking range
+	netip.MustParsePrefix("64:ff9b::/96"),   // RFC 6052 NAT64 (maps onto IPv4)
+	netip.MustParsePrefix("64:ff9b:1::/48"), // RFC 8215 local-use NAT64 (maps onto IPv4)
 }
 
 // Fetcher errors.
@@ -473,16 +475,16 @@ func validateURL(ctx context.Context, rawURL string, allowHTTP bool) error {
 		return ErrUnsupportedScheme
 	}
 
-	// Parse to extract host
-	host := extractHost(rawURL)
-	if host == "" {
-		return ErrInvalidHost
+	parsedURL, err := neturl.Parse(rawURL)
+	if err != nil {
+		return fmt.Errorf("%w: %w", ErrInvalidHost, err)
 	}
 
-	// Remove port if present
-	h, _, err := net.SplitHostPort(host)
-	if err == nil {
-		host = h
+	// Hostname drops any port, and the brackets of an IPv6 address whether
+	// or not a port follows them
+	host := parsedURL.Hostname()
+	if host == "" {
+		return ErrInvalidHost
 	}
 
 	// Block obvious localhost patterns
@@ -543,6 +545,9 @@ func isPrivateIP(ip net.IP) bool {
 		return true
 	}
 
+	// An IPv4-compatible IPv6 address is judged as the IPv4 address it holds
+	ip = ipv4CompatibleToIPv4(ip)
+
 	// Check for loopback
 	if ip.IsLoopback() {
 		return true
@@ -594,7 +599,12 @@ 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
+// IPv4-compatible address (in ::/96) is refused when a prefix contains the
+// address as written or the IPv4 address it holds. An IPv4-mapped address (in
+// ::ffff:0:0/96) is matched only as the IPv4 address it holds: Go keeps a
+// plain IPv4 address in that form too, and an IPv6 prefix such as ::/0 must
+// not refuse every IPv4 address.
 func isBlockedIP(ip net.IP, blocked []netip.Prefix) bool {
 	if isPrivateIP(ip) {
 		return true
@@ -607,11 +617,28 @@ func isBlockedIP(ip net.IP, blocked []netip.Prefix) bool {
 
 	addr = addr.Unmap()
 
+	// ip read as IPv4 where it is or holds an IPv4 address, otherwise the
+	// zero netip.Addr, which no prefix contains. Matching asIPv4 as well as
+	// addr refuses more only for an IPv4-compatible address.
+	asIPv4, _ := netip.AddrFromSlice(ipv4CompatibleToIPv4(ip).To4())
+
 	return slices.ContainsFunc(blocked, func(prefix netip.Prefix) bool {
-		return prefix.Contains(addr)
+		return prefix.Contains(addr) || prefix.Contains(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.
+func ipv4CompatibleToIPv4(ip net.IP) net.IP {
+	addr, ok := netip.AddrFromSlice(ip)
+	if ok && netip.MustParsePrefix("::/96").Contains(addr) {
+		return ip[12:]
+	}
+
+	return ip
+}
+
 // ssrfSafeDialer validates IP addresses against the built-in blocked ranges
 // before connecting. New wraps dialSSRFSafe with the operator-supplied
 // blocklist; this entry point enforces the built-in ranges alone.

internal/httpfetcher/httpfetcher_internal_test.go

@@ -186,6 +186,44 @@ func TestIsPrivateIP(t *testing.T) {
 	}
 }
 
+// TestIsPrivateIPJudgesEmbeddedIPv4Address checks IPv6 addresses that hold an
+// IPv4 address in their last four bytes: the IPv4-compatible form (::/96) and
+// the NAT64 form (64:ff9b::/96). One holding a loopback, private or
+// link-local IPv4 address is refused as that address is, and one holding a
+// public IPv4 address in the IPv4-compatible form passes.
+func TestIsPrivateIPJudgesEmbeddedIPv4Address(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		ip   string
+		want bool
+	}{
+		{"::127.0.0.1", true},       // loopback
+		{"::10.0.0.1", true},        // private
+		{"::169.254.169.254", true}, // link-local
+		{"64:ff9b::7f00:1", true},   // NAT64 form of 127.0.0.1
+		{"::8.8.8.8", false},        // public
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.ip, func(t *testing.T) {
+			t.Parallel()
+
+			ip := net.ParseIP(tc.ip)
+			if ip == nil {
+				t.Fatalf("failed to parse IP %q", tc.ip)
+			}
+
+			got := isPrivateIP(ip)
+			t.Logf("isPrivateIP(%s) = %v", ip, got)
+
+			if got != tc.want {
+				t.Errorf("isPrivateIP(%q) = %v, want %v", tc.ip, got, tc.want)
+			}
+		})
+	}
+}
+
 func TestValidateURL_RejectsNonHTTPS(t *testing.T) {
 	t.Parallel()
 

