diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 76a974257..c3f335050 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -2,3 +2,8 @@
 **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`.
+
+## 2026-07-08 - Prevent Null Byte Injection in File Validation
+**Vulnerability:** The document validation service relied on the original filename without verifying the presence of null bytes (`\u0000`). This allows attackers to bypass file extension checks via truncation (e.g., `malicious.hwp\u0000.pdf`).
+**Learning:** Checking the file extension using standard string operations can be bypassed if the underlying system API (like a C-based filesystem or a library handling byte arrays) truncates the string at the first null byte.
+**Prevention:** Explicitly check for the presence of null bytes (`\u0000`) in the filename as early as possible and throw an exception to reject the input entirely, rather than attempting to sanitize it.
diff --git a/pom.xml b/pom.xml
index 6b7e40df2..6a8f6d9b0 100644
--- a/pom.xml
+++ b/pom.xml
@@ -26,6 +26,31 @@
     <pdfjs.dist.version>4.10.38</pdfjs.dist.version>
   </properties>
 
+  <dependencyManagement>
+    <dependencies>
+      <dependency>
+        <groupId>org.bouncycastle</groupId>
+        <artifactId>bcprov-jdk18on</artifactId>
+        <version>1.84</version>
+      </dependency>
+      <dependency>
+        <groupId>org.bouncycastle</groupId>
+        <artifactId>bcpkix-jdk18on</artifactId>
+        <version>1.84</version>
+      </dependency>
+      <dependency>
+        <groupId>org.bouncycastle</groupId>
+        <artifactId>bcutil-jdk18on</artifactId>
+        <version>1.84</version>
+      </dependency>
+      <dependency>
+        <groupId>org.bouncycastle</groupId>
+        <artifactId>bcjmail-jdk18on</artifactId>
+        <version>1.84</version>
+      </dependency>
+    </dependencies>
+  </dependencyManagement>
+
   <dependencies>
     <dependency>
       <groupId>org.springframework.boot</groupId>
diff --git a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
index b898f7ac8..a7c7e6c0b 100644
--- a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
+++ b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentValidationService.java
@@ -49,12 +49,18 @@ public void validateOrThrow(MultipartFile file) {
      * {@inheritDoc}
      */
     @Override
-    public void validateOrThrow(MultipartFile file, PolicyOverrideRequest overrideRequest) {
+    public void validateOrThrow(final MultipartFile file, final PolicyOverrideRequest overrideRequest) {
         if (file == null || file.isEmpty()) {
             throw new IllegalArgumentException("File is required.");
         }
 
         String fileName = file.getOriginalFilename();
+        if (fileName != null && fileName.indexOf('\u0000') >= 0) {
+            throw new IllegalArgumentException(
+                    "File name contains invalid characters."
+            );
+        }
+
         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 dc1df7934..d0eb7c4ad 100644
--- a/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java
+++ b/src/test/java/com/clearfolio/viewer/service/DefaultDocumentValidationServiceTest.java
@@ -283,6 +283,22 @@ void rejectsMultipartWithNullOriginalFilename() {
         assertEquals("File extension is required.", ex.getMessage());
     }
 
+    @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\u0000.pdf", "application/octet-stream", new byte[] {1})
+                )
+        );
+
+        assertEquals("File name contains invalid characters.", ex.getMessage());
+    }
+
     @Test
     void rejectsFilenameEndingWithDot() {
         ConversionProperties conversionProperties = new ConversionProperties();