diff --git a/user/src/com/google/gwt/core/server/StackTraceDeobfuscator.java b/user/src/com/google/gwt/core/server/StackTraceDeobfuscator.java
index 25d851c7c0c..0a7f61cc079 100644
--- a/user/src/com/google/gwt/core/server/StackTraceDeobfuscator.java
+++ b/user/src/com/google/gwt/core/server/StackTraceDeobfuscator.java
@@ -145,6 +145,9 @@ Map<String, String> getAll(String strongName, Set<String> symbols) {
 
   private static final Pattern JsniRefPattern = Pattern.compile("@?([^:]+)::([^(]+)(\\((.*)\\))?");
   private static final Pattern fragmentIdPattern = Pattern.compile(".*(\\d+)\\.js");
+  // Matches ServerSerializationStreamReader: the strong name reaches us straight from the
+  // client (X-GWT-Permutation header) and is concatenated into symbol/source map file names.
+  private static final Pattern strongNamePattern = Pattern.compile("[a-zA-Z0-9_]+");
   private static final int LINE_NUMBER_UNKNOWN = -1;
   private static final String SYMBOL_DATA_UNKNOWN = "";
 
@@ -365,9 +368,13 @@ protected InputStream getSymbolMapInputStream(String permutationStrongName) thro
    */
   protected abstract InputStream openInputStream(String fileName) throws IOException;
 
+  private static boolean isValidStrongName(String strongName) {
+    return strongName != null && strongNamePattern.matcher(strongName).matches();
+  }
+
   private SourceMapping loadSourceMap(String permutationStrongName, int fragmentId) {
     SourceMapping toReturn = sourceMaps.get(permutationStrongName + fragmentId);
-    if (toReturn == null) {
+    if (toReturn == null && isValidStrongName(permutationStrongName)) {
       try {
         String sourceMapString = loadStreamAsString(
             getSourceMapInputStream(permutationStrongName, fragmentId));
@@ -407,6 +414,9 @@ private Map<String, String> loadSymbolMap(
     String line;
 
     try {
+      if (!isValidStrongName(strongName)) {
+        throw new IOException("Invalid permutation strong name: " + strongName);
+      }
       BufferedReader bin = new BufferedReader(
           new InputStreamReader(getSymbolMapInputStream(strongName)));
       try {
diff --git a/user/test/com/google/gwt/core/server/StackTraceDeobfuscatorTest.java b/user/test/com/google/gwt/core/server/StackTraceDeobfuscatorTest.java
new file mode 100644
index 00000000000..5aac2ffc1db
--- /dev/null
+++ b/user/test/com/google/gwt/core/server/StackTraceDeobfuscatorTest.java
@@ -0,0 +1,56 @@
+/*
+ * Copyright 2026 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.gwt.core.server;
+
+import junit.framework.TestCase;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Tests for {@link StackTraceDeobfuscator}.
+ */
+public class StackTraceDeobfuscatorTest extends TestCase {
+
+  private static class RecordingDeobfuscator extends StackTraceDeobfuscator {
+    final List<String> opened = new ArrayList<String>();
+
+    @Override
+    protected InputStream openInputStream(String fileName) throws IOException {
+      opened.add(fileName);
+      throw new IOException("no such resource: " + fileName);
+    }
+  }
+
+  private static StackTraceElement[] trace() {
+    return new StackTraceElement[] {new StackTraceElement("C", "m", "C.java", 1)};
+  }
+
+  public void testTraversalStrongNameIsNotUsedToBuildPath() {
+    RecordingDeobfuscator d = new RecordingDeobfuscator();
+    d.resymbolize(trace(), "../../../../../../etc/passwd");
+    assertTrue("strong name with path separators must not reach openInputStream: " + d.opened,
+        d.opened.isEmpty());
+  }
+
+  public void testValidStrongNameStillLoadsSymbolMap() {
+    RecordingDeobfuscator d = new RecordingDeobfuscator();
+    d.resymbolize(trace(), "0F2C4A6E8B1D3F5709ABCDEF12345678");
+    assertEquals("0F2C4A6E8B1D3F5709ABCDEF12345678.symbolMap", d.opened.get(0));
+  }
+}