feat(security): trust a forwarded IP only from a proxy, and set COOP

The rate limiter and the audit log key on the caller's address, and that address
was read with chi's middleware.RealIP — which believes X-Forwarded-For from
anyone. So a caller could set it to a new value on every request and get a fresh
password-guess bucket each time, and could write any address they liked into the
log the panel's watchers read. The one control meant for when a password is
already gone was keyed on a number the attacker got to pick.

Now the header is believed only when the machine that *connected* is a trusted
proxy — by default anything on a private network, which is the shape of a reverse
proxy or tunnel sharing the host or the docker bridge. A connection straight off
the internet is trusted for nothing and is keyed on the address it actually came
from. VERDANDE_TRUSTED_PROXIES overrides the set (CIDRs, or "none" to trust no
proxy at all); VERDANDE_REAL_IP_HEADER names the header.

Verified in the current deployment before the change: behind the Cloudflare
Tunnel a spoofed X-Real-IP already got no fresh bucket, because cloudflared
overwrites it. This makes that safety a property of the code, not of the
deployment — expose the instance directly, or put a header-passing proxy in
front, and it would have been a live hole.

Also COOP: Cross-Origin-Opener-Policy: same-origin on every response, isolating
the browsing context. verdande scripts no other window, so there is nothing to
lose and a class of cross-window attacks to close.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/configuration.md

@@ -12,6 +12,25 @@ nowhere to put one that the operator would ever see.
 | `VERDANDE_DATA_DIR` | `/data` | Database, uploaded files and backups. |
 | `VERDANDE_DEV` | `false` | Human-readable logs at debug level, and a relaxed origin check so a Vite dev server can connect. Never in production. |
 
+## Behind a proxy
+
+verdande keys its login rate limit and its audit log on the caller's address, so
+that address has to be one the caller cannot forge. A forwarded-for header is
+believed only when the machine that connected is a trusted proxy; a connection
+straight off the internet is keyed on the address it really came from.
+
+| Variable | Default | What it does |
+|---|---|---|
+| `VERDANDE_TRUSTED_PROXIES` | private networks | Whose `X-Forwarded-For` to believe, as comma-separated CIDRs. The default trusts loopback and the private ranges — the shape of a reverse proxy or tunnel sharing the host or the docker bridge. Set to `none` for an instance exposed directly with no proxy, so no forwarded header is ever believed. |
+| `VERDANDE_REAL_IP_HEADER` | `X-Forwarded-For` | The header the client address is read from, for a trusted proxy. |
+
+!!! warning "A proxy that does not overwrite the header"
+    The default is safe when your proxy replaces a client-supplied
+    `X-Forwarded-For` with the real address — nginx, Caddy and Cloudflare Tunnel all
+    do. If yours passes the client's own header through, narrow
+    `VERDANDE_TRUSTED_PROXIES` to just your proxy's address so a visitor cannot
+    choose the address they are rate-limited and logged as.
+
 ## Sessions and links
 
 | Variable | Default | What it does |

internal/config/config.go

@@ -7,6 +7,7 @@ package config
 
 import (
 	"fmt"
+	"net/netip"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -80,6 +81,26 @@ type Config struct {
 	// TrashRetention is how long a soft-deleted task or project stays recoverable.
 	TrashRetention time.Duration
 
+	// TrustedProxies are the peers whose forwarded-for header may be believed.
+	//
+	// The rate limiter and the audit log key on the caller's address, and that
+	// address is only as honest as wherever it was read. Read it from a header that
+	// anybody can set, and an attacker rotates it to get a fresh guess-bucket for
+	// every request and to write someone else's IP into the log. So the header is
+	// believed only when the machine that *connected* is one of these — a reverse
+	// proxy or tunnel the operator put there — and ignored when the connection came
+	// straight off the internet.
+	//
+	// The default is "the peer is on a private network", which is exactly the shape
+	// of a homelab behind Caddy, nginx or a Cloudflare Tunnel: the proxy shares the
+	// host or the docker bridge, so its address is loopback or RFC1918. A directly
+	// exposed instance sees a public peer, trusts nothing, and keys on the real
+	// connection. Override with VERDANDE_TRUSTED_PROXIES (comma-separated CIDRs),
+	// or "none" to trust no proxy at all.
+	TrustedProxies []netip.Prefix
+	// RealIPHeader is where the client address is read from, for a trusted peer.
+	RealIPHeader string
+
 	Dev bool
 }
 
