diff --git a/src/main/java/org/codelibs/fess/filter/StaticThemeFilter.java b/src/main/java/org/codelibs/fess/filter/StaticThemeFilter.java
index 5eae4d008..855ef6f02 100644
--- a/src/main/java/org/codelibs/fess/filter/StaticThemeFilter.java
+++ b/src/main/java/org/codelibs/fess/filter/StaticThemeFilter.java
@@ -20,6 +20,7 @@
 import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Locale;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.Collectors;
@@ -29,6 +30,7 @@
 import org.codelibs.core.lang.StringUtil;
 import org.codelibs.fess.helper.VirtualHostHelper;
 import org.codelibs.fess.mylasta.direction.FessConfig;
+import org.codelibs.fess.theme.StaticThemeInstaller;
 import org.codelibs.fess.theme.StaticThemeResponder;
 import org.codelibs.fess.theme.Theme;
 import org.codelibs.fess.theme.ThemeRegistry;
@@ -62,6 +64,8 @@
  *
  * <p>Behavior summary:
  * <ul>
+ *   <li>A request for a JSP (or similar server-side page) under {@code /themes/} is answered
+ *       with 404, whatever the method or active theme: a static theme contains plain files only.</li>
  *   <li>Requests other than GET and HEAD pass through unchanged.</li>
  *   <li>Requests that are neither a {@code /themes/...} asset nor an allowlisted UI path
  *       pass through unchanged (without even resolving the active theme).</li>
@@ -169,6 +173,14 @@ public void doFilter(final ServletRequest request, final ServletResponse respons
         }
         final HttpServletResponse res = (HttpServletResponse) response;
 
+        // A static theme is plain files. A JSP (or similar) page under /themes/ is never
+        // part of one, so it is not handed to the servlet container, whatever the method or
+        // active theme. Checked on the container-decoded path, which is what it maps to a servlet.
+        if (isServerSideThemePage(req)) {
+            res.sendError(HttpServletResponse.SC_NOT_FOUND);
+            return;
+        }
+
         // GET and HEAD are the SPA's read paths. A HEAD response is built exactly like the GET
         // one — the container drops the body and keeps the headers, which is what a HEAD client
         // asks for. Everything else is a Fess route.
@@ -358,6 +370,21 @@ private static String stripContextPath(final HttpServletRequest req) {
         return uri;
     }
 
+    /**
+     * Returns whether the request targets a server-side page (JSP and the like) under
+     * {@code /themes/}. Uses the servlet path and path info, which the container has already
+     * decoded and normalized, rather than the raw request URI.
+     *
+     * @param req the request
+     * @return true when the request should not reach a servlet
+     */
+    static boolean isServerSideThemePage(final HttpServletRequest req) {
+        final String servletPath = req.getServletPath() == null ? "" : req.getServletPath();
+        final String pathInfo = req.getPathInfo() == null ? "" : req.getPathInfo();
+        final String path = servletPath + pathInfo;
+        return path.toLowerCase(Locale.ROOT).startsWith("/themes/") && StaticThemeInstaller.isServerSidePage(path);
+    }
+
     private static boolean isThemeUiPath(final String uri) {
         if ("/".equals(uri)) {
             return true;
diff --git a/src/main/java/org/codelibs/fess/theme/StaticThemeInstaller.java b/src/main/java/org/codelibs/fess/theme/StaticThemeInstaller.java
index 5c6e6082f..e8be24a22 100644
--- a/src/main/java/org/codelibs/fess/theme/StaticThemeInstaller.java
+++ b/src/main/java/org/codelibs/fess/theme/StaticThemeInstaller.java
@@ -28,6 +28,8 @@
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
 import java.util.Comparator;
+import java.util.List;
+import java.util.Locale;
 import java.util.Set;
 import java.util.UUID;
 import java.util.function.Supplier;
@@ -158,6 +160,33 @@ public void init() {
      */
     private static final Set<String> DENIED_SEGMENTS = Set.of(".git", ".svn", ".hg", "__MACOSX", ".DS_Store");
 
+    /**
+     * File extensions of server-side pages the servlet container would evaluate rather than
+     * serve as files. A static theme is plain files only, so these are refused at install time
+     * and never served from {@code /themes/}.
+     */
+    private static final List<String> SERVER_SIDE_EXTENSIONS = List.of(".jsp", ".jspx", ".jspf");
+
+    /**
+     * Returns whether the path names a server-side page (JSP and the like) rather than a static
+     * file. Compared case-insensitively.
+     *
+     * @param path a ZIP entry name or request path
+     * @return true when the path ends with one of the server-side page extensions
+     */
+    public static boolean isServerSidePage(final String path) {
+        if (path == null) {
+            return false;
+        }
+        final String lower = path.toLowerCase(Locale.ROOT);
+        for (final String ext : SERVER_SIDE_EXTENSIONS) {
+            if (lower.endsWith(ext)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     /** Retention duration in days for attic dirs (used when fessConfig is absent). */
     private static final int DEFAULT_ATTIC_RETENTION_DAYS = 7;
 
