esm.sh CDN service has JS Template Literal Injection in CSS-to-JavaScript
Date: 2025-11
Affected: go/github.com/esm-dev/esm.sh
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2025-65026
CWE: CWE-94
CVSS: 6.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N)

### Summary The esm.sh CDN service contains a Template Literal Injection vulnerability (CWE-94) in its CSS-to-JavaScript module conversion feature. When a CSS file is requested with the `?module` query parameter, esm.sh converts it to a JavaScript module by embedding the CSS content directly into a template literal without proper sanitization. An attacker can inject malicious JavaScript code using `${...}` expressions within CSS files, which will execute when the module is imported by victim applications. This enables Cross-Site Scripting (XSS) in browsers and Remote Code Execution (RCE) in Electron applications. **Root Cause:** The CSS module conversion logic (`router.go:1112-1119`) performs incomplete sanitization - it only checks for backticks (\`) but fails to escape template literal expressions (`${...}`), allowing arbitrary JavaScript execution when the CSS content is inserted into a template literal string. ### Details **File:** `server/router.go` **Lines:** 1112-1119 ```go // Convert CSS to JavaScript module when ?module query is present if pathKind == RawFile && strings.HasSuffix(esm.SubPath, ".css") && query.Has("module") { filename := path.Join(npmrc.StoreDir(), esm.Name(), "node_modules", esm.PkgName, esm.SubPath) css, err := os.ReadFile(filename) if err != nil { return rex.Status(500, err.Error()) } buf := bytes.NewBufferString("/* esm.sh - css module */\n") buf.WriteString("const stylesheet = new CSSStyleSheet();\n"


Mitigations:

Architectural layers (MAESTRO):