Coder: Unauthenticated SSRF via Azure Instance Identity Endpoint

@@ -8,7 +8,9 @@ import (
 	"encoding/pem"
 	"errors"
 	"io"
+	"net"
 	"net/http"
+	"net/url"
 	"regexp"
 	"sync"
 	"time"
@@ -25,6 +27,158 @@ var allowedSigners = regexp.MustCompile(`^(.*\.)?metadata\.(azure\.(com|us|cn)|m
 // each time a parse occurs.
 var pkcs7Mutex sync.Mutex
 
+// allowedCertHosts contains the hosts Azure intermediate
+// certificates are served from. Only these hosts are permitted
+// when fetching issuing certificates referenced in the signer
+// certificate. This prevents SSRF via crafted
+// IssuingCertificateURL values.
+//
+// Source: https://learn.microsoft.com/en-us/azure/security/fundamentals/azure-ca-details
+var allowedCertHosts = map[string]bool{
+	"www.microsoft.com":    true,
+	"cacerts.digicert.com": true,
+}
+
+// maxCertResponseBytes is the maximum size of a certificate
+// response body we will read. Azure intermediate certificates
+// are typically under 4 KiB; 1 MiB is a generous upper bound
+// that prevents memory exhaustion from malicious responses.
+const maxCertResponseBytes = 1 << 20 // 1 MiB
+
+// extraBlockedNetworks lists special-use CIDR ranges that the
+// stdlib classification methods (IsLoopback, IsPrivate, etc.) do
+// not cover. Blocking these prevents SSRF against carrier-grade
+// NAT, network-benchmarking, documentation, discard-only, and
+// the all-zeros "this network" range.
+//
+// IPv6 ranges already handled by stdlib:
+//   - ::1/128        (IsLoopback)
+//   - fc00::/7       (IsPrivate, ULA)
+//   - fe80::/10      (IsLinkLocalUnicast)
+//   - ff00::/8       (IsMulticast)
+//   - ::/128         (IsUnspecified)
+var extraBlockedNetworks []*net.IPNet
+
+func init() {
+	for _, cidr := range []string{
+		// IPv4 special-use ranges.
+		"0.0.0.0/8",     // RFC 1122 "this network".
+		"100.64.0.0/10", // RFC 6598 carrier-grade NAT.
+		"198.18.0.0/15", // RFC 2544 benchmarking.
+
+		// IPv6 special-use ranges not covered by stdlib.
+		"64:ff9b:1::/48", // RFC 8215 IPv4/IPv6 translation.
+		"100::/64",       // RFC 6666 discard-only.
+		"2001:2::/48",    // RFC 5180 benchmarking.
+		"2001:db8::/32",  // RFC 3849 documentation.
+	} {
+		_, network, _ := net.ParseCIDR(cidr)
+		extraBlockedNetworks = append(extraBlockedNetworks, network)
+	}
+}
+
+// isPrivateIP reports whether the IP is on a network that must
+// not be reachable when fetching certificates. IPv4-mapped IPv6
+// addresses are canonicalized to IPv4 first so a literal like
+// ::ffff:169.254.169.254 cannot bypass the IPv4 ranges.
+func isPrivateIP(ip net.IP) bool {
+	if v4 := ip.To4(); v4 != nil {
+		ip = v4
+	}
+	if ip.IsLoopback() ||
+		ip.IsPrivate() ||
+		ip.IsLinkLocalUnicast() ||
+		ip.IsLinkLocalMulticast() ||
+		ip.IsMulticast() ||
+		ip.IsUnspecified() ||
+		ip.IsInterfaceLocalMulticast() {
+		return true
+	}
+	for _, network := range extraBlockedNetworks {
+		if network.Contains(ip) {
+			return true
+		}
+	}
+	return false
+}
+
+// certFetchClient is an HTTP client that refuses to connect
+// to private or link-local IP addresses. This provides
+// defense-in-depth against SSRF even if the host allowlist is
+// somehow bypassed (e.g. via DNS rebinding).
+var certFetchClient = &http.Client{
+	Timeout: 5 * time.Second,
+	Transport: &http.Transport{
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			host, port, err := net.SplitHostPort(addr)
+			if err != nil {
+				return nil, xerrors.Errorf("split host/port: %w", err)
+			}
+			ips, err := net.DefaultResolver.LookupIPAddr(ctx, host)
+			if err != nil {
+				return nil, xerrors.Errorf("resolve host: %w", err)
+			}
+			if len(ips) == 0 {
+				return nil, xerrors.Errorf("no addresses for %q", host)
+			}
+			// Reject up front so a single tainted answer
+			// short-circuits the dial rather than racing it.
+			for _, ip := range ips {
+				if isPrivateIP(ip.IP) {
+					return nil, xerrors.Errorf(
+						"certificate fetch blocked: %q resolved to private IP %s",
+						host, ip.IP,
+					)
+				}
+			}
+			// Dial the validated IP directly. If we dialed by
+			// hostname here, Go's stdlib would re-resolve and a
+			// hostile resolver could swap in a private IP after
+			// validation (DNS rebinding). TLS verification still
+			// uses the URL host via the Transport's TLS config.
+			var d net.Dialer
+			var firstErr error
+			for _, ip := range ips {
+				conn, derr := d.DialContext(ctx, network, net.JoinHostPort(ip.IP.String(), port))
+				if derr == nil {
+					return conn, nil
+				}
+				if firstErr == nil {
+					firstErr = derr
+				}
+			}
+			return nil, firstErr
+		},
+	},
+}
+
+// IsAllowedCertificateURL reports whether rawURL points to a
+// host on the allowlist, uses http or https, and targets a
+// standard PKI distribution port. Microsoft and DigiCert serve
+// these artifacts on 80/443 only; any other port is rejected to
+// keep the SSRF surface as narrow as the hostname itself.
+func IsAllowedCertificateURL(rawURL string) bool {
+	if rawURL == "" {
+		return false
+	}
+	u, err := url.Parse(rawURL)
+	if err != nil {
+		return false
+	}
+	if u.Scheme != "http" && u.Scheme != "https" {
+		return false
+	}
+	if !allowedCertHosts[u.Hostname()] {
+		return false
+	}
+	switch u.Port() {
+	case "", "80", "443":
+		return true
+	default:
+		return false
+	}
+}
+
 type metadata struct {
 	VMID string `json:"vmId"`
 }
@@ -95,29 +249,42 @@ func Validate(ctx context.Context, signature string, options Options) (string, e
 		ctx, cancelFunc := context.WithTimeout(ctx, 5*time.Second)
 		defer cancelFunc()
 		for _, certURL := range signer.IssuingCertificateURL {
+			if !IsAllowedCertificateURL(certURL) {
+				return "", xerrors.New("issuing certificate URL not on allowlist")
+			}
 			req, err := http.NewRequestWithContext(ctx, "GET", certURL, nil)
 			if err != nil {
-				return "", xerrors.Errorf("new request %q: %w", certURL, err)
+				return "", xerrors.New("construct certificate request")
 			}
-			res, err := http.DefaultClient.Do(req)
+			res, err := certFetchClient.Do(req)
 			if err != nil {
-				return "", xerrors.Errorf("no cached certificate for %q found. error fetching: %w", certURL, err)
+				return "", xerrors.New("certificate fetch unsuccessful")
 			}
-			data, err := io.ReadAll(res.Body)
+			limited := io.LimitReader(res.Body, maxCertResponseBytes+1)
+			data, err := io.ReadAll(limited)
+			_ = res.Body.Close()
 			if err != nil {
-				_ = res.Body.Close()
-				return "", xerrors.Errorf("read body %q: %w", certURL, err)
+				return "", xerrors.New("read certificate response body")
+			}
+			if int64(len(data)) > maxCertResponseBytes {
+				return "", xerrors.New(
+					"certificate response exceeds maximum size",
+				)
 			}
-			_ = res.Body.Close()
 			cert, err := x509.ParseCertificate(data)
 			if err != nil {
-				return "", xerrors.Errorf("parse certificate %q: %w", certURL, err)
+				// Do not wrap the parse error; it may contain
+				// fragments of the HTTP response body, which
+				// could leak internal data to the caller.
+				return "", xerrors.New(
+					"fetched data is not a valid certificate",
+				)
 			}
 			options.Intermediates.AddCert(cert)
 		}
 		_, err = signer.Verify(options.VerifyOptions)
 		if err != nil {
-			return "", err
+			return "", xerrors.New("signature verification failed after fetching issuing certificates")
 		}
 	}
 

