diff --git a/src/main/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilter.java b/src/main/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilter.java
index 71cd7392..7902bfe5 100644
--- a/src/main/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilter.java
+++ b/src/main/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilter.java
@@ -108,6 +108,10 @@ private String normalizeFrameAncestors(String configured) {
         if (configured == null) {
             return "'self'";
         }
+        if (configured.indexOf(';') >= 0 || configured.chars().anyMatch(Character::isISOControl)) {
+            throw new IllegalArgumentException(
+                    "viewer.security.frame-ancestors contains an unsafe CSP character");
+        }
         String trimmed = configured.trim();
         if (trimmed.isEmpty()) {
             return "'self'";
@@ -117,4 +121,4 @@ private String normalizeFrameAncestors(String configured) {
         }
         return trimmed;
     }
-}
+}
\ No newline at end of file
diff --git a/src/test/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilterTest.java b/src/test/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilterTest.java
index 0e62117e..4090574d 100644
--- a/src/test/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilterTest.java
+++ b/src/test/java/com/clearfolio/viewer/config/ViewerSecurityHeadersWebFilterTest.java
@@ -1,28 +1,30 @@
 package com.clearfolio.viewer.config;
 
-import static org.junit.jupiter.api.Assertions.assertNull;
-import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.util.concurrent.atomic.AtomicBoolean;
+
 import org.junit.jupiter.api.Test;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
+import org.springframework.http.server.RequestPath;
+import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.mock.http.server.reactive.MockServerHttpRequest;
 import org.springframework.mock.http.server.reactive.MockServerHttpResponse;
 import org.springframework.mock.web.server.MockServerWebExchange;
-import org.springframework.web.server.WebFilterChain;
 import org.springframework.web.server.ServerWebExchange;
-import org.springframework.http.server.reactive.ServerHttpRequest;
-import org.springframework.http.server.RequestPath;
+import org.springframework.web.server.WebFilterChain;
 
 import reactor.core.publisher.Mono;
 
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
-import java.util.concurrent.atomic.AtomicBoolean;
 
 class ViewerSecurityHeadersWebFilterTest {
 
@@ -133,6 +135,23 @@ void usesCustomFrameAncestorsValueWhenConfigured() {
         assertTrue(csp.contains("frame-ancestors https://example.test"));
     }
 
+    @Test
+    void rejectsFrameAncestorDirectiveAndHeaderInjection() {
+        IllegalArgumentException directiveInjection = assertThrows(
+                IllegalArgumentException.class,
+                () -> new ViewerSecurityHeadersWebFilter("https://trusted.example; script-src *")
+        );
+        assertEquals("viewer.security.frame-ancestors contains an unsafe CSP character",
+                directiveInjection.getMessage());
+
+        IllegalArgumentException headerInjection = assertThrows(
+                IllegalArgumentException.class,
+                () -> new ViewerSecurityHeadersWebFilter("https://trusted.example\nX-Injected: true")
+        );
+        assertEquals("viewer.security.frame-ancestors contains an unsafe CSP character",
+                headerInjection.getMessage());
+    }
+
     @Test
     void supportsHeadRequestsForViewerSurface() {
         ViewerSecurityHeadersWebFilter filter = new ViewerSecurityHeadersWebFilter("self");
@@ -198,4 +217,4 @@ void doesNotAddHeadersForNonViewerPaths() {
         assertNull(headers.getFirst("X-Content-Type-Options"));
         assertNull(headers.getFirst("Referrer-Policy"));
     }
-}
+}
\ No newline at end of file