feat: add path validation for commit_files MCP tool (#796)

Add validatePathWithinRepo helper to ensure file paths resolve within the repository root directory. This hardens the commit_files tool by validating paths before file operations.

Changes:
- Add src/mcp/path-validation.ts with async path validation using realpath
- Update commit_files to validate all paths before reading files
- Prevent symlink-based path escapes by resolving real paths
- Add comprehensive test coverage including symlink attack scenarios

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

test/github-file-ops-path-validation.test.ts

@@ -0,0 +1,214 @@
+import { describe, expect, it, beforeAll, afterAll } from "bun:test";
+import { validatePathWithinRepo } from "../src/mcp/path-validation";
+import { resolve } from "path";
+import { mkdir, writeFile, symlink, rm, realpath } from "fs/promises";
+import { tmpdir } from "os";
+
+describe("validatePathWithinRepo", () => {
+  // Use a real temp directory for tests that need filesystem access
+  let testDir: string;
+  let repoRoot: string;
+  let outsideDir: string;
+  // Real paths after symlink resolution (e.g., /tmp -> /private/tmp on macOS)
+  let realRepoRoot: string;
+
+  beforeAll(async () => {
+    // Create test directory structure
+    testDir = resolve(tmpdir(), `path-validation-test-${Date.now()}`);
+    repoRoot = resolve(testDir, "repo");
+    outsideDir = resolve(testDir, "outside");
+
+    await mkdir(repoRoot, { recursive: true });
+    await mkdir(resolve(repoRoot, "src"), { recursive: true });
+    await mkdir(outsideDir, { recursive: true });
+
+    // Create test files
+    await writeFile(resolve(repoRoot, "file.txt"), "inside repo");
+    await writeFile(resolve(repoRoot, "src", "main.js"), "console.log('hi')");
+    await writeFile(resolve(outsideDir, "secret.txt"), "sensitive data");
+
+    // Get real paths after symlink resolution
+    realRepoRoot = await realpath(repoRoot);
+  });
+
+  afterAll(async () => {
+    // Cleanup
+    await rm(testDir, { recursive: true, force: true });
+  });
+
+  describe("valid paths", () => {
+    it("should accept simple relative paths", async () => {
+      const result = await validatePathWithinRepo("file.txt", repoRoot);
+      expect(result).toBe(resolve(realRepoRoot, "file.txt"));
+    });
+
+    it("should accept nested relative paths", async () => {
+      const result = await validatePathWithinRepo("src/main.js", repoRoot);
+      expect(result).toBe(resolve(realRepoRoot, "src/main.js"));
+    });
+
+    it("should accept paths with single dot segments", async () => {
+      const result = await validatePathWithinRepo("./src/main.js", repoRoot);
+      expect(result).toBe(resolve(realRepoRoot, "src/main.js"));
+    });
+
+    it("should accept paths that use .. but resolve inside repo", async () => {
+      // src/../file.txt resolves to file.txt which is still inside repo
+      const result = await validatePathWithinRepo("src/../file.txt", repoRoot);
+      expect(result).toBe(resolve(realRepoRoot, "file.txt"));
+    });
+
+    it("should accept absolute paths within the repo root", async () => {
+      const absolutePath = resolve(repoRoot, "file.txt");
+      const result = await validatePathWithinRepo(absolutePath, repoRoot);
+      expect(result).toBe(resolve(realRepoRoot, "file.txt"));
+    });
+
+    it("should accept the repo root itself", async () => {
+      const result = await validatePathWithinRepo(".", repoRoot);
+      expect(result).toBe(realRepoRoot);
+    });
+
+    it("should handle new files (non-existent) in valid directories", async () => {
+      const result = await validatePathWithinRepo("src/newfile.js", repoRoot);
+      // For non-existent files, we validate the parent but return the initial path
+      // (can't realpath a file that doesn't exist yet)
+      expect(result).toBe(resolve(repoRoot, "src/newfile.js"));
+    });
+  });
+
+  describe("path traversal attacks", () => {
+    it("should reject simple parent directory traversal", async () => {
+      await expect(
+        validatePathWithinRepo("../outside/secret.txt", repoRoot),
+      ).rejects.toThrow(/resolves outside the repository root/);
+    });
+
+    it("should reject deeply nested parent directory traversal", async () => {
+      await expect(
+        validatePathWithinRepo("../../../etc/passwd", repoRoot),
+      ).rejects.toThrow(/resolves outside the repository root/);
+    });
+
+    it("should reject traversal hidden within path", async () => {
+      await expect(
