fix: File upload Content-Type override via extension mismatch ([GHSA-vr5f-2r24-w5hc](https://github.com/parse-community/parse-server/security/advisories/GHSA-vr5f-2r24-w5hc)) (#10384)

spec/vulnerabilities.spec.js

@@ -2769,6 +2769,69 @@ describe('Vulnerabilities', () => {
       }
     });
 
+    describe('(GHSA-vr5f-2r24-w5hc) Stored XSS via Content-Type and file extension mismatch', () => {
+      const headers = {
+        'X-Parse-Application-Id': 'test',
+        'X-Parse-REST-API-Key': 'rest',
+      };
+
+      it('overrides mismatched Content-Type with extension-derived MIME type on buffered upload', async () => {
+        const adapter = Config.get('test').filesController.adapter;
+        const spy = spyOn(adapter, 'createFile').and.callThrough();
+        const content = Buffer.from('<script>alert(1)</script>').toString('base64');
+        await request({
+          method: 'POST',
+          url: 'http://localhost:8378/1/files/evil.txt',
+          body: JSON.stringify({
+            _ApplicationId: 'test',
+            _JavaScriptKey: 'test',
+            _ContentType: 'text/html',
+            base64: content,
+          }),
+          headers,
+        });
+        expect(spy).toHaveBeenCalled();
+        const contentTypeArg = spy.calls.mostRecent().args[2];
+        expect(contentTypeArg).toBe('text/plain');
+      });
+
+      it('preserves Content-Type when no file extension is present', async () => {
+        const adapter = Config.get('test').filesController.adapter;
+        const spy = spyOn(adapter, 'createFile').and.callThrough();
+        await request({
+          method: 'POST',
+          url: 'http://localhost:8378/1/files/noextension',
+          headers: {
+            ...headers,
+            'Content-Type': 'image/png',
+          },
+          body: Buffer.from('fake png content'),
+        });
+        expect(spy).toHaveBeenCalled();
+        const contentTypeArg = spy.calls.mostRecent().args[2];
+        expect(contentTypeArg).toBe('image/png');
+      });
+
+      it('infers Content-Type from extension when none is provided', async () => {
+        const adapter = Config.get('test').filesController.adapter;
+        const spy = spyOn(adapter, 'createFile').and.callThrough();
+        const content = Buffer.from('test content').toString('base64');
+        await request({
+          method: 'POST',
+          url: 'http://localhost:8378/1/files/data.txt',
+          body: JSON.stringify({
+            _ApplicationId: 'test',
+            _JavaScriptKey: 'test',
+            base64: content,
+          }),
+          headers,
+        });
+        expect(spy).toHaveBeenCalled();
+        const contentTypeArg = spy.calls.mostRecent().args[2];
+        expect(contentTypeArg).toBe('text/plain');
+      });
+    });
+
     describe('(GHSA-9ccr-fpp6-78qf) Schema poisoning via __proto__ bypassing requestKeywordDenylist and addField CLP', () => {
       const headers = {
         'Content-Type': 'application/json',

src/Controllers/FilesController.js

@@ -21,8 +21,8 @@ export class FilesController extends AdaptableController {
     const mime = (await import('mime')).default
     if (!hasExtension && contentType && mime.getExtension(contentType)) {
       filename = filename + '.' + mime.getExtension(contentType);
-    } else if (hasExtension && !contentType) {
-      contentType = mime.getType(filename);
+    } else if (hasExtension) {
+      contentType = mime.getType(filename) || contentType;
     }
 
     if (!this.options.preserveFileName) {