diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index a885865d..2efe1b50 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -99,3 +99,7 @@
 **Root cause:** The protected implementation added canonical names to the exclusion set but did not compare each observed directory entry through a locale-stable normalized key.
 **Prevention:** Build one `Locale.ROOT` lowercase set from the canonical sensitive names, compare every observed name against it, and add the original spelling to the exclusion set so downstream exact membership remains correct.
 **Evidence:** `testProcessIgnoreFileTreatsSensitiveNamesCaseInsensitively` failed on test-only commit `472b916cd40f70693c4e1eb48956042a25353feb` (CI run `31469596932`) and passed with the source fix at `bb113d858ccfc42ddaecf6729749b238e5ade2d0` (CI run `31469921661`).
+## 2024-09-24 - [CRITICAL] Fail-closed Security Policy for .html4ignore Files
+**Vulnerability:** The application failed to properly enforce security policies defined in `.html4ignore`. If the file was unreadable due to restrictive permissions or due to a TOCTOU race condition after enumeration, the system failed-open. This could lead to information exposure by indexing and publishing directories containing sensitive files that were intended to be ignored.
+**Learning:** Checking for `.html4ignore` existence without verifying read access and intentionally failing if access is denied violates the principle of failing securely. Security controls must enforce a fail-closed behavior to avoid bypassing intended protections.
+**Prevention:** Always employ fail-closed logic when evaluating security boundaries or access control lists (like `.html4ignore`). Ensure that unreadable policy files throw a dedicated exception (`IgnoreFileReadException`) to stop the operation, instead of silently continuing. Also, avoid TOCTOU by ensuring the presence of the policy file from the directory snapshot is strictly verified and safely read.
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 0972fa2c..c468eb64 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -200,20 +200,26 @@ internal fun crawl_directories(
         val dirFilesNames = dirFiles?.let { files ->
             Array(files.size) { index -> files[index].name }
         }
-        val exclude = processIgnoreFile(lle.file, dirFilesNames)
-
-        if(maxLevel == -1 || currentLevel <= maxLevel)
-           processDirectory(lle.file, exclude, dirFiles)
-
-        if(maxLevel == -1 || currentLevel < maxLevel) {
-            dirFiles?.forEach {
-                // ⚡ Bolt Performance Optimization: Short-circuit OS stat calls
-                // by checking cheap in-memory string exclusion rules first
-                if(!it.name.isHiddenFile() && it.name !in exclude) {
-                    val childAttrs = readAttributes(it)
-                    if(childAttrs != null && childAttrs.isDirectory && !childAttrs.isSymbolicLink) {
-                        val childEntry = LinkedListEntry(it, currentLevel+1, readIdentity(it).key)
-                        ll.push(childEntry)
+        val exclude = try {
+            processIgnoreFile(lle.file, dirFilesNames)
+        } catch (e: IgnoreFileReadException) {
+            null
+        }
+
+        if(exclude != null) {
+            if(maxLevel == -1 || currentLevel <= maxLevel)
+               processDirectory(lle.file, exclude, dirFiles)
+
+            if(maxLevel == -1 || currentLevel < maxLevel) {
+                dirFiles?.forEach {
+                    // ⚡ Bolt Performance Optimization: Short-circuit OS stat calls
+                    // by checking cheap in-memory string exclusion rules first
+                    if(!it.name.isHiddenFile() && it.name !in exclude) {
+                        val childAttrs = readAttributes(it)
+                        if(childAttrs != null && childAttrs.isDirectory && !childAttrs.isSymbolicLink) {
+                            val childEntry = LinkedListEntry(it, currentLevel+1, readIdentity(it).key)
+                            ll.push(childEntry)
+                        }
                     }
                 }
             }
@@ -293,6 +299,12 @@ fun String.urlEncodePath(): String {
     return encoded?.toString() ?: this
 }
 
+/**
+ * Exception thrown when a policy file like .html4ignore exists but cannot be safely read.
+ * Enforces a fail-closed security contract to prevent TOCTOU bypasses and information exposure.
+ */
+class IgnoreFileReadException(message: String, cause: Throwable? = null) : RuntimeException(message, cause)
+
 fun process_ignore_file(curr_dir: File, dirFilesNames: Array<String>? = null): Set<String> {
 
     val ignore_filename = ".html4ignore"
@@ -303,10 +315,13 @@ fun process_ignore_file(curr_dir: File, dirFilesNames: Array<String>? = null): S
 
     val files_to_exclude = mutableSetOf<String>()
 