@@ -149,9 +170,70 @@ func Load() (*Config, error) {
 	if c.DataDir, err = filepath.Abs(c.DataDir); err != nil {
 		return nil, fmt.Errorf("VERDANDE_DATA_DIR: %w", err)
 	}
+
+	c.RealIPHeader = env("VERDANDE_REAL_IP_HEADER", "X-Forwarded-For")
+	if c.TrustedProxies, err = parseTrustedProxies(env("VERDANDE_TRUSTED_PROXIES", "")); err != nil {
+		return nil, err
+	}
 	return c, nil
 }
 
+// defaultTrustedProxies is "the peer is not on the public internet": loopback,
+// the three RFC1918 ranges, Tailscale's CGNAT borrow, IPv6 unique-local and both
+// families' link-local. A proxy an operator stands up in front of this server is
+// on one of these; a stranger connecting directly is on none.
+var defaultTrustedProxies = mustPrefixes(
+	"127.0.0.0/8", "::1/128",
+	"10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16",
+	"100.64.0.0/10",
+	"fc00::/7",
+	"169.254.0.0/16", "fe80::/10",
+)
+
+// parseTrustedProxies reads the override. Empty keeps the private-network default;
+// the literal "none" trusts no proxy at all, for an instance that is exposed
+// directly and must never believe a forwarded header.
+func parseTrustedProxies(raw string) ([]netip.Prefix, error) {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return defaultTrustedProxies, nil
+	}
+	if strings.EqualFold(raw, "none") {
+		return nil, nil
+	}
+	var out []netip.Prefix
+	for _, part := range strings.Split(raw, ",") {
+		part = strings.TrimSpace(part)
+		if part == "" {
+			continue
+		}
+		// A bare address is a host route: 10.0.0.5 means 10.0.0.5/32, so an operator
+		// naming one proxy does not have to know CIDR notation to do it.
+		if !strings.Contains(part, "/") {
+			addr, err := netip.ParseAddr(part)
+			if err != nil {
+				return nil, fmt.Errorf("VERDANDE_TRUSTED_PROXIES: %q is not an address or CIDR", part)
+			}
+			out = append(out, netip.PrefixFrom(addr, addr.BitLen()))
+			continue
+		}
+		p, err := netip.ParsePrefix(part)
+		if err != nil {
+			return nil, fmt.Errorf("VERDANDE_TRUSTED_PROXIES: %q is not a CIDR", part)
+		}
+		out = append(out, p.Masked())
+	}
+	return out, nil
+}
+
+func mustPrefixes(cidrs ...string) []netip.Prefix {
+	out := make([]netip.Prefix, 0, len(cidrs))
+	for _, c := range cidrs {
+		out = append(out, netip.MustParsePrefix(c))
+	}
+	return out
+}
+
 // DBPath, FilesDir and BackupsDir are the three things that live on the /data volume.
 func (c *Config) DBPath() string     { return filepath.Join(c.DataDir, "verdande.db") }
 func (c *Config) FilesDir() string   { return filepath.Join(c.DataDir, "files") }

internal/httpapi/csp_header_test.go

@@ -79,6 +79,10 @@ func TestHSTSTracksHTTPS(t *testing.T) {
 	if got := resp.Header.Get("Permissions-Policy"); !strings.Contains(got, "camera=()") {
 		t.Errorf("Permissions-Policy lukker ikke for kameraet: %q", got)
 	}
+	// COOP isolerer browserkonteksten og står på hvert svar.
+	if got := resp.Header.Get("Cross-Origin-Opener-Policy"); got != "same-origin" {
+		t.Errorf("Cross-Origin-Opener-Policy er ikke same-origin: %q", got)
+	}
 }
 
