From 615be176b82070a729e51852ede41bc6f7151257 Mon Sep 17 00:00:00 2001
From: Jakub Domeracki <jakub@coder.com>
Date: Mon, 20 Apr 2026 13:01:46 +0200
Subject: [PATCH] fix(coderd): add frame-ancestors CSP directive to prevent
 clickjacking (#24474)

---
 coderd/coderd.go          | 57 +++++++++++++++++++++++++--------------
 coderd/httpmw/csp.go      |  7 +++++
 coderd/httpmw/csp_test.go | 39 +++++++++++++++++++++++++++
 3 files changed, 83 insertions(+), 20 deletions(-)

diff --git a/coderd/coderd.go b/coderd/coderd.go
index 700a5d8a88..f07a085dc9 100644
--- a/coderd/coderd.go
+++ b/coderd/coderd.go
@@ -2002,29 +2002,46 @@ func New(options *Options) *API {
 
 	// Add CSP headers to all static assets and pages. CSP headers only affect
 	// browsers, so these don't make sense on api routes.
-	cspMW := httpmw.CSPHeaders(
-		options.Telemetry.Enabled(), func() []*proxyhealth.ProxyHost {
-			if api.DeploymentValues.Dangerous.AllowAllCors {
-				// In this mode, allow all external requests.
-				return []*proxyhealth.ProxyHost{
-					{
-						Host:    "*",
-						AppHost: "*",
-					},
-				}
-			}
-			// Always add the primary, since the app host may be on a sub-domain.
-			proxies := []*proxyhealth.ProxyHost{
+	cspProxyHosts := func() []*proxyhealth.ProxyHost {
+		if api.DeploymentValues.Dangerous.AllowAllCors {
+			// In this mode, allow all external requests.
+			return []*proxyhealth.ProxyHost{
 				{
-					Host:    api.AccessURL.Host,
-					AppHost: appurl.ConvertAppHostForCSP(api.AccessURL.Host, api.AppHostname),
+					Host:    "*",
+					AppHost: "*",
 				},
 			}
-			if f := api.WorkspaceProxyHostsFn.Load(); f != nil {
-				proxies = append(proxies, (*f)()...)
-			}
-			return proxies
-		}, additionalCSPHeaders)
+		}
+		// Always add the primary, since the app host may be on a sub-domain.
+		proxies := []*proxyhealth.ProxyHost{
+			{
+				Host:    api.AccessURL.Host,
+				AppHost: appurl.ConvertAppHostForCSP(api.AccessURL.Host, api.AppHostname),
+			},
+		}
+		if f := api.WorkspaceProxyHostsFn.Load(); f != nil {
+			proxies = append(proxies, (*f)()...)
+		}
+		return proxies
+	}
+	cspMW := httpmw.CSPHeaders(options.Telemetry.Enabled(), cspProxyHosts, additionalCSPHeaders)
+
+	// Embed routes (e.g. VS Code extension chat) are designed to be
+	// loaded inside iframes, so they need a relaxed frame-ancestors
+	// policy instead of the default 'self'. However, if the operator
+	// explicitly configured frame-ancestors via CODER_ADDITIONAL_CSP_POLICY,
+	// respect that setting.
+	embedCSPHeaders := make(map[httpmw.CSPFetchDirective][]string, len(additionalCSPHeaders))
+	for k, v := range additionalCSPHeaders {
+		embedCSPHeaders[k] = v
+	}
+	if _, ok := additionalCSPHeaders[httpmw.CSPFrameAncestors]; !ok {
+		embedCSPHeaders[httpmw.CSPFrameAncestors] = []string{"*"}
+	}
+	embedCSPMW := httpmw.CSPHeaders(options.Telemetry.Enabled(), cspProxyHosts, embedCSPHeaders)
+	embedHandler := embedCSPMW(compressHandler(httpmw.HSTS(api.SiteHandler, options.StrictTransportSecurityCfg)))
+	r.Get("/agents/{agentId}/embed", embedHandler.ServeHTTP)
+	r.Get("/agents/{agentId}/embed/*", embedHandler.ServeHTTP)
 
 	// Static file handler must be wrapped with HSTS handler if the
 	// StrictTransportSecurityAge is set. We only need to set this header on
diff --git a/coderd/httpmw/csp.go b/coderd/httpmw/csp.go
index f39781ad51..a42567e342 100644
--- a/coderd/httpmw/csp.go
+++ b/coderd/httpmw/csp.go
@@ -142,6 +142,13 @@ func CSPHeaders(telemetry bool, proxyHosts func() []*proxyhealth.ProxyHost, stat
 				cspSrcs.Append(directive, values...)
 			}
 
+			// Default to 'self' to prevent clickjacking unless
+			// explicitly overridden via staticAdditions (e.g. for
+			// embeddable routes).
+			if _, ok := cspSrcs[CSPFrameAncestors]; !ok {
+				cspSrcs[CSPFrameAncestors] = []string{"'self'"}
+			}
+
 			var csp strings.Builder
 			for src, vals := range cspSrcs {
 				_, _ = fmt.Fprintf(&csp, "%s %s; ", src, strings.Join(vals, " "))
diff --git a/coderd/httpmw/csp_test.go b/coderd/httpmw/csp_test.go
index ba88320e6f..8af2755514 100644
--- a/coderd/httpmw/csp_test.go
+++ b/coderd/httpmw/csp_test.go
@@ -12,6 +12,45 @@ import (
 	"github.com/coder/coder/v2/coderd/proxyhealth"
 )
 
+func TestCSPFrameAncestors(t *testing.T) {
+	t.Parallel()
+
+	t.Run("DefaultSelf", func(t *testing.T) {
+		t.Parallel()
+
+		r := httptest.NewRequest(http.MethodGet, "/", nil)
+		rw := httptest.NewRecorder()
+
+		httpmw.CSPHeaders(false, func() []*proxyhealth.ProxyHost {
+			return nil
+		}, nil)(http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+			rw.WriteHeader(http.StatusOK)
+		})).ServeHTTP(rw, r)
+
+		csp := rw.Header().Get("Content-Security-Policy")
+		require.Contains(t, csp, "frame-ancestors 'self'")
+	})
+
+	t.Run("OverrideViaStaticAdditions", func(t *testing.T) {
+		t.Parallel()
+
+		r := httptest.NewRequest(http.MethodGet, "/", nil)
+		rw := httptest.NewRecorder()
+
+		httpmw.CSPHeaders(false, func() []*proxyhealth.ProxyHost {
+			return nil
+		}, map[httpmw.CSPFetchDirective][]string{
+			httpmw.CSPFrameAncestors: {"*"},
+		})(http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+			rw.WriteHeader(http.StatusOK)
+		})).ServeHTTP(rw, r)
+
+		csp := rw.Header().Get("Content-Security-Policy")
+		require.Contains(t, csp, "frame-ancestors *")
+		require.NotContains(t, csp, "frame-ancestors 'self'")
+	})
+}
+
 func TestCSP(t *testing.T) {
 	t.Parallel()