internal/server/image_proxy_internal_test.go

@@ -14,10 +14,12 @@ import (
 	"net"
 	"net/http"
 	"net/http/httptest"
+	"net/netip"
 	"os"
 	"path/filepath"
 	"sync/atomic"
 	"testing"
+	"time"
 
 	"go.uber.org/fx"
 	"go.uber.org/fx/fxtest"
@@ -183,6 +185,57 @@ func startImageProxy(
 	return s, db.DB(), stateDir
 }
 
+// startImageProxyWithBlockedNetworks starts the components pixad's fx app
+// builds, from a config with a fresh state directory and blockedNetworks as
+// blocked_networks. Unlike startImageProxy it provides no upstream fetcher, so
+// the handlers build the one pixad uses, which checks every address it
+// connects to against the built-in blocked ranges and blocked_networks. It
+// returns the server with its routes.
+func startImageProxyWithBlockedNetworks(
+	t *testing.T, blockedNetworks []netip.Prefix,
+) *Server {
+	t.Helper()
+
+	stateDir := t.TempDir()
+	cfg := &config.Config{
+		SigningKey:      testSigningKey,
+		StateDir:        stateDir,
+		DBURL:           "file:" + filepath.Join(stateDir, "state.sqlite3"),
+		BlockedNetworks: blockedNetworks,
+		// A fetch that gets past the checks and finds nothing at the address
+		// fails within this time at the latest.
+		UpstreamFetchTimeout: 2 * time.Second,
+		// With no connection slots, the fetch would fail before connecting.
+		UpstreamConnections: config.DefaultUpstreamConnections,
+		DownstreamTimeout:   config.DefaultDownstreamTimeout,
+	}
+
+	var (
+		h  *handlers.Handlers
+		mw *middleware.Middleware
+	)
+
+	app := fxtest.New(t,
+		fx.Supply(cfg),
+		fx.Provide(
+			globals.New,
+			logger.New,
+			database.New,
+			healthcheck.New,
+			handlers.New,
+			middleware.New,
+		),
+		fx.Populate(&h, &mw),
+	)
+	app.RequireStart()
+	t.Cleanup(app.RequireStop)
+
+	s := &Server{config: cfg, mw: mw, h: h}
+	s.SetupRoutes()
+
+	return s
+}
+
 // getImage sends a GET for target to s and fails unless it answers 200.
 func getImage(t *testing.T, s *Server, target string) *httptest.ResponseRecorder {
 	t.Helper()

internal/server/source_url_internal_test.go

@@ -3,15 +3,20 @@ package server
 import (
 	"net/http"
 	"net/http/httptest"
+	"net/netip"
 	"net/url"
 	"regexp"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 	"testing"
+	"time"
 
+	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/imgcache"
 	"sneak.berlin/go/pixa/internal/session"
+	"sneak.berlin/go/pixa/internal/signature"
 )
 
