CVE-2026-27730

esm.sh has SSRF localhost/private-network bypass in /http(s) module route

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.

Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.

CVE-2026-27730 has a CVSS score of 8.6 (High). The vector is network-reachable, no 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 (0.0.0-20250616164159-0593516c4cfa); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27730? CVE-2026-27730 is a high-severity server-side request forgery (SSRF) vulnerability in github.com/esm-dev/esm.sh (go), affecting versions < 0.0.0-20250616164159-0593516c4cfa. It is fixed in 0.0.0-20250616164159-0593516c4cfa. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
2. How severe is CVE-2026-27730? CVE-2026-27730 has a CVSS score of 8.6 (High). 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.
3. Which versions of github.com/esm-dev/esm.sh are affected by CVE-2026-27730? github.com/esm-dev/esm.sh (go) versions < 0.0.0-20250616164159-0593516c4cfa is affected.
4. Is there a fix for CVE-2026-27730? Yes. CVE-2026-27730 is fixed in 0.0.0-20250616164159-0593516c4cfa. Upgrade to this version or later.
5. Is CVE-2026-27730 exploitable, and should I be worried? Whether CVE-2026-27730 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
6. What actually determines whether CVE-2026-27730 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.
7. How do I fix CVE-2026-27730? Upgrade github.com/esm-dev/esm.sh to 0.0.0-20250616164159-0593516c4cfa or later.