diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 323abb9..110dbdf 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -78,3 +78,7 @@
 **Vulnerability:** The application lists all files in a directory, including hidden files (those starting with `.`), which could inadvertently expose sensitive information like `.env`, `.git`, or `.ssh` directories.
 **Learning:** Default directory listing implementations without hidden file filtering can lead to information disclosure vulnerabilities when serving directories containing configuration or sensitive files.
 **Prevention:** Automatically exclude hidden files (files starting with `.`) from the generated directory listing by default.
+## 2023-10-27 - [Information Exposure via Case-Sensitive Matches]
+**Vulnerability:** The `process_ignore_file` routine used case-sensitive matching for sensitive files (like `id_rsa`, `secrets.yml`), allowing case variations (e.g., `ID_RSA`) to bypass filtering and be included in the generated HTML.
+**Learning:** Exact string matching is fragile for file systems that are case-insensitive or for users who create mixed-case variations of sensitive files.
+**Prevention:** Always normalize file names (e.g., using `toLowerCase()`) when matching against exclusion lists, taking care to preserve expected outputs like exact sizes in tests or edge cases (like `.DS_Store`).
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index c606e58..2da5302 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -222,8 +222,17 @@ fun process_ignore_file(curr_dir: File, dirFilesNames: Array<String>? = null): S
        files_to_exclude.add("index.html")
 
     // 보안 향상: 민감한 시스템, 설정, 시크릿 파일을 디렉토리 목록에서 기본적으로 제외하여 정보 노출(Information Exposure) 방지
-    val defaultSensitiveFiles = listOf(".git", ".env", ".ssh", ".htpasswd", ".htaccess", "id_rsa", "id_ed25519", "secrets.yml", ".html4ignore", ".DS_Store", ".aws", ".kube", ".npmrc", ".gnupg", "config.json", "credentials.json")
+    val defaultSensitiveFiles = listOf(".git", ".env", ".ssh", ".htpasswd", ".htaccess", "id_rsa", "id_ed25519", "secrets.yml", ".html4ignore", ".ds_store", ".aws", ".kube", ".npmrc", ".gnupg", "config.json", "credentials.json")
     files_to_exclude.addAll(defaultSensitiveFiles)
+    files_to_exclude.add(".DS_Store") // Ensure original case is also explicitly excluded
+
+    // 🛡️ Sentinel: Make sensitive file checks case-insensitive
+    (dirFilesNames ?: curr_dir.list())?.forEach {
+        val lowerCaseName = it.toLowerCase()
+        if (lowerCaseName in defaultSensitiveFiles) {
+            files_to_exclude.add(it)
+        }
+    }
 
     // 보안 향상: .env, .git 등 민감한 정보가 포함될 수 있는 숨김 파일(.으로 시작하는 모든 항목)을 기본적으로 노출하지 않도록 제외 (정보 노출 방지)
     (dirFilesNames ?: curr_dir.list())?.forEach {
diff --git a/src/test/kotlin/html4tree/MainTest.kt b/src/test/kotlin/html4tree/MainTest.kt
index a4d2ba5..3ec6a16 100644
--- a/src/test/kotlin/html4tree/MainTest.kt
+++ b/src/test/kotlin/html4tree/MainTest.kt
@@ -267,7 +267,7 @@ class MainTest {
     fun testProcessIgnoreFileNoIgnore() {
         val excluded = process_ignore_file(tempDir, null)
         assertTrue(excluded.contains("index.html"))
-        assertEquals(17, excluded.size) // index.html + 16 default sensitive files
+        assertEquals(18, excluded.size) // updated // index.html + 16 default sensitive files
     }
 
     @Test
@@ -277,7 +277,7 @@ class MainTest {
 
         val excluded = process_ignore_file(tempDir, arrayOf("test1.txt", "test3.txt"))
         assertTrue(excluded.contains("index.html"))
-        assertEquals(18, excluded.size) // index.html + 16 default sensitive + test1.txt
+        assertEquals(19, excluded.size)
     }
 
     @Test
diff --git a/src/test/kotlin/html4tree/SecurityTest.kt b/src/test/kotlin/html4tree/SecurityTest.kt
new file mode 100644
index 0000000..02bbb6a
--- /dev/null
+++ b/src/test/kotlin/html4tree/SecurityTest.kt
@@ -0,0 +1,27 @@
+package html4tree
+
+import org.junit.Test
+import org.junit.Assert.*
+import java.io.File
+import java.nio.file.Files
+
+class SecurityTest {
+    @Test
+    fun testSensitiveFilesCaseInsensitive() {
+        val tempDir = Files.createTempDirectory("sectest").toFile()
+        try {
+            val file1 = File(tempDir, "ID_RSA")
+            file1.createNewFile()
+
+            val excluded = process_ignore_file(tempDir, null)
+            assertTrue("ID_RSA should be excluded", excluded.contains("ID_RSA"))
+
+            process_dir(tempDir)
+
+            val indexContent = File(tempDir, "index.html").readText()
+            assertFalse("ID_RSA should be excluded from index.html", indexContent.contains("ID_RSA"))
+        } finally {
+            tempDir.deleteRecursively()
+        }
+    }
+}