diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index a21dad7..bc2ec38 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -45,3 +45,7 @@
 **Vulnerability:** The `.html4ignore` parser still allowed excessively large files and root-directory crawls could generate unbounded filesystem output.
 **Learning:** Local CLI configuration inputs and traversal roots need explicit resource ceilings, not only syntactic validation.
 **Prevention:** Limit `.html4ignore` file size, parsed line count, compiled pattern count, and regex length; reject filesystem root traversal using `File.parentFile != null`.
+## 2024-06-29 - [html4tree] App Crashing on Unreadable Files/Unwritable Directories
+**Vulnerability:** DoS (Denial of Service) via Permissions
+**Learning:** Application blindly reading `.html4ignore` or attempting to create `index.html` without verifying `canRead()` or catching `IOException`/`AccessDeniedException` allows a user/process with sufficient privileges to create directories/files with restricted permissions, crashing the entire tree traversal crawler.
+**Prevention:** Always verify `canRead()` on configuration files before parsing them, and wrap I/O operations (like file writes) in `try-catch` blocks for `IOException` to ensure the crawler skips restricted areas gracefully rather than terminating.
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 936c2b7..6263a91 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -120,8 +120,9 @@ fun process_ignore_file(curr_dir: File): Set<String> {
     val files_to_exclude = mutableSetOf<String>()
 
     // 보안 향상: .html4ignore 파일이 일반 파일인지 확인하고, 심볼릭 링크인 경우 무시하여 DoS 및 경로 조작을 방지합니다.
+    // 보안 향상: 권한 기반 서비스 거부(DoS) 방지를 위해 읽기 권한을 확인합니다.
     // 보안 향상: 파일 크기(1MB 제한) 및 줄 수(1000줄), 정규식 길이(100자)를 제한하여 ReDoS 및 메모리 고갈(OOM) 방지
-    if(ignore_file.isFile && !Files.isSymbolicLink(ignore_file.toPath()) && ignore_file.length() <= 1048576){
+    if(ignore_file.isFile && !Files.isSymbolicLink(ignore_file.toPath()) && ignore_file.canRead() && ignore_file.length() <= 1048576){
        val ignored_regexes = mutableListOf<Regex>()
 
        ignore_file.useLines { lines ->
@@ -160,12 +161,16 @@ fun process_ignore_file(curr_dir: File): Set<String> {
 
 fun write_index_file(curr_dir: File, content: String) {
     val indexPath = curr_dir.toPath().resolve("index.html")
-    val tempPath = Files.createTempFile(curr_dir.toPath(), ".index-", ".html")
     try {
-        Files.write(tempPath, content.toByteArray(Charsets.UTF_8))
-        Files.move(tempPath, indexPath, StandardCopyOption.REPLACE_EXISTING)
-    } finally {
-        Files.deleteIfExists(tempPath)
+        val tempPath = Files.createTempFile(curr_dir.toPath(), ".index-", ".html")
+        try {
+            Files.write(tempPath, content.toByteArray(Charsets.UTF_8))
+            Files.move(tempPath, indexPath, StandardCopyOption.REPLACE_EXISTING)
+        } finally {
+            Files.deleteIfExists(tempPath)
+        }
+    } catch (e: java.io.IOException) {
+        // 보안 향상: 쓰기 권한이 없는 디렉토리에서 크래시(가용성 저하)가 발생하지 않도록 예외를 잡아서 무시합니다.
     }
 }
  
diff --git a/src/test/kotlin/html4tree/MainTest.kt b/src/test/kotlin/html4tree/MainTest.kt
index 9988cee..895edc2 100644
--- a/src/test/kotlin/html4tree/MainTest.kt
+++ b/src/test/kotlin/html4tree/MainTest.kt
@@ -172,9 +172,9 @@ class MainTest {
         indexDir.mkdir()
         File(indexDir, "occupant.txt").writeText("keep")
 
-        assertFailsWith<Exception> {
-            write_index_file(tempDir, "content")
-        }
+        // Since we now catch IOException and AccessDeniedException in write_index_file,
+        // it should no longer throw an exception.
+        write_index_file(tempDir, "content")
 
         assertTrue(indexDir.isDirectory)
         assertEquals("keep", File(indexDir, "occupant.txt").readText())
@@ -453,4 +453,39 @@ class MainTest {
         // Line 1001 should be ignored due to line limit
         assertFalse(excluded.contains("test.txt1001"))
     }
+
+    @Test
+    fun testProcessIgnoreFileUnreadable() {
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText(".*\\.txt\n")
+        File(tempDir, "test.txt").createNewFile()
+
+        try {
+            Assume.assumeTrue(ignoreFile.setReadable(false, false))
+            val excluded = process_ignore_file(tempDir)
+            // unreadable ignore file is ignored, so the file is not excluded
+            assertFalse(excluded.contains("test.txt"))
+            assertTrue(excluded.contains("index.html"))
+        } finally {
+            ignoreFile.setReadable(true, false)
+        }
+    }
+
+    @Test
+    fun testProcessDirUnwritable() {
+        val unwritableDir = File(tempDir, "unwritable")
+        unwritableDir.mkdir()
+        File(unwritableDir, "test.txt").createNewFile()
+
+        try {
+            Assume.assumeTrue(unwritableDir.setWritable(false, false))
+            // This should not throw an exception
+            process_dir(unwritableDir)
+
+            // The index.html should not have been created
+            assertFalse(File(unwritableDir, "index.html").exists())
+        } finally {
+            unwritableDir.setWritable(true, false)
+        }
+    }
 }