 // percentSignPath is a source path holding a "%", which a URL writes %25.
@@ -433,3 +438,228 @@ func TestGeneratorRefusesSourceURLItCannotFetchAsWritten(t *testing.T) {
 		})
 	}
 }
+
+// ipv6SourcePath is the source path of the image URLs signedImageURL and
+// encryptedImageURL make.
+const ipv6SourcePath = "/ipv6.png"
+
+// TestImageRoutesFetchIPv6SourceHostWithOrWithoutPort requests ipv6SourcePath
+// on an IPv6 source host, written with and without a port, through a
+// /v1/image/ URL with a signature and through an encrypted URL. The public
+// address 2001:db8::a, a documentation address (RFC 3849) the fetcher accepts
+// as public, answers 200, and the upstream receives the request for
+// ipv6SourcePath. The private address fd00::1 answers 403, and the upstream
+// receives no request: the test's fetcher connects every address to the test
+// upstream, so only the fetcher's check of the source URL can refuse it.
+func TestImageRoutesFetchIPv6SourceHostWithOrWithoutPort(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		sourceHost string
+		wantStatus int
+		// wantRequests are the request targets the upstream must receive
+		wantRequests []string
+	}{
+		{"public address without a port", "[2001:db8::a]",
+			http.StatusOK, []string{ipv6SourcePath}},
+		{"public address with a port", "[2001:db8::a]:8443",
+			http.StatusOK, []string{ipv6SourcePath}},
+		{"private address without a port", "[fd00::1]",
+			http.StatusForbidden, nil},
+		{"private address with a port", "[fd00::1]:8443",
+			http.StatusForbidden, nil},
+	}
+
+	for _, tt := range tests {
+		imageURLs := map[string]string{
+			"image URL with a signature": signedImageURL(t, tt.sourceHost),
+			"encrypted URL":              encryptedImageURL(t, tt.sourceHost),
+		}
+
+		for route, imageURL := range imageURLs {
+			t.Run(tt.name+" "+route, func(t *testing.T) {
+				t.Parallel()
+
+				upstream := startRecordingUpstream(t)
+				s, _, _ := startImageProxy(t, upstream.Server)
+
+				rec := httptest.NewRecorder()
+				s.ServeHTTP(rec, httptest.NewRequestWithContext(
+					t.Context(), http.MethodGet, imageURL, nil))
+				t.Logf("GET %s: %d", imageURL, rec.Code)
+
+				if rec.Code != tt.wantStatus {
+					t.Errorf("status = %d, want %d; body %s",
+						rec.Code, tt.wantStatus, rec.Body.String())
+				}
+
+				got := upstream.requestTargets()
+				t.Logf("upstream received %q", got)
+
+				if !slices.Equal(got, tt.wantRequests) {
+					t.Errorf("upstream received %q, want %q", got, tt.wantRequests)
+				}
+			})
+		}
+	}
+}
+
+// signedImageURL returns the /v1/image/ URL of ipv6SourcePath on sourceHost at
+// its original size as PNG, with a signature made with the test signing key,
+// which a host not on the allowlist needs. The URL has no q or fit, so it is
+// signed with the quality 85 and the fit cover.
+func signedImageURL(t *testing.T, sourceHost string) string {
+	t.Helper()
+
+	path, sig, exp := signature.New(testSigningKey).GenerateSignedURL(
+		&signature.Request{
+			SourceHost: sourceHost,
+			SourcePath: ipv6SourcePath,
+			Format:     string(imgcache.FormatPNG),
+			Quality:    85,
+			FitMode:    string(imgcache.FitCover),
+		}, time.Hour)
+
+	return path + "?sig=" + sig + "&exp=" + strconv.FormatInt(exp, 10)
+}
+
+// encryptedImageURL returns the encrypted URL of ipv6SourcePath on sourceHost
+// at its original size as PNG, made with the test signing key.
+func encryptedImageURL(t *testing.T, sourceHost string) string {
+	t.Helper()
+
+	generator, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator: %v", err)
+	}
+
+	token, err := generator.Generate(&encurl.Payload{
+		SourceHost: sourceHost, SourcePath: ipv6SourcePath,
+		Format: imgcache.FormatPNG,
+	})
+	if err != nil {
+		t.Fatalf("encurl Generate: %v", err)
+	}
+
+	return "/v1/e/" + token + "/img.png"
+}
+
+// TestImageRoutesRefuseIPv6SourceHostHoldingInternalIPv4Address requests
+// ipv6SourcePath on IPv6 source hosts that hold a loopback or private IPv4
+// address in their last four bytes, in the IPv4-compatible form (::/96) or the
+// NAT64 form (64:ff9b::/96), through a /v1/image/ URL with a signature and
+// through an encrypted URL. Each answers 403, and the upstream receives no
