fix(coding-agent): sanitize markdown links in exported session HTML (#3532)

marked v15 does not filter dangerous URL protocols. The default link
renderer passes href values through verbatim, so markdown like
`[click](javascript:alert(1))` renders as a clickable XSS link in
shared/exported session HTML.

Add custom link and image renderers that:
- Block javascript:, vbscript:, and data: protocol URLs
- Escape href/title/alt attributes via escapeHtml()

Also escape img.mimeType in session image rendering to prevent
attribute breakout from crafted session JSONL.

Fixes #3531

packages/coding-agent/src/core/export-html/template.js

@@ -881,7 +881,7 @@
           const images = getResultImages();
           if (images.length === 0) return '';
           return '<div class="tool-images">' +
-            images.map(img => `<img src="data:${img.mimeType};base64,${img.data}" class="tool-image" />`).join('') +
+            images.map(img => `<img src="data:${escapeHtml(img.mimeType || 'image/png')};base64,${img.data}" class="tool-image" />`).join('') +
             '</div>';
         };
 
@@ -1148,7 +1148,7 @@
               if (images.length > 0) {
                 html += '<div class="message-images">';
                 for (const img of images) {
-                  html += `<img src="data:${img.mimeType};base64,${img.data}" class="message-image" />`;
+                  html += `<img src="data:${escapeHtml(img.mimeType || 'image/png')};base64,${img.data}" class="message-image" />`;
                 }
                 html += '</div>';
               }
@@ -1491,6 +1491,32 @@
           }
         },
         renderer: {
+          // Sanitize link URLs to prevent javascript:/vbscript:/data: XSS
+          link(token) {
+            const href = (token.href || '').trim();
+            if (/^\s*(javascript|vbscript|data):/i.test(href)) {
+              return this.parser.parseInline(token.tokens);
+            }
+            let out = '<a href="' + escapeHtml(href) + '"';
+            if (token.title) {
+              out += ' title="' + escapeHtml(token.title) + '"';
+            }
+            out += '>' + this.parser.parseInline(token.tokens) + '</a>';
+            return out;
+          },
+          // Sanitize image src URLs
+          image(token) {
+            const href = (token.href || '').trim();
+            if (/^\s*(javascript|vbscript|data):/i.test(href)) {
+              return escapeHtml(token.text || '');
+            }
+            let out = '<img src="' + escapeHtml(href) + '" alt="' + escapeHtml(token.text || '') + '"';
+            if (token.title) {
+              out += ' title="' + escapeHtml(token.title) + '"';
+            }
+            out += '>';
+            return out;
+          },
           // Code blocks: syntax highlight, no HTML escaping
           code(token) {
             const code = token.text;

packages/coding-agent/test/export-html-xss.test.ts

@@ -0,0 +1,28 @@
+import { readFileSync } from "fs";
+import { describe, expect, it } from "vitest";
+
+describe("export HTML markdown link sanitization", () => {
+	const templateJs = readFileSync(new URL("../src/core/export-html/template.js", import.meta.url), "utf-8");
+
+	it("overrides the marked link renderer to block javascript: protocol", () => {
+		// The custom link renderer must check for dangerous protocols
+		expect(templateJs).toMatch(/link\s*\(\s*token\s*\)/);
+		expect(templateJs).toMatch(/javascript/i);
+		expect(templateJs).toMatch(/vbscript/i);
+	});
+
+	it("overrides the marked image renderer to block javascript: protocol", () => {
+		expect(templateJs).toMatch(/image\s*\(\s*token\s*\)/);
+	});
+
+	it("escapes href attributes in the custom link renderer", () => {
+		// The link renderer must escape href values to prevent attribute breakout
+		expect(templateJs).toMatch(/escapeHtml\(href\)/);
+	});
+
+	it("escapes image mimeType attributes", () => {
+		// Image mimeType must be escaped to prevent attribute breakout
+		expect(templateJs).not.toMatch(/\$\{img\.mimeType\}/);
+		expect(templateJs).toMatch(/escapeHtml\(img\.mimeType/);
+	});
+});