 // Et bilag må ikke kunne blive læst som et dokument.

internal/httpapi/realip.go

@@ -0,0 +1,88 @@
+package httpapi
+
+import (
+	"net"
+	"net/http"
+	"net/netip"
+	"strings"
+
+	"github.com/kristianwind/verdande/internal/config"
+)
+
+// realIP rewrites r.RemoteAddr to the client address a trusted proxy reported,
+// and does nothing at all when the peer is not trusted.
+//
+// This replaces chi's middleware.RealIP, which is deprecated precisely because it
+// believes a forwarded header from anyone: an attacker sets X-Forwarded-For to a
+// new value on every request and gets a fresh rate-limit bucket each time, and
+// writes whatever address they like into the audit log. The one control that
+// exists for when a password is already gone is keyed on this address, so it has
+// to be an address the caller cannot choose.
+//
+// The gate is the peer. A header is read only when the machine that opened the
+// connection is one of cfg.TrustedProxies — a reverse proxy or tunnel the operator
+// put there. A connection straight off the internet is trusted for nothing, so its
+// header is ignored and it is keyed on the address it actually came from. Behind a
+// proxy that overwrites the client's own header (nginx, Caddy and cloudflared all
+// do) the value is then the real client and cannot be forged from outside.
+func realIP(cfg *config.Config) func(http.Handler) http.Handler {
+	trusted := cfg.TrustedProxies
+	header := cfg.RealIPHeader
+	if header == "" {
+		header = "X-Forwarded-For"
+	}
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if ip := trustedClientIP(r, trusted, header); ip != "" {
+				// Port preserved off the original peer only for shape; downstream
+				// clientIP() splits it away, and the forwarded value carries none.
+				r.RemoteAddr = ip
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
+// trustedClientIP returns the forwarded client address if the peer may be believed,
+// or "" to leave RemoteAddr untouched.
+func trustedClientIP(r *http.Request, trusted []netip.Prefix, header string) string {
+	peer := r.RemoteAddr
+	if host, _, err := net.SplitHostPort(peer); err == nil {
+		peer = host
+	}
+	peerAddr, err := netip.ParseAddr(peer)
+	if err != nil || !isTrusted(peerAddr, trusted) {
+		return ""
+	}
+
+	raw := r.Header.Get(header)
+	if raw == "" {
+		return ""
+	}
+	// The leftmost entry is the original client. That entry is only as trustworthy
+	// as the proxy that wrote it — but the trusted proxy is exactly the one that
+	// replaces a client-supplied header, so on the deployments this is written for
+	// it is the real client, and on a direct connection we never reach this line.
+	first := raw
+	if i := strings.IndexByte(raw, ','); i >= 0 {
+		first = raw[:i]
+	}
+	first = strings.TrimSpace(first)
+	if _, err := netip.ParseAddr(first); err != nil {
+		return ""
+	}
+	return first
+}
+
+func isTrusted(addr netip.Addr, trusted []netip.Prefix) bool {
+	if !addr.IsValid() {
+		return false
+	}
+	addr = addr.Unmap()
+	for _, p := range trusted {
+		if p.Contains(addr) {
+			return true
+		}
+	}
+	return false
+}