+// request: the test's fetcher connects every address to the test upstream, so
+// only the fetcher's check of the source URL can refuse it.
+func TestImageRoutesRefuseIPv6SourceHostHoldingInternalIPv4Address(t *testing.T) {
+	t.Parallel()
+
+	upstream := startRecordingUpstream(t)
+	s, _, _ := startImageProxy(t, upstream.Server)
+
+	sourceHosts := []string{
+		"[::127.0.0.1]",     // IPv4-compatible form of a loopback address
+		"[::10.0.0.1]",      // IPv4-compatible form of a private address
+		"[64:ff9b::7f00:1]", // NAT64 form of 127.0.0.1
+	}
+
+	for _, sourceHost := range sourceHosts {
+		imageURLs := []string{
+			signedImageURL(t, sourceHost),
+			encryptedImageURL(t, sourceHost),
+		}
+
+		for _, imageURL := range imageURLs {
+			rec := httptest.NewRecorder()
+			s.ServeHTTP(rec, httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, imageURL, nil))
+			t.Logf("GET %s (source host %s): %d", imageURL, sourceHost, rec.Code)
+
+			if rec.Code != http.StatusForbidden {
+				t.Errorf("source host %s: status = %d, want %d; body %s",
+					sourceHost, rec.Code, http.StatusForbidden, rec.Body.String())
+			}
+		}
+	}
+
+	if got := upstream.requestTargets(); len(got) != 0 {
+		t.Errorf("upstream received %q, want no request", got)
+	}
+}
+
+// TestImageRoutesCheckIPv4AddressInIPv6SourceHost requests ipv6SourcePath on
+// source hosts that hold an IPv4 address, through a /v1/image/ URL with a
+// signature and through an encrypted URL, with the fetcher pixad uses.
+// [64:ff9b:1::a00:1], in the local-use NAT64 range (64:ff9b:1::/48, RFC 8215),
+// answers 403. [::203.0.113.5] holds the public address 203.0.113.5 in the
+// IPv4-compatible form (::/96) and answers 403 when blocked_networks lists
+// 203.0.113.0/24 or ::/96. [::ffff:203.0.113.5] holds it in the IPv4-mapped
+// form (::ffff:0:0/96) and answers 403 when blocked_networks lists
+// 203.0.113.0/24. Go keeps the plain address 203.0.113.5 in the mapped form
+// too, and an IPv6 range must not refuse it: with ::/0 listed it is not
+// refused. Neither is [::203.0.113.5] with blocked_networks empty. Where a
+// host is not refused, the fetcher tries to connect to it, finds nothing
+// there, and the request fails with another status.
+func TestImageRoutesCheckIPv4AddressInIPv6SourceHost(t *testing.T) {
+	t.Parallel()
+
+	const (
+		ipv4CompatibleHost = "[::203.0.113.5]"
+		ipv4MappedHost     = "[::ffff:203.0.113.5]"
+	)
+
+	tests := []struct {
+		name            string
+		sourceHost      string
+		blockedNetworks []netip.Prefix
+		wantForbidden   bool
+	}{
+		{"local-use NAT64 form of 10.0.0.1", "[64:ff9b:1::a00:1]", nil, true},
+		{"IPv4-compatible form of an address in blocked_networks",
+			ipv4CompatibleHost,
+			[]netip.Prefix{netip.MustParsePrefix("203.0.113.0/24")}, true},
+		{"IPv4-compatible form with ::/96 in blocked_networks",
+			ipv4CompatibleHost,
+			[]netip.Prefix{netip.MustParsePrefix("::/96")}, true},
+		{"IPv4-mapped form of an address in blocked_networks",
+			ipv4MappedHost,
+			[]netip.Prefix{netip.MustParsePrefix("203.0.113.0/24")}, true},
+		{"plain IPv4 address with ::/0 in blocked_networks",
+			"203.0.113.5",
+			[]netip.Prefix{netip.MustParsePrefix("::/0")}, false},
+		{"IPv4-compatible form of an address with blocked_networks empty",
+			ipv4CompatibleHost, nil, false},
+	}
+
+	for _, tt := range tests {
+		imageURLs := map[string]string{
+			"image URL with a signature": signedImageURL(t, tt.sourceHost),
+			"encrypted URL":              encryptedImageURL(t, tt.sourceHost),
+		}
+
+		for route, imageURL := range imageURLs {
+			t.Run(tt.name+" "+route, func(t *testing.T) {
+				t.Parallel()
+
+				s := startImageProxyWithBlockedNetworks(t, tt.blockedNetworks)
+
+				rec := httptest.NewRecorder()
+				s.ServeHTTP(rec, httptest.NewRequestWithContext(
+					t.Context(), http.MethodGet, imageURL, nil))
+				t.Logf("GET %s: %d", imageURL, rec.Code)
+
+				forbidden := rec.Code == http.StatusForbidden
+				if tt.wantForbidden && !forbidden {
+					t.Errorf("status = %d, want %d; body %s",
+						rec.Code, http.StatusForbidden, rec.Body.String())
+				}
+
+				if !tt.wantForbidden && forbidden {
+					t.Errorf("status = %d, want any other status; body %s",
+						rec.Code, rec.Body.String())
+				}
+			})
+		}
+	}
+}