fix(SEC-API-17): Block data: URI scheme in markdown renderer

Remove data: from allowedSchemesByTag for img tags and add transformTags
filters for both <a> and <img> elements that strip data: URI schemes
(including mixed-case and whitespace-padded variants). This prevents
XSS/CSRF attacks via embedded data URIs in markdown content.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/knowledge/utils/markdown.spec.ts

@@ -146,13 +146,12 @@ plain text code
       expect(html).toContain('alt="Alt text"');
     });
 
-    it("should allow data URIs for images", async () => {
+    it("should block data URIs for images", async () => {
       const markdown =
         "![Image](data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==)";
       const html = await renderMarkdown(markdown);
 
-      expect(html).toContain("<img");
+      expect(html).not.toContain("data:");
-      expect(html).toContain('src="data:image/png;base64');
     });
   });
 
@@ -317,6 +316,45 @@ plain text code
       expect(html).not.toContain("<svg");
       expect(html).not.toContain("<script>");
     });
+
+    it("should block data: URI scheme in image src", async () => {
+      const markdown = "![XSS](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4=)";
+      const html = await renderMarkdown(markdown);
+
+      expect(html).not.toContain("data:");
+      expect(html).not.toContain("text/html");
+    });
+
+    it("should block data: URI scheme in links", async () => {
+      const markdown = "[Click me](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4=)";
+      const html = await renderMarkdown(markdown);
+
+      expect(html).not.toContain("data:");
+      expect(html).not.toContain("text/html");
+    });
+
+    it("should block data: URI with mixed case in images", async () => {
+      const markdown =
+        "![XSS](Data:image/svg+xml;base64,PHN2Zz48c2NyaXB0PmFsZXJ0KCdYU1MnKTwvc2NyaXB0Pjwvc3ZnPg==)";
+      const html = await renderMarkdown(markdown);
+
+      expect(html).not.toContain("data:");
+      expect(html).not.toContain("Data:");
+    });
+
+    it("should block data: URI with leading whitespace", async () => {
+      const markdown = "![XSS]( data:image/png;base64,abc123)";
+      const html = await renderMarkdown(markdown);
+
+      expect(html).not.toContain("data:");
+    });
+
+    it("should block data: URI in sync renderer", () => {
+      const markdown = "![XSS](data:image/png;base64,abc123)";
+      const html = renderMarkdownSync(markdown);
+
+      expect(html).not.toContain("data:");
+    });
   });
 
   describe("Edge Cases", () => {

apps/api/src/knowledge/utils/markdown.ts

@@ -107,7 +107,7 @@ const SANITIZE_OPTIONS: sanitizeHtml.IOptions = {
   },
   allowedSchemes: ["http", "https", "mailto"],
   allowedSchemesByTag: {
-    img: ["http", "https", "data"],
+    img: ["http", "https"],
   },
   allowedClasses: {
     code: ["hljs", "language-*"],
@@ -115,9 +115,18 @@ const SANITIZE_OPTIONS: sanitizeHtml.IOptions = {
   },
   allowedIframeHostnames: [], // No iframes allowed
   // Enforce target="_blank" and rel="noopener noreferrer" for external links
+  // Block data: URIs in links and images to prevent XSS/CSRF attacks
   transformTags: {
     a: (tagName: string, attribs: sanitizeHtml.Attributes) => {
       const href = attribs.href;
+      // Strip data: URI scheme from links
+      if (href?.trim().toLowerCase().startsWith("data:")) {
+        const { href: _removed, ...safeAttribs } = attribs;
+        return {
+          tagName,
+          attribs: safeAttribs,
+        };
+      }
       if (href && (href.startsWith("http://") || href.startsWith("https://"))) {
         return {
           tagName,
@@ -133,6 +142,21 @@ const SANITIZE_OPTIONS: sanitizeHtml.IOptions = {
         attribs,
       };
     },
+    // Strip data: URI scheme from images to prevent XSS/CSRF
+    img: (tagName: string, attribs: sanitizeHtml.Attributes) => {
+      const src = attribs.src;
+      if (src?.trim().toLowerCase().startsWith("data:")) {
+        const { src: _removed, ...safeAttribs } = attribs;
+        return {
+          tagName,
+          attribs: safeAttribs,
+        };
+      }
+      return {
+        tagName,
+        attribs,
+      };
+    },
     // Disable task list checkboxes (make them read-only)
     input: (tagName: string, attribs: sanitizeHtml.Attributes) => {
       if (attribs.type === "checkbox") {