diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 76a97425..251c2fb5 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -2,3 +2,7 @@
 **Vulnerability:** Untrusted paths from API responses were directly assigned to `a.href` and used in `iframe` generation, which allows execution of malicious URIs like `javascript:` or `data:`.
 **Learning:** Even when avoiding `innerHTML`, directly setting URL-like strings to DOM attributes without protocol validation introduces XSS vectors. The payload can be executed when the link is clicked or the iframe is loaded.
 **Prevention:** Implement an `isSafeUrl` verification function to ensure the protocol is strictly `http:` or `https:` (using `new URL()`) before assigning untrusted inputs to DOM attributes like `href` or `src`.
+## 2026-07-06 - Prevent Truncation Bypass Attacks via Null Bytes
+**Vulnerability:** File validation logic (`DefaultDocumentValidationService.java`) failed to properly check for null bytes (`\u0000`) in filenames, relying on a naive replace/sanitize step in a different method that didn't prevent an attacker from bypassing the extension check.
+**Learning:** Never silently sanitize or truncate input when performing security validations like file extensions. Attackers can use null bytes (`.hwp\0.pdf`) to trick the extension check (`.pdf`) while the underlying OS or downstream services process the truncated file (`.hwp`).
+**Prevention:** Explicitly reject any input containing a null byte by throwing an exception immediately. Do not attempt to strip or sanitize it.
diff --git a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
index b898f7ac..61a704a6 100644
--- a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
+++ b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
@@ -55,6 +55,12 @@ public void validateOrThrow(MultipartFile file, PolicyOverrideRequest overrideRe
         }
 
         String fileName = file.getOriginalFilename();
+        if (fileName != null && fileName.indexOf('\0') != -1) {
+            throw new IllegalArgumentException(
+                    "Filename cannot contain null bytes."
+            );
+        }
+
         String extension = extensionOf(fileName);
         if (extension.isEmpty()) {
             throw new IllegalArgumentException("File extension is required.");
diff --git a/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java b/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java
index dc1df793..1410964e 100644
--- a/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java
+++ b/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java
@@ -401,4 +401,20 @@ void throwsWhenSha256DigestIsUnavailableForOverrideAuditFingerprint() {
             }
         }
     }
+
+    @Test
+    void rejectsFilenameWithNullByte() {
+        ConversionProperties conversionProperties = new ConversionProperties();
+        conversionProperties.setBlockedExtensions(Set.of("hwp", "hwpx"));
+        DefaultDocumentValidationService validationService = new DefaultDocumentValidationService(conversionProperties);
+
+        IllegalArgumentException ex = assertThrows(
+                IllegalArgumentException.class,
+                () -> validationService.validateOrThrow(
+                        new MockMultipartFile("file", "contract.hwp\0.pdf", "application/octet-stream", new byte[] {1})
+                )
+        );
+
+        assertEquals("Filename cannot contain null bytes.", ex.getMessage());
+    }
 }