@@ -0,0 +1,76 @@
+package azureidentity
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestIsPrivateIP(t *testing.T) {
+	t.Parallel()
+	cases := []struct {
+		name    string
+		ip      string
+		blocked bool
+	}{
+		{"loopback v4", "127.0.0.1", true},
+		{"loopback v6", "::1", true},
+		{"link local v4 (azure metadata)", "169.254.169.254", true},
+		{"link local v6", "fe80::1", true},
+		{"rfc1918 10/8", "10.0.0.1", true},
+		{"rfc1918 172.16/12", "172.16.0.1", true},
+		{"rfc1918 192.168/16", "192.168.0.1", true},
+		{"ipv6 ula", "fc00::1", true},
+		{"unspecified v4", "0.0.0.0", true},
+		{"unspecified v6", "::", true},
+		{"this-network 0.0.0.0/8", "0.1.2.3", true},
+		{"cgnat 100.64/10", "100.64.0.1", true},
+		{"benchmarking 198.18/15", "198.18.0.1", true},
+		{"multicast v4", "224.0.0.1", true},
+		{"ipv6 nat64 well-known", "64:ff9b:1::1", true},
+		{"ipv6 discard-only", "100::1", true},
+		{"ipv6 benchmarking", "2001:2::1", true},
+		{"ipv6 documentation", "2001:db8::1", true},
+		// IPv4-mapped IPv6: must canonicalize to v4 before
+		// classification, otherwise an attacker could bypass
+		// the metadata block via ::ffff:169.254.169.254.
+		{"ipv4-mapped metadata", "::ffff:169.254.169.254", true},
+		{"ipv4-mapped rfc1918", "::ffff:10.0.0.1", true},
+
+		{"public v4", "8.8.8.8", false},
+		{"public v6", "2606:4700:4700::1111", false},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			ip := net.ParseIP(tc.ip)
+			require.NotNil(t, ip, "parse %q", tc.ip)
+			require.Equal(t, tc.blocked, isPrivateIP(ip))
+		})
+	}
+}
+
+// TestCertFetchClientRejectsLoopback proves the dialer refuses
+// to connect even when the URL itself would have passed an
+// allowlist (httptest.Server always binds to 127.0.0.1, so a
+// successful fetch here would mean the SSRF guard had failed).
+func TestCertFetchClientRejectsLoopback(t *testing.T) {
+	t.Parallel()
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write([]byte("should never be reached"))
+	}))
+	t.Cleanup(srv.Close)
+
+	req, err := http.NewRequestWithContext(context.Background(), http.MethodGet, srv.URL, nil)
+	require.NoError(t, err)
+	resp, err := certFetchClient.Do(req)
+	if resp != nil {
+		defer resp.Body.Close()
+	}
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "private IP")
+}

