security: forbid \\ in local dev server requests

Author: evanw

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## Unreleased
 
+* Disallow `\\` in local development server HTTP requests ([GHSA-g7r4-m6w7-qqqr](https://github.com/evanw/esbuild/security/advisories/GHSA-g7r4-m6w7-qqqr))
+
+    This release fixes a security issue where HTTP requests to esbuild's local development server could traverse outside of the serve directory on Windows using a `\\` backslash character. It happened due to the use of Go's `path.Clean()` function, which only handles Unix-style `/` characters. HTTP requests with paths containing `\\` are no longer allowed.
+
+    Thanks to [@dellalibera](https://github.com/dellalibera) for reporting this issue.
+
 * Avoid inlining `using` and `await using` declarations ([#4482](https://github.com/evanw/esbuild/issues/4482))
 
     Previously esbuild's minifier sometimes incorrectly inlined `using` and `await using` declarations into subsequent uses of that declaration, which then fails to dispose of the resource correctly. This bug happened because inlining was done for `let` and `const` declarations by avoiding doing it for `var` declarations, which no longer worked when more declaration types were added. Here's an example:

pkg/api/serve_other.go

@@ -154,6 +154,14 @@ func (h *apiHandler) ServeHTTP(res http.ResponseWriter, req *http.Request) {
 		}
 	}
 
+	// All requests containing Windows-style path separators are invalid
+	if strings.ContainsRune(req.URL.Path, '\\') {
+		go h.notifyRequest(time.Since(start), req, http.StatusBadRequest)
+		res.WriteHeader(http.StatusBadRequest)
+		maybeWriteResponseBody([]byte("400 - Bad Request"))
+		return
+	}
+
 	// Special-case the esbuild event stream
 	if req.Method == "GET" && req.URL.Path == "/esbuild" && req.Header.Get("Accept") == "text/event-stream" {
 		h.serveEventStream(start, req, res)

scripts/js-api-tests.js

@@ -5602,6 +5602,62 @@ let serveTests = {
       await context.dispose();
     }
   },
+
+  // https://github.com/evanw/esbuild/security/advisories/GHSA-g7r4-m6w7-qqqr
+  async serveDirectoryTraversalUsingBackslash({ esbuild, testDir }) {
+    const failure = path.join(testDir, 'failure.txt')
+    const servedir = path.join(testDir, 'root')
+    const input = path.join(servedir, 'in.ts')
+    await mkdirAsync(servedir)
+    await writeFileAsync(failure, `TEST FAILURE`)
+    await writeFileAsync(input, `console.log(123)`)
+
+    let onRequest;
+
+    const context = await esbuild.context({
+      entryPoints: [input],
+      format: 'esm',
+      outdir: servedir,
+      write: false,
+    });
+    try {
+      const result = await context.serve({
+        host: '127.0.0.1',
+        servedir,
+        onRequest: args => onRequest(args),
+      })
+      assert.deepStrictEqual(result.hosts, ['127.0.0.1']);
+      assert.strictEqual(typeof result.port, 'number');
+
+      // GET ..\failure.txt
+      {
+        const singleRequestPromise = new Promise(resolve => { onRequest = resolve });
+        try {
+          const buffer = await fetch(result.hosts[0], result.port, '/..\\failure.txt')
+          throw new Error('Unexpected response: ' + buffer)
+        } catch (e) {
+          if (e.statusCode !== 400) throw e
+        }
+        const args = await singleRequestPromise
+        if (args.status !== 400) throw new Error('Unexpected args: ' + JSON.stringify(args))
+      }
+
+      // GET ..%5cfailure.txt
+      {
+        const singleRequestPromise = new Promise(resolve => { onRequest = resolve });
+        try {
+          const buffer = await fetch(result.hosts[0], result.port, '/..%5cfailure.txt')
+          throw new Error('Unexpected response: ' + buffer)
+        } catch (e) {
+          if (e.statusCode !== 400) throw e
+        }
+        const args = await singleRequestPromise
+        if (args.status !== 400) throw new Error('Unexpected args: ' + JSON.stringify(args))
+      }
+    } finally {
+      await context.dispose();
+    }
+  },
 }
 
 async function futureSyntax(esbuild, js, targetBelow, targetAbove) {