internal/httpapi/realip_test.go

@@ -0,0 +1,94 @@
+package httpapi
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/kristianwind/verdande/internal/config"
+)
+
+// The address the rate limiter keys on must be one the caller cannot choose. A
+// forwarded header is believed only from a trusted proxy, and ignored from a
+// stranger connecting directly — the difference between a control and a suggestion.
+func TestRealIPTrustsOnlyTheProxy(t *testing.T) {
+	cfg, err := config.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	seen := ""
+	h := realIP(cfg)(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		seen = clientIP(r)
+	}))
+
+	cases := []struct {
+		name       string
+		remoteAddr string
+		xff        string
+		want       string
+	}{
+		{
+			// A private peer is a proxy the operator put there: believe it.
+			name:       "trusted proxy is believed",
+			remoteAddr: "10.1.2.3:5000",
+			xff:        "203.0.113.9",
+			want:       "203.0.113.9",
+		},
+		{
+			// A public peer connected straight to us; its header is a guess.
+			name:       "public peer header ignored",
+			remoteAddr: "198.51.100.7:5000",
+			xff:        "203.0.113.9",
+			want:       "198.51.100.7",
+		},
+		{
+			// No header from a trusted proxy leaves the peer as the answer.
+			name:       "trusted peer without header",
+			remoteAddr: "127.0.0.1:5000",
+			xff:        "",
+			want:       "127.0.0.1",
+		},
+		{
+			// A spoofed value from a stranger cannot become the key.
+			name:       "spoof from outside does not win",
+			remoteAddr: "198.51.100.7:5000",
+			xff:        "127.0.0.1",
+			want:       "198.51.100.7",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			r := httptest.NewRequest("POST", "/api/v1/auth/login", nil)
+			r.RemoteAddr = tc.remoteAddr
+			if tc.xff != "" {
+				r.Header.Set("X-Forwarded-For", tc.xff)
+			}
+			h.ServeHTTP(httptest.NewRecorder(), r)
+			if seen != tc.want {
+				t.Errorf("keyed on %q, want %q", seen, tc.want)
+			}
+		})
+	}
+}
+
+// "none" turns the trust off entirely, for an instance exposed directly.
+func TestRealIPTrustNone(t *testing.T) {
+	t.Setenv("VERDANDE_TRUSTED_PROXIES", "none")
+	cfg, err := config.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	seen := ""
+	h := realIP(cfg)(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		seen = clientIP(r)
+	}))
+	r := httptest.NewRequest("POST", "/", nil)
+	r.RemoteAddr = "10.1.2.3:5000" // even a private peer is not trusted now
+	r.Header.Set("X-Forwarded-For", "203.0.113.9")
+	h.ServeHTTP(httptest.NewRecorder(), r)
+	if seen != "10.1.2.3" {
+		t.Errorf("keyed on %q, want the peer 10.1.2.3", seen)
+	}
+}

internal/httpapi/server.go

@@ -110,7 +110,10 @@ func New(cfg *config.Config, db *store.DB, log *slog.Logger, web fs.FS) *Server
 
 	r := chi.NewRouter()
 	r.Use(middleware.RequestID)
-	r.Use(middleware.RealIP)
+	// Not middleware.RealIP: that believes a forwarded header from anyone, which
+	// lets a caller pick the address the rate limiter and audit log key on. This
+	// one reads the header only from a trusted proxy — see realip.go.
+	r.Use(realIP(cfg))
 	r.Use(requestLogger(log))
 	r.Use(middleware.Recoverer)
 	r.Use(middleware.Timeout(30 * time.Second))
@@ -685,6 +688,11 @@ func (s *Server) securityHeaders(next http.Handler) http.Handler {
 		// browser ignores is not a control.
 		h.Set("Permissions-Policy", "camera=(), microphone=(), geolocation=(), "+
 			"payment=(), usb=(), interest-cohort=()")
+		// Isolate the browsing context: a window this page opened, or one that
+		// opened it, cannot keep a handle across the origin boundary. verdande
+		// never scripts another window, so there is nothing to lose and a whole
+		// class of cross-window attacks to close.
+		h.Set("Cross-Origin-Opener-Policy", "same-origin")
 		next.ServeHTTP(w, r)
 	})
 }