fix(a2a-core): eliminate ReDoS in Message code-block parsing

libs/a2a-core/src/react/components/Message/Message.tsx

@@ -92,6 +92,61 @@ marked.use({
   },
 });
 
+interface FencedCodeBlock {
+  index: number;
+  endIndex: number;
+  language: string;
+  code: string;
+}
+
+// Extract fenced code blocks (```lang\n...\n```) using a single linear scan.
+// This intentionally avoids a lazy `([\s\S]*?)` regular expression, which runs
+// in polynomial time (ReDoS) on untrusted content containing many unterminated
+// code fences. `String.indexOf` keeps the whole pass linear in the input length.
+// Exported for unit testing of the ReDoS-safe extraction.
+export function extractFencedCodeBlocks(content: string): FencedCodeBlock[] {
+  const blocks: FencedCodeBlock[] = [];
+  const fence = '```';
+  let searchFrom = 0;
+
+  while (searchFrom < content.length) {
+    const fenceStart = content.indexOf(fence, searchFrom);
+    if (fenceStart === -1) {
+      break;
+    }
+
+    // The opening fence's info string (language) runs up to the next newline.
+    const infoEnd = content.indexOf('\n', fenceStart + fence.length);
+    if (infoEnd === -1) {
+      break;
+    }
+
+    // A valid opening fence only carries word characters as its language.
+    const language = content.slice(fenceStart + fence.length, infoEnd);
+    if (!/^\w*$/.test(language)) {
+      searchFrom = fenceStart + 1;
+      continue;
+    }
+
+    const codeStart = infoEnd + 1;
+    const closingFence = content.indexOf(`\n${fence}`, codeStart);
+    if (closingFence === -1) {
+      break;
+    }
+
+    const endIndex = closingFence + 1 + fence.length;
+    blocks.push({
+      index: fenceStart,
+      endIndex,
+      language,
+      code: content.slice(codeStart, closingFence),
+    });
+    searchFrom = endIndex;
+  }
+
+  return blocks;
+}
+
 const useStyles = makeStyles({
   '@keyframes fadeIn': {
     '0%': { opacity: 0, transform: 'translateY(10px)' },
@@ -446,9 +501,9 @@ function MessageComponent({
       let contentToDownload = message.metadata?.rawContent;
 
       if (!contentToDownload) {
-        const codeBlockMatch = message.content.match(/```[\w]*\n([\s\S]*?)\n```/);
-        if (codeBlockMatch) {
-          contentToDownload = codeBlockMatch[1];
+        const codeBlocks = extractFencedCodeBlocks(message.content);
+        if (codeBlocks.length > 0) {
+          contentToDownload = codeBlocks[0].code;
         } else {
           contentToDownload = message.content.replace(/^\*\*.*?\*\*\n\n/, '');
         }
@@ -527,24 +582,24 @@ function MessageComponent({
     // For regular markdown content, we need to parse and render code blocks with headers
     const processedContent = useMemo(() => {
       // Extract code blocks and render them separately
-      const codeBlockRegex = /```(\w*)\n([\s\S]*?)\n```/g;
+      const content = message.content;
+      const codeBlocks = extractFencedCodeBlocks(content);
       let lastIndex = 0;
       const elements: React.ReactNode[] = [];
-      let match;
 
-      while ((match = codeBlockRegex.exec(message.content)) !== null) {
+      for (const block of codeBlocks) {
         // Add content before the code block
-        if (match.index > lastIndex) {
-          const textContent = message.content.slice(lastIndex, match.index);
+        if (block.index > lastIndex) {
+          const textContent = content.slice(lastIndex, block.index);
           const html = sanitizeHtml(marked.parse(textContent, { gfm: true, breaks: true }) as string);
           elements.push(
             <div key={`text-${lastIndex}`} dangerouslySetInnerHTML={{ __html: html }} />
           );
         }
 
         // Add the code block with header
-        const language = match[1] || '';
-        const code = match[2];
+        const language = block.language;
+        const code = block.code;
         let highlighted = code;
 
         if (language && Prism.languages[language]) {
@@ -556,7 +611,7 @@ function MessageComponent({
         }
 
         elements.push(
-          <div key={`code-${match.index}`} className={styles.codeBlockWrapper}>
+          <div key={`code-${block.index}`} className={styles.codeBlockWrapper}>
             <CodeBlockHeader language={language} code={code} />
             <div className={styles.codeBlockContent}>
               <pre>
@@ -569,19 +624,19 @@ function MessageComponent({
           </div>
         );
 
-        lastIndex = match.index + match[0].length;
+        lastIndex = block.endIndex;
       }
 