+        validatePathWithinRepo("src/../../outside/secret.txt", repoRoot),
+      ).rejects.toThrow(/resolves outside the repository root/);
+    });
+
+    it("should reject traversal at the end of path", async () => {
+      await expect(
+        validatePathWithinRepo("src/../..", repoRoot),
+      ).rejects.toThrow(/resolves outside the repository root/);
+    });
+
+    it("should reject absolute paths outside the repo root", async () => {
+      await expect(
+        validatePathWithinRepo("/etc/passwd", repoRoot),
+      ).rejects.toThrow(/resolves outside the repository root/);
+    });
+
+    it("should reject absolute paths to sibling directories", async () => {
+      await expect(
+        validatePathWithinRepo(resolve(outsideDir, "secret.txt"), repoRoot),
+      ).rejects.toThrow(/resolves outside the repository root/);
+    });
+  });
+
+  describe("symlink attacks", () => {
+    it("should reject symlinks pointing outside the repo", async () => {
+      // Create a symlink inside the repo that points to a file outside
+      const symlinkPath = resolve(repoRoot, "evil-link");
+      await symlink(resolve(outsideDir, "secret.txt"), symlinkPath);
+
+      try {
+        // The symlink path looks like it's inside the repo, but points outside
+        await expect(
+          validatePathWithinRepo("evil-link", repoRoot),
+        ).rejects.toThrow(/resolves outside the repository root/);
+      } finally {
+        await rm(symlinkPath, { force: true });
+      }
+    });
+
+    it("should reject symlinks to parent directories", async () => {
+      // Create a symlink to the parent directory
+      const symlinkPath = resolve(repoRoot, "parent-link");
+      await symlink(testDir, symlinkPath);
+
+      try {
+        await expect(
+          validatePathWithinRepo("parent-link/outside/secret.txt", repoRoot),
+        ).rejects.toThrow(/resolves outside the repository root/);
+      } finally {
+        await rm(symlinkPath, { force: true });
+      }
+    });
+
+    it("should accept symlinks that resolve within the repo", async () => {
+      // Create a symlink inside the repo that points to another file inside
+      const symlinkPath = resolve(repoRoot, "good-link");
+      await symlink(resolve(repoRoot, "file.txt"), symlinkPath);
+
+      try {
+        const result = await validatePathWithinRepo("good-link", repoRoot);
+        // Should resolve to the actual file location
+        expect(result).toBe(resolve(realRepoRoot, "file.txt"));
+      } finally {
+        await rm(symlinkPath, { force: true });
+      }
+    });
+
+    it("should reject directory symlinks that escape the repo", async () => {
+      // Create a symlink to outside directory
+      const symlinkPath = resolve(repoRoot, "escape-dir");
+      await symlink(outsideDir, symlinkPath);
+
+      try {
+        await expect(
+          validatePathWithinRepo("escape-dir/secret.txt", repoRoot),
+        ).rejects.toThrow(/resolves outside the repository root/);
+      } finally {
+        await rm(symlinkPath, { force: true });
+      }
+    });
+  });
+
+  describe("edge cases", () => {
+    it("should handle empty path (current directory)", async () => {
+      const result = await validatePathWithinRepo("", repoRoot);
+      expect(result).toBe(realRepoRoot);
+    });
+
+    it("should handle paths with multiple consecutive slashes", async () => {
+      const result = await validatePathWithinRepo("src//main.js", repoRoot);
+      expect(result).toBe(resolve(realRepoRoot, "src/main.js"));
+    });
+
+    it("should handle paths with trailing slashes", async () => {
+      const result = await validatePathWithinRepo("src/", repoRoot);
+      expect(result).toBe(resolve(realRepoRoot, "src"));
+    });
+
+    it("should reject prefix attack (repo root as prefix but not parent)", async () => {
+      // Create a sibling directory with repo name as prefix
+      const evilDir = repoRoot + "-evil";
+      await mkdir(evilDir, { recursive: true });
+      await writeFile(resolve(evilDir, "file.txt"), "evil");
+
+      try {
+        await expect(
+          validatePathWithinRepo(resolve(evilDir, "file.txt"), repoRoot),
+        ).rejects.toThrow(/resolves outside the repository root/);
+      } finally {
+        await rm(evilDir, { recursive: true, force: true });
+      }
+    });
+
+    it("should throw error for non-existent repo root", async () => {
+      await expect(
+        validatePathWithinRepo("file.txt", "/nonexistent/repo"),
+      ).rejects.toThrow(/does not exist/);
+    });
+  });
+});