@@ -461,6 +490,10 @@ private void extract(final InputStream in, final Path target) throws IOException
                     zis.closeEntry();
                     continue;
                 }
+                if (isServerSidePage(name)) {
+                    throw new InstallException(InstallException.Code.EXTRACT_FAILED,
+                            "Server-side page not allowed in a static theme: " + name);
+                }
                 final Path parent = resolved.getParent();
                 if (parent != null) {
                     Files.createDirectories(parent);
diff --git a/src/test/java/org/codelibs/fess/filter/StaticThemeFilterTest.java b/src/test/java/org/codelibs/fess/filter/StaticThemeFilterTest.java
index 834239d86..2ac451abb 100644
--- a/src/test/java/org/codelibs/fess/filter/StaticThemeFilterTest.java
+++ b/src/test/java/org/codelibs/fess/filter/StaticThemeFilterTest.java
@@ -95,6 +95,62 @@ public void test_passesThroughForApi() throws Exception {
         assertFalse(stub.servedAsset);
     }
 
+    @Test
+    public void test_doFilter_answersServerSidePageUnderThemesWith404() throws Exception {
+        // A static theme is plain files, so a JSP under /themes/ must never reach the
+        // container's JSP servlet: not for the active theme, not for another theme, not for
+        // any method, and not when no static theme is active.
+        final Theme staticTheme = new Theme("t", Paths.get("/tmp/t"), null);
+        for (final Theme active : new Theme[] { staticTheme, null }) {
+            for (final String method : new String[] { "GET", "HEAD", "POST" }) {
+                for (final String path : new String[] { "/themes/t/x.jsp", "/themes/other/x.jsp", "/themes/other/sub/x.JSPX",
+                        "/themes/other/x.jspf" }) {
+                    final StaticThemeFilter f = new StaticThemeFilter();
+                    f.setThemeRegistry(new StubRegistry(active));
+                    final StubResponder stub = new StubResponder();
+                    f.setStaticThemeResponder(stub);
+                    final StubResponse res = new StubResponse();
+                    final StubChain chain = new StubChain();
+                    f.doFilter(new StubRequest(method, path), res, chain);
+                    final String label = method + " " + path + " active=" + (active == null ? null : active.getName());
+                    assertFalse(chain.called, label + " must not pass through to the container");
+                    assertFalse(stub.servedAsset, label + " must not be served as an asset");
+                    assertEquals(HttpServletResponse.SC_NOT_FOUND, res.errorStatus, label);
+                }
+            }
+        }
+    }
+
+    @Test
+    public void test_doFilter_matchesServerSidePageOnTheDecodedPath() throws Exception {
+        // The container maps the decoded path to a servlet, so the check must use it rather
+        // than the raw request URI (an encoded or ;param-suffixed URI decodes to x.jsp).
+        final StaticThemeFilter f = new StaticThemeFilter();
+        f.setThemeRegistry(new StubRegistry(null));
+        f.setStaticThemeResponder(new StubResponder());
+        final StubResponse res = new StubResponse();
+        final StubChain chain = new StubChain();
+        f.doFilter(new StubRequest("GET", "/%74hemes/other/x.jsp;a=b").withServletPath("/themes/other/x.jsp"), res, chain);
+        assertFalse(chain.called);
+        assertEquals(HttpServletResponse.SC_NOT_FOUND, res.errorStatus);
+    }
+
+    @Test
+    public void test_doFilter_passesThroughStaticFileOfInactiveTheme() throws Exception {
+        // Only server-side pages are stopped; a plain file of a theme that is not active
+        // still passes through to the container as before.
+        final Theme staticTheme = new Theme("t", Paths.get("/tmp/t"), null);
+        final StaticThemeFilter f = new StaticThemeFilter();
+        f.setThemeRegistry(new StubRegistry(staticTheme));
+        final StubResponder stub = new StubResponder();
+        f.setStaticThemeResponder(stub);
+        final StubResponse res = new StubResponse();
+        final StubChain chain = new StubChain();
+        f.doFilter(new StubRequest("GET", "/themes/other/app.js"), res, chain);
+        assertTrue(chain.called);
+        assertEquals(0, res.errorStatus);
+    }
+
     @Test
     public void test_doFilter_servesHeadLikeGet() throws Exception {
         // HEAD must be served exactly like GET: same allowlist match, same serveIndex call.
@@ -679,6 +735,7 @@ public void doFilter(final ServletRequest req, final ServletResponse res) {
     static class StubResponse implements HttpServletResponse {
         // Implement the bare minimum; methods we don't call throw UnsupportedOperationException.
         String redirectLocation;
+        int errorStatus;
 