       // Add any remaining content after the last code block
-      if (lastIndex < message.content.length) {
-        const remainingContent = message.content.slice(lastIndex);
+      if (lastIndex < content.length) {
+        const remainingContent = content.slice(lastIndex);
         const html = sanitizeHtml(marked.parse(remainingContent, { gfm: true, breaks: true }) as string);
         elements.push(<div key={`text-${lastIndex}`} dangerouslySetInnerHTML={{ __html: html }} />);
       }
 
       // If no code blocks were found, just return the parsed markdown
       if (elements.length === 0) {
-        const html = sanitizeHtml(marked.parse(message.content, { gfm: true, breaks: true }) as string);
+        const html = sanitizeHtml(marked.parse(content, { gfm: true, breaks: true }) as string);
         return <div dangerouslySetInnerHTML={{ __html: html }} />;
       }
 

libs/a2a-core/src/react/components/Message/__tests__/Message.codeblock.test.tsx

@@ -1,7 +1,7 @@
 import { render, screen, fireEvent } from '@testing-library/react';
 import { vi, describe, beforeEach, it, expect } from 'vitest';
 import '@testing-library/jest-dom';
-import { Message } from '../Message';
+import { Message, extractFencedCodeBlocks } from '../Message';
 import type { Message as MessageType } from '../../../types';
 
 // Mock clipboard API
@@ -138,4 +138,75 @@ greeting = "Hello"
     // Check for copy button
     expect(screen.getByRole('button', { name: /copy/i })).toBeInTheDocument();
   });
+
+  it('should render text that follows a code block', () => {
+    const message: MessageType = {
+      id: '1',
+      content: '```javascript\nconst x = 1;\n```\n\nTrailing explanation text.',
+      sender: 'assistant',
+      timestamp: new Date(),
+      status: 'sent',
+    };
+
+    render(<Message message={message} />);
+
+    // The code block header is rendered...
+    expect(screen.getByText('javascript')).toBeInTheDocument();
+    // ...and the text after the closing fence is preserved.
+    expect(screen.getByText('Trailing explanation text.')).toBeInTheDocument();
+  });
+});
+
+describe('extractFencedCodeBlocks', () => {
+  it('extracts a single code block with language', () => {
+    const blocks = extractFencedCodeBlocks('```javascript\nconst x = 1;\n```');
+    expect(blocks).toHaveLength(1);
+    expect(blocks[0].language).toBe('javascript');
+    expect(blocks[0].code).toBe('const x = 1;');
+    expect(blocks[0].index).toBe(0);
+  });
+
+  it('extracts a code block without a language', () => {
+    const blocks = extractFencedCodeBlocks('```\nplain code\n```');
+    expect(blocks).toHaveLength(1);
+    expect(blocks[0].language).toBe('');
+    expect(blocks[0].code).toBe('plain code');
+  });
+
+  it('extracts multiple code blocks with the text between them intact', () => {
+    const content = 'intro\n```js\na();\n```\nmiddle\n```py\nb()\n```';
+    const blocks = extractFencedCodeBlocks(content);
+    expect(blocks.map((b) => b.language)).toEqual(['js', 'py']);
+    expect(blocks.map((b) => b.code)).toEqual(['a();', 'b()']);
+    // The gap between the two blocks maps back to the original 'middle' text.
+    expect(content.slice(blocks[0].endIndex, blocks[1].index)).toBe('\nmiddle\n');
+  });
+
+  it('ignores an unterminated fence', () => {
+    expect(extractFencedCodeBlocks('```js\nno closing fence here')).toEqual([]);
+  });
+
+  it('does not treat a fence with a non-word info string as an opening', () => {
+    // The original regex required the language to be word characters only.
+    expect(extractFencedCodeBlocks('```not a lang\ncode\n```')).toEqual([]);
+  });
+
+  it('stops the first code block at the first closing fence (lazy match parity)', () => {
+    const blocks = extractFencedCodeBlocks('```\nfirst\n```\nsecond\n```');
+    expect(blocks[0].code).toBe('first');
+  });
+
+  it('handles pathological unterminated fences in linear time (ReDoS guard)', () => {
+    // The exact shape flagged by CodeQL js/polynomial-redos: a string starting
+    // with '```\n' followed by many repetitions of '```\na' and no closing
+    // fence. A polynomial-time regex scans to the end from every fence start
+    // (O(n^2)); the linear scanner returns almost instantly.
+    const content = `\`\`\`\n${'```\na'.repeat(200000)}`;
+    const start = performance.now();
+    const blocks = extractFencedCodeBlocks(content);
+    const elapsed = performance.now() - start;
+
+    expect(blocks).toEqual([]);
+    expect(elapsed).toBeLessThan(1000);
+  });
 });