diff --git a/runtime/secrets/converter.go b/runtime/secrets/converter.go
index b312c55b..1e340bb1 100644
--- a/runtime/secrets/converter.go
+++ b/runtime/secrets/converter.go
@@ -133,6 +133,8 @@ func TLSConfigFromSecret(ctx context.Context, secret *corev1.Secret, opts ...TLS
 // proxy URL. Optional "username" and "password" fields can be provided
 // for proxy authentication.
 func ProxyURLFromSecret(ctx context.Context, secret *corev1.Secret) (*url.URL, error) {
+	ref := client.ObjectKeyFromObject(secret)
+
 	addressData, exists := secret.Data[KeyAddress]
 	if !exists {
 		return nil, &KeyNotFoundError{Key: KeyAddress, Secret: secret}
@@ -140,16 +142,25 @@ func ProxyURLFromSecret(ctx context.Context, secret *corev1.Secret) (*url.URL, e
 
 	address := string(addressData)
 	if address == "" {
-		ref := client.ObjectKeyFromObject(secret)
 		return nil, fmt.Errorf("secret '%s': proxy address is empty", ref)
 	}
 
+	// Validate length before parsing to avoid parsing large invalid URLs.
+	// The 2048 character limit matches the validation used in notification-controller's
+	// spec.address field.
+	if len(address) > 2048 {
+		return nil, fmt.Errorf("secret '%s': proxy URL exceeds maximum length of 2048 characters", ref)
+	}
+
 	proxyURL, err := url.Parse(address)
 	if err != nil {
-		ref := client.ObjectKeyFromObject(secret)
 		return nil, fmt.Errorf("secret '%s': failed to parse proxy address '%s': %w", ref, address, err)
 	}
 
+	if proxyURL.Scheme != "http" && proxyURL.Scheme != "https" {
+		return nil, fmt.Errorf("secret '%s': proxy URL must use http or https scheme, got '%s'", ref, proxyURL.Scheme)
+	}
+
 	username, hasUsername := secret.Data[KeyUsername]
 	password, hasPassword := secret.Data[KeyPassword]
 
diff --git a/runtime/secrets/converter_test.go b/runtime/secrets/converter_test.go
index 30f54d00..021d0239 100644
--- a/runtime/secrets/converter_test.go
+++ b/runtime/secrets/converter_test.go
@@ -752,6 +752,36 @@ func TestProxyURLFromSecret(t *testing.T) {
 			),
 			errMsg: "secret 'default/proxy-secret': failed to parse proxy address",
 		},
+		{
+			name: "invalid scheme - ftp",
+			secret: testSecret(
+				withName("proxy-secret"),
+				withData(map[string][]byte{
+					secrets.KeyAddress: []byte("ftp://ftp.example.com:21"),
+				}),
+			),
+			errMsg: "secret 'default/proxy-secret': proxy URL must use http or https scheme, got 'ftp'",
+		},
+		{
+			name: "invalid scheme - no scheme",
+			secret: testSecret(
+				withName("proxy-secret"),
+				withData(map[string][]byte{
+					secrets.KeyAddress: []byte("proxy.example.com:8080"),
+				}),
+			),
+			errMsg: "secret 'default/proxy-secret': proxy URL must use http or https scheme",
+		},
+		{
+			name: "URL exceeds maximum length",
+			secret: testSecret(
+				withName("proxy-secret"),
+				withData(map[string][]byte{
+					secrets.KeyAddress: []byte("http://proxy.example.com/" + string(make([]byte, 2049))),
+				}),
+			),
+			errMsg: "secret 'default/proxy-secret': proxy URL exceeds maximum length of 2048 characters",
+		},
 	}
 
 	for _, tt := range tests {
diff --git a/tests/integration/go.mod b/tests/integration/go.mod
index 84e26db3..cd13441d 100644
--- a/tests/integration/go.mod
+++ b/tests/integration/go.mod
@@ -22,7 +22,7 @@ require (
 	github.com/fluxcd/pkg/cache v0.11.0
 	github.com/fluxcd/pkg/git v0.36.0
 	github.com/fluxcd/pkg/git/gogit v0.40.0
-	github.com/fluxcd/pkg/runtime v0.86.0
+	github.com/fluxcd/pkg/runtime v0.87.0
 	github.com/fluxcd/test-infra/tftestenv v0.0.0-20250626232827-e0ca9c3f8d7b
 	github.com/go-git/go-git/v5 v5.16.2
 	github.com/google/go-containerregistry v0.20.6