         @Override
         public String getCharacterEncoding() {
@@ -787,7 +844,7 @@ public void sendError(final int sc, final String msg) {
 
         @Override
         public void sendError(final int sc) {
-            throw new UnsupportedOperationException();
+            this.errorStatus = sc;
         }
 
         @Override
@@ -874,12 +931,19 @@ static class StubRequest implements HttpServletRequest {
         private String contextPath = "";
         private String queryString;
         private final Map<String, String[]> params = new HashMap<>();
+        private String servletPath;
 
         StubRequest(final String method, final String uri) {
             this.method = method;
             this.uri = uri;
         }
 
+        /** Sets the container-decoded servlet path when it differs from the raw request URI. */
+        StubRequest withServletPath(final String path) {
+            this.servletPath = path;
+            return this;
+        }
+
         StubRequest withContextPath(final String ctx) {
             this.contextPath = ctx;
             return this;
@@ -1036,7 +1100,7 @@ public StringBuffer getRequestURL() {
 
         @Override
         public String getServletPath() {
-            return uri;
+            return servletPath != null ? servletPath : uri;
         }
 
         @Override
diff --git a/src/test/java/org/codelibs/fess/theme/StaticThemeInstallerTest.java b/src/test/java/org/codelibs/fess/theme/StaticThemeInstallerTest.java
index fb1cefad6..cc917b7b9 100644
--- a/src/test/java/org/codelibs/fess/theme/StaticThemeInstallerTest.java
+++ b/src/test/java/org/codelibs/fess/theme/StaticThemeInstallerTest.java
@@ -252,6 +252,48 @@ public void test_install_rejectsDenylistedDotfileEntries() throws Exception {
         }
     }
 
+    @Test
+    public void test_install_rejectsServerSidePages() throws Exception {
+        // A static theme is plain files; a JSP (or similar) entry would be evaluated by the
+        // servlet container instead of served, so the whole archive is refused and nothing is
+        // left behind in the themes directory.
+        for (final String entryName : new String[] { "x.jsp", "sub/x.jspx", "inc/x.jspf", "X.JSP" }) {
+            final Path themesDir = Files.createTempDirectory("themes-installer-");
+            try {
+                final StaticThemeInstaller installer = newInstaller(themesDir);
+                final ByteArrayOutputStream bao = new ByteArrayOutputStream();
+                try (ZipOutputStream zos = new ZipOutputStream(bao)) {
+                    final String yml = String.join("\n", "apiVersion: fess.codelibs.org/v1", "kind: StaticTheme", "name: withpage",
+                            "displayName: \"withpage\"", "version: 1.0.0");
+                    putEntry(zos, "theme.yml", yml.getBytes(StandardCharsets.UTF_8));
+                    putEntry(zos, "index.html", "<html></html>".getBytes(StandardCharsets.UTF_8));
+                    putEntry(zos, entryName, "<%= 1 %>".getBytes(StandardCharsets.UTF_8));
+                }
+                final StaticThemeInstaller.InstallException ex = assertThrows(StaticThemeInstaller.InstallException.class,
+                        () -> installer.installZip(new ByteArrayInputStream(bao.toByteArray())), entryName);
+                assertEquals(entryName, StaticThemeInstaller.InstallException.Code.EXTRACT_FAILED, ex.code());
+                assertTrue(ex.getMessage().contains(entryName), ex.getMessage());
+                assertFalse(Files.exists(themesDir.resolve("withpage")), entryName);
+                try (java.util.stream.Stream<Path> left = Files.list(themesDir)) {
+                    assertEquals(entryName, 0L, left.filter(p -> p.getFileName().toString().startsWith(".staging-")).count());
+                }
+            } finally {
+                deleteRecursively(themesDir);
+            }
+        }
+    }
+
+    @Test
+    public void test_isServerSidePage() {
+        assertTrue(StaticThemeInstaller.isServerSidePage("x.jsp"));
+        assertTrue(StaticThemeInstaller.isServerSidePage("/themes/t/a/b.JSPX"));
+        assertTrue(StaticThemeInstaller.isServerSidePage("frag.jspf"));
+        assertFalse(StaticThemeInstaller.isServerSidePage("index.html"));
+        assertFalse(StaticThemeInstaller.isServerSidePage("app.js"));
+        assertFalse(StaticThemeInstaller.isServerSidePage("notes.jsp.txt"));
+        assertFalse(StaticThemeInstaller.isServerSidePage(null));
+    }
+
     @Test
     public void test_install_rejectsDenylistedDirectories() throws Exception {
         // Denylist policy: .git, .svn, .hg, __MACOSX, .DS_Store path segments are rejected