Summary
esbuild allows arbitrary file read when running the development server on Windows
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
Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.
GHSA-G7R4-M6W7-QQQR has a CVSS score of 2.5 (Low). The vector is requires local access, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.28.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is GHSA-G7R4-M6W7-QQQR? GHSA-G7R4-M6W7-QQQR is a low-severity path traversal vulnerability in esbuild (npm), affecting versions >= 0.27.3, < 0.28.1. It is fixed in 0.28.1. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
- How severe is GHSA-G7R4-M6W7-QQQR? GHSA-G7R4-M6W7-QQQR has a CVSS score of 2.5 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of esbuild are affected by GHSA-G7R4-M6W7-QQQR? esbuild (npm) versions >= 0.27.3, < 0.28.1 is affected.
- Is there a fix for GHSA-G7R4-M6W7-QQQR? Yes. GHSA-G7R4-M6W7-QQQR is fixed in 0.28.1. Upgrade to this version or later.
- Is GHSA-G7R4-M6W7-QQQR exploitable, and should I be worried? Whether GHSA-G7R4-M6W7-QQQR is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-G7R4-M6W7-QQQR is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-G7R4-M6W7-QQQR? Upgrade
esbuildto 0.28.1 or later.