-    // 보안 향상: .html4ignore 파일이 일반 파일인지 확인하고, 심볼릭 링크인 경우 무시하여 DoS 및 경로 조작을 방지합니다.
-    // 보안 향상: 파일 크기(1MB 제한) 및 줄 수(1000줄), 정규식 길이(100자)를 제한하여 ReDoS 및 메모리 고갈(OOM) 방지
-    // 보안 향상: 권한이 없는 파일 접근 시 발생하는 예외(DoS)를 방지하기 위해 canRead() 추가 확인
-    if(ignore_file.isFile && !Files.isSymbolicLink(ignore_file.toPath()) && ignore_file.canRead() && ignore_file.length() <= 1048576){
+    // 보안 향상: TOCTOU(Time-of-check to time-of-use) 취약점 방지 및 Fail-closed 정책 적용
+    val hasIgnoreFile = (dirFilesNames != null && dirFilesNames.contains(ignore_filename)) || ignore_file.exists() || Files.isSymbolicLink(ignore_file.toPath())
+
+    if(hasIgnoreFile){
+       if(!ignore_file.isFile || Files.isSymbolicLink(ignore_file.toPath()) || !ignore_file.canRead() || ignore_file.length() > 1048576){
+           throw IgnoreFileReadException("Policy file cannot be safely read: fail closed.")
+       }
        val ignored_matchers = mutableListOf<java.nio.file.PathMatcher>()
 
        ignore_file.useLines { lines ->
diff --git a/src/test/kotlin/html4tree/MainTest.kt b/src/test/kotlin/html4tree/MainTest.kt
index 5b76cc5d..f983aa1c 100644
--- a/src/test/kotlin/html4tree/MainTest.kt
+++ b/src/test/kotlin/html4tree/MainTest.kt
@@ -718,9 +718,13 @@ class MainTest {
         val ignoreDir = File(tempDir, ".html4ignore")
         ignoreDir.mkdir()
 
-        // This should not crash or parse the directory
-        val excluded = process_ignore_file(tempDir, null)
-        assertTrue(excluded.contains("index.html"))
+        var thrown = false
+        try {
+            process_ignore_file(tempDir, null)
+        } catch (e: IgnoreFileReadException) {
+            thrown = true
+        }
+        assertTrue(thrown)
     }
 
     @Test
@@ -762,10 +766,13 @@ class MainTest {
 
         File(tempDir, "test.txt").createNewFile()
 
-        // Should ignore the symlink and NOT parse it
-        val excluded = process_ignore_file(tempDir, null)
-        assertFalse(excluded.contains("test.txt"))
-        assertTrue(excluded.contains("index.html"))
+        var thrown = false
+        try {
+            process_ignore_file(tempDir, null)
+        } catch (e: IgnoreFileReadException) {
+            thrown = true
+        }
+        assertTrue(thrown)
     }
 
     @Test
@@ -777,10 +784,13 @@ class MainTest {
 
         File(tempDir, "test.txt").createNewFile()
 
-        // Should ignore the file because it's too large
-        val excluded = process_ignore_file(tempDir, null)
-        assertFalse(excluded.contains("test.txt"))
-        assertTrue(excluded.contains("index.html"))
+        var thrown = false
+        try {
+            process_ignore_file(tempDir, null)
+        } catch (e: IgnoreFileReadException) {
+            thrown = true
+        }
+        assertTrue(thrown)
     }
 
     @Test
@@ -946,4 +956,32 @@ class MainTest {
         assertTrue(content.contains("<h1>Root</h1>"))
     }
 
+    @Test
+    fun testCrawlDirectoriesThrowsIgnoreFileReadException() {
+        val subdir = File(tempDir, "ignore_exception_test")
+        subdir.mkdir()
+        val ll = LinkedList()
+        val entry = LinkedListEntry(subdir, 0)
+        entry.fileKey = "test-key"
+        ll.push(entry)
+
+        var processed = false
+        var listed = false
+
+        crawl_directories(
+            ll,
+            -1,
+            processDirectory = { _, _, _ -> processed = true },
+            processIgnoreFile = { _, _ -> throw IgnoreFileReadException("mock") },
+            listFiles = {
+                listed = true
+                emptyArray()
+            },
+            readAttributes = { _ -> createMockAttributes(isDir = true, isSymlink = false) },
+            readIdentity = { FileIdentity("test-key", true) }
+        )
+
+        assertFalse(processed, "Directory must not be processed if IgnoreFileReadException is thrown")
+    }
+
 }