diff --git a/CHANGELOG.md b/CHANGELOG.md
index e8e8c980..6ab083fe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,9 @@
   - KPI 스냅샷 증거를 다시 불러오는 `refreshKpiEvidence` 동작 중에 "Refresh evidence" 버튼을 비활성화하고 "Refreshing..." 이라는 피드백을 제공하여 사용자의 중복 클릭을 방지했습니다.
   - 버튼 상태 변경 시 내부 DOM 구조를 보존하기 위해 `Array.from(button.childNodes)`로 원래 노드를 저장하고, 성공 및 실패 후 `finally` 블록에서 `replaceChildren(...)`으로 안전하게 복원하도록 구현했습니다.
 
+### Fixed
+- `DefaultDocumentValidationService`의 `java.nio.file.Path.of` 기반 파일명 추출 로직을 `StringUtils.cleanPath()`로 변경하여 경로 탐색(Path Traversal) 공격 방지 강화
+- `tokenFingerprint` 메서드에서 인자가 null일 때 발생하는 `NullPointerException` 방지 로직 추가
 
 ## [0.1.0] - 2026-06-25
 
diff --git a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
index 95e67228..a0178635 100644
--- a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
+++ b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
@@ -145,11 +145,12 @@ private String extensionOf(final String fileName) {
             throw new IllegalArgumentException("File name contains null byte.");
         }
 
-        java.nio.file.Path leafName = java.nio.file.Path.of(fileName.strip()).getFileName();
-        if (leafName == null) {
+        String cleanPath = org.springframework.util.StringUtils.cleanPath(fileName.strip());
+        int lastSlash = cleanPath.lastIndexOf('/');
+        String normalized = lastSlash != -1 ? cleanPath.substring(lastSlash + 1) : cleanPath;
+        if (normalized.isEmpty()) {
             return "";
         }
-        String normalized = leafName.toString();
         int lastDot = normalized.lastIndexOf('.');
         if (lastDot <= 0 || lastDot == normalized.length() - 1) {
             return "";
@@ -199,6 +200,9 @@ private byte[] computeSignature(String approverId, String extension, String secr
     }
 
     private String tokenFingerprint(String approvalToken) {
+        if (approvalToken == null) {
+            return null;
+        }
         try {
             MessageDigest digest = MessageDigest.getInstance("SHA-256");
             byte[] hashed = digest.digest(approvalToken.getBytes(StandardCharsets.UTF_8));
diff --git a/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java b/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java
index 4f27bdcc..d37b4ff5 100644
--- a/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java
+++ b/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java
@@ -32,7 +32,7 @@ void sanitizeFilenameReturnsNullWhenFilenameIsNull() throws Exception {
         DefaultDocumentValidationService validationService = new DefaultDocumentValidationService(conversionProperties);
         java.lang.reflect.Method method = DefaultDocumentValidationService.class.getDeclaredMethod("sanitizeFilename", String.class);
         method.setAccessible(true);
-        String sanitized = (String) method.invoke(validationService, new Object[] {null});
+        String sanitized = (String) method.invoke(validationService, (Object) null);
         assertNull(sanitized);
     }
 
@@ -47,6 +47,46 @@ void sanitizeFilenameReturnsCleanPathWhenNoSlashIsPresent() throws Exception {
         assertEquals("simple-file.txt", sanitized);
     }
 
+    @Test
+    void sanitizeFilenameReturnsCleanPathWhenMultipleSlashesArePresent() throws Exception {
+        ConversionProperties conversionProperties = new ConversionProperties();
+        DefaultDocumentValidationService validationService = new DefaultDocumentValidationService(conversionProperties);
+        java.lang.reflect.Method method = DefaultDocumentValidationService.class.getDeclaredMethod("sanitizeFilename", String.class);
+        method.setAccessible(true);
+        String sanitized = (String) method.invoke(validationService, "some/nested/path/file.txt");
+        assertEquals("file.txt", sanitized);
+    }
+
+    @Test
+    void sanitizeFilenameReturnsCleanPathWhenPathTraversalIsAttempted() throws Exception {
+        ConversionProperties conversionProperties = new ConversionProperties();
+        DefaultDocumentValidationService validationService = new DefaultDocumentValidationService(conversionProperties);
+        java.lang.reflect.Method method = DefaultDocumentValidationService.class.getDeclaredMethod("sanitizeFilename", String.class);
+        method.setAccessible(true);
+        String sanitized = (String) method.invoke(validationService, "../../../etc/passwd.txt");
+        assertEquals("passwd.txt", sanitized);
+    }
+
+    @Test
+    void tokenFingerprintReturnsNullWhenApprovalTokenIsNull() throws Exception {
+        ConversionProperties conversionProperties = new ConversionProperties();
+        DefaultDocumentValidationService validationService = new DefaultDocumentValidationService(conversionProperties);
+        java.lang.reflect.Method method = DefaultDocumentValidationService.class.getDeclaredMethod("tokenFingerprint", String.class);
+        method.setAccessible(true);
+        String fingerprint = (String) method.invoke(validationService, (Object) null);
+        assertNull(fingerprint);
+    }
+
+    @Test
+    void tokenFingerprintReturnsHashWhenApprovalTokenIsProvided() throws Exception {
+        ConversionProperties conversionProperties = new ConversionProperties();
+        DefaultDocumentValidationService validationService = new DefaultDocumentValidationService(conversionProperties);
+        java.lang.reflect.Method method = DefaultDocumentValidationService.class.getDeclaredMethod("tokenFingerprint", String.class);
+        method.setAccessible(true);
+        String fingerprint = (String) method.invoke(validationService, "some-token");
+        // The exact hash doesn't matter for this test, just that it's not null and of expected length
+        org.junit.jupiter.api.Assertions.assertNotNull(fingerprint);
+    }
 
     @Test
     void stripsDirectoryTraversalFromFilename() {