diff --git a/common/src/main/scala/org/apache/celeborn/common/rpc/RpcEndpoint.scala b/common/src/main/scala/org/apache/celeborn/common/rpc/RpcEndpoint.scala
index 8d12c227af6..3b219372113 100644
--- a/common/src/main/scala/org/apache/celeborn/common/rpc/RpcEndpoint.scala
+++ b/common/src/main/scala/org/apache/celeborn/common/rpc/RpcEndpoint.scala
@@ -20,6 +20,7 @@ package org.apache.celeborn.common.rpc
 import org.apache.celeborn.common.exception.CelebornException
 import org.apache.celeborn.common.network.client.TransportClient
 import org.apache.celeborn.common.rpc.netty.RemoteNettyRpcCallContext
+import org.apache.celeborn.common.util.Utils
 
 /**
  * A factory class to create the [[RpcEnv]]. It must have an empty constructor so that it can be
@@ -138,6 +139,11 @@ trait RpcEndpoint {
   }
 
   def checkAuth(context: RpcCallContext, appId: String): Unit = {
+    // Validate the application id at the single auth chokepoint so every current
+    // and future RPC handler that calls checkAuth is covered, and so it runs even
+    // when auth is disabled (clientId == null). This guards the worker against
+    // path traversal via appId (e.g. "../foo") before any filesystem path is built.
+    Utils.validateAppId(appId)
     context match {
       case remoteContext: RemoteNettyRpcCallContext =>
         checkAuth(remoteContext.transportClient, appId)
diff --git a/common/src/main/scala/org/apache/celeborn/common/util/Utils.scala b/common/src/main/scala/org/apache/celeborn/common/util/Utils.scala
index 5b2dd6a1097..2c4f3e310e4 100644
--- a/common/src/main/scala/org/apache/celeborn/common/util/Utils.scala
+++ b/common/src/main/scala/org/apache/celeborn/common/util/Utils.scala
@@ -700,6 +700,18 @@ object Utils extends Logging {
     (appId, shuffleId)
   }
 
+  private val appIdPattern = "[A-Za-z0-9_-]+".r.pattern
+
+  def validateAppId(applicationId: String): Unit = {
+    // matches() anchors the whole input, so a trailing newline (which `$` would
+    // otherwise tolerate) is rejected along with any other traversal character.
+    if (applicationId == null || !appIdPattern.matcher(applicationId).matches()) {
+      throw new IllegalArgumentException(
+        s"Invalid application id: '$applicationId'. " +
+          "Application id must be non-empty and match [A-Za-z0-9_-]+.")
+    }
+  }
+
   def splitPartitionLocationUniqueId(uniqueId: String): (Int, Int) = {
     val splits = uniqueId.split("-")
     val partitionId = splits.dropRight(1).mkString("-").toInt
diff --git a/common/src/test/scala/org/apache/celeborn/common/util/UtilsSuite.scala b/common/src/test/scala/org/apache/celeborn/common/util/UtilsSuite.scala
index 8be472b6447..e59296fc6b9 100644
--- a/common/src/test/scala/org/apache/celeborn/common/util/UtilsSuite.scala
+++ b/common/src/test/scala/org/apache/celeborn/common/util/UtilsSuite.scala
@@ -289,4 +289,30 @@ class UtilsSuite extends CelebornFunSuite {
       celebornConf)
     assert(testInstance.isInstanceOf[DefaultIdentityProvider])
   }
+
+  test("validateAppId rejects path traversal and accepts valid ids") {
+    Seq(
+      "application_1234567890123_0001",
+      "local-1234567890123",
+      "app1",
+      "my-app-id",
+      "app_with_underscores").foreach { id =>
+      Utils.validateAppId(id)
+    }
+
+    Seq(
+      "../etc/passwd",
+      "app/../secret",
+      "app/id",
+      "app\\id",
+      "app id",
+      "app\n",
+      "valid_app\n",
+      "",
+      null).foreach { id =>
+      intercept[IllegalArgumentException] {
+        Utils.validateAppId(id)
+      }
+    }
+  }
 }
diff --git a/worker/src/main/scala/org/apache/celeborn/service/deploy/worker/storage/StorageManager.scala b/worker/src/main/scala/org/apache/celeborn/service/deploy/worker/storage/StorageManager.scala
index 9a2d4a8a740..a8341659510 100644
--- a/worker/src/main/scala/org/apache/celeborn/service/deploy/worker/storage/StorageManager.scala
+++ b/worker/src/main/scala/org/apache/celeborn/service/deploy/worker/storage/StorageManager.scala
@@ -1161,6 +1161,9 @@ final private[worker] class StorageManager(conf: CelebornConf, workerSource: Abs
         throw new IOException(s"No available disks! suggested mountPoint $suggestedMountPoint")
       }
 
+      // NOTE: the DFS branches below (HDFS/S3/OSS) also build "$appId/$shuffleId"
+      // paths but rely solely on the upstream Utils.validateAppId guard at the RPC
+      // entry points
       if (storageType == Type.HDFS && location.getStorageInfo.HDFSAvailable()) {
         val shuffleDir =
           new Path(new Path(hdfsDir, conf.workerWorkingDir), s"$appId/$shuffleId")
@@ -1216,8 +1219,15 @@ final private[worker] class StorageManager(conf: CelebornConf, workerSource: Abs
         val dir = dirs(getNextIndex % dirs.size)
         val mountPoint = DeviceInfo.getMountPoint(dir.getAbsolutePath, mountPoints)
         val shuffleDir = new File(dir, s"$appId/$shuffleId")
-        shuffleDir.mkdirs()
         val file = new File(shuffleDir, fileName)
+        // Defense in depth: ensure the resolved path stays under the working dir
+        // even if appId / shuffleId / fileName contained traversal characters.
+        val dirCanonical = dir.getCanonicalPath + File.separator
+        if (!file.getCanonicalPath.startsWith(dirCanonical)) {
+          throw new IOException(
+            s"Refusing to create shuffle file outside working dir: ${file.getCanonicalPath}")
+        }
+        shuffleDir.mkdirs()
         try {
           if (file.exists()) {
             throw new FileAlreadyExistsException(