From 2ce222c3cd8a1b0837344fc1b7d1cbe5e4989ac8 Mon Sep 17 00:00:00 2001
From: bensynapse <118375461+bensynapse@users.noreply.github.com>
Date: Wed, 7 Oct 2026 23:28:57 +0300
Subject: [PATCH] fix: preserve CSP values during nonce substitution

---
 csp_test.go | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 secure.go   | 17 +++---------
 2 files changed, 78 insertions(+), 14 deletions(-)

diff --git a/csp_test.go b/csp_test.go
index f09042b..8d88863 100644
--- a/csp_test.go
+++ b/csp_test.go
@@ -63,3 +63,78 @@ func TestWithCSPNonce(t *testing.T) {
 
 	expect(t, CSPNonce(withCSPNonce(req, nonce).Context()), nonce)
 }
+
+func TestCSPNoncePreservesStaticPolicy(t *testing.T) {
+	policies := []string{
+		"default-src 'self'",
+		"default-src 'self'; report-uri https://example.com/a%20b%2Fc%25",
+	}
+
+	for _, policy := range policies {
+		for _, nonceHeader := range []string{cspHeader, cspReportOnlyHeader} {
+			t.Run(nonceHeader+"/"+policy, func(t *testing.T) {
+				options := Options{ContentSecurityPolicy: policy, ContentSecurityPolicyReportOnly: policy}
+				staticHeader := cspReportOnlyHeader
+				if nonceHeader == cspHeader {
+					options.ContentSecurityPolicy = "script-src $NONCE"
+				} else {
+					options.ContentSecurityPolicyReportOnly = "script-src $NONCE"
+					staticHeader = cspHeader
+				}
+
+				res := httptest.NewRecorder()
+				req := httptest.NewRequest(http.MethodGet, "/foo", nil)
+				New(options).Handler(cspHandler).ServeHTTP(res, req)
+
+				nonce := res.Body.String()
+				if nonce == "" {
+					t.Fatal("handler received no nonce")
+				}
+				expect(t, res.Header().Get(nonceHeader), "script-src 'nonce-"+nonce+"'")
+				expect(t, res.Header().Get(staticHeader), policy)
+			})
+		}
+	}
+}
+
+func TestCSPNoncePreservesPercentEncodedURLs(t *testing.T) {
+	const policy = "script-src $NONCE; style-src $NONCE; report-uri https://example.com/a%20b%2Fc%25"
+	cases := []struct {
+		name    string
+		options Options
+		headers []string
+	}{
+		{"enforced", Options{ContentSecurityPolicy: policy}, []string{cspHeader}},
+		{"report only", Options{ContentSecurityPolicyReportOnly: policy}, []string{cspReportOnlyHeader}},
+		{
+			"both",
+			Options{ContentSecurityPolicy: policy, ContentSecurityPolicyReportOnly: policy},
+			[]string{cspHeader, cspReportOnlyHeader},
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			handler := New(c.options).Handler(cspHandler)
+			var previousNonce string
+			for request := 0; request < 2; request++ {
+				res := httptest.NewRecorder()
+				req := httptest.NewRequest(http.MethodGet, "/foo", nil)
+				handler.ServeHTTP(res, req)
+
+				nonce := res.Body.String()
+				if nonce == "" || nonce == previousNonce {
+					t.Fatal("handler did not receive a fresh nonce")
+				}
+				previousNonce = nonce
+
+				want := "script-src 'nonce-" + nonce + "'; style-src 'nonce-" + nonce +
+					"'; report-uri https://example.com/a%20b%2Fc%25"
+				for _, header := range c.headers {
+					expect(t, res.Header().Get(header), want)
+				}
+				expect(t, CSPNonce(req.Context()), "")
+			}
+		})
+	}
+}
diff --git a/secure.go b/secure.go
index 122c43c..302ec15 100644
--- a/secure.go
+++ b/secure.go
@@ -156,10 +156,7 @@ func New(options ...Options) *Secure {
 		o = options[0]
 	}
 
-	o.ContentSecurityPolicy = strings.ReplaceAll(o.ContentSecurityPolicy, "$NONCE", "'nonce-%[1]s'")
-	o.ContentSecurityPolicyReportOnly = strings.ReplaceAll(o.ContentSecurityPolicyReportOnly, "$NONCE", "'nonce-%[1]s'")
-
-	o.nonceEnabled = strings.Contains(o.ContentSecurityPolicy, "%[1]s") || strings.Contains(o.ContentSecurityPolicyReportOnly, "%[1]s")
+	o.nonceEnabled = strings.Contains(o.ContentSecurityPolicy, "$NONCE") || strings.Contains(o.ContentSecurityPolicyReportOnly, "$NONCE")
 
 	s := &Secure{
 		opt:               o,
@@ -445,20 +442,12 @@ func (s *Secure) processRequest(w http.ResponseWriter, r *http.Request) (http.He
 
 	// Content Security Policy header.
 	if len(s.opt.ContentSecurityPolicy) > 0 {
-		if s.opt.nonceEnabled {
-			responseHeader.Set(cspHeader, fmt.Sprintf(s.opt.ContentSecurityPolicy, CSPNonce(r.Context())))
-		} else {
-			responseHeader.Set(cspHeader, s.opt.ContentSecurityPolicy)
-		}
+		responseHeader.Set(cspHeader, strings.ReplaceAll(s.opt.ContentSecurityPolicy, "$NONCE", "'nonce-"+CSPNonce(r.Context())+"'"))
 	}
 
 	// Content Security Policy Report Only header.
 	if len(s.opt.ContentSecurityPolicyReportOnly) > 0 {
-		if s.opt.nonceEnabled {
-			responseHeader.Set(cspReportOnlyHeader, fmt.Sprintf(s.opt.ContentSecurityPolicyReportOnly, CSPNonce(r.Context())))
-		} else {
-			responseHeader.Set(cspReportOnlyHeader, s.opt.ContentSecurityPolicyReportOnly)
-		}
+		responseHeader.Set(cspReportOnlyHeader, strings.ReplaceAll(s.opt.ContentSecurityPolicyReportOnly, "$NONCE", "'nonce-"+CSPNonce(r.Context())+"'"))
 	}
 
 	// Referrer Policy header.