@@ -116,3 +116,37 @@ func TestExpiresSoon(t *testing.T) {
 		}
 	}
 }
+
+func TestIsAllowedCertificateURL(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name    string
+		url     string
+		allowed bool
+	}{
+		{"microsoft http", "http://www.microsoft.com/pki/mscorp/cert.crt", true},
+		{"microsoft https", "https://www.microsoft.com/pkiops/certs/cert.crt", true},
+		{"digicert http", "http://cacerts.digicert.com/DigiCertGlobalRootG2.crt", true},
+		{"digicert https", "https://cacerts.digicert.com/DigiCertGlobalRootG3.crt", true},
+		{"evil domain", "http://evil.example.com/cert.crt", false},
+		{"metadata endpoint", "http://169.254.169.254/latest/meta-data/", false},
+		{"localhost", "http://localhost/secret", false},
+		{"subdomain trick", "http://www.microsoft.com.evil.com/cert.crt", false},
+		{"empty string", "", false},
+		{"ftp scheme", "ftp://www.microsoft.com/cert.crt", false},
+		{"no scheme", "www.microsoft.com/cert.crt", false},
+		{"javascript scheme", "javascript:alert(1)", false},
+		{"microsoft with path", "http://www.microsoft.com/pkiops/certs/cert.crt", true},
+		{"microsoft explicit port 80", "http://www.microsoft.com:80/cert.crt", true},
+		{"microsoft explicit port 443", "https://www.microsoft.com:443/cert.crt", true},
+		{"microsoft non-standard port", "http://www.microsoft.com:8080/cert.crt", false},
+		{"microsoft port 22", "http://www.microsoft.com:22/cert.crt", false},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			result := azureidentity.IsAllowedCertificateURL(tc.url)
+			require.Equal(t, tc.allowed, result, "URL: %s", tc.url)
+		})
+	}
+}

@@ -8,6 +8,7 @@ import (
 
 	"github.com/mitchellh/mapstructure"
 
+	"cdr.dev/slog/v3"
 	"github.com/coder/coder/v2/coderd/awsidentity"
 	"github.com/coder/coder/v2/coderd/azureidentity"
 	"github.com/coder/coder/v2/coderd/database/dbauthz"
@@ -38,9 +39,17 @@ func (api *API) postWorkspaceAuthAzureInstanceIdentity(rw http.ResponseWriter, r
 		VerifyOptions: api.AzureCertificates,
 	})
 	if err != nil {
+		// Log the full error for operators but return only a
+		// generic message to the caller. Errors from the
+		// certificate fetch path may contain fragments of
+		// internal HTTP responses, so exposing them would be
+		// an information disclosure risk.
+		api.Logger.Warn(ctx, "azure identity validation failed",
+			slog.Error(err),
+		)
 		httpapi.Write(ctx, rw, http.StatusUnauthorized, codersdk.Response{
 			Message: "Invalid Azure identity.",
-			Detail:  err.Error(),
+			Detail:  "Signature verification failed.",
 		})
 		return
 	}