esm.sh is a no-build CDN service widely used in web development to serve JavaScript modules without requiring local builds. Versions up to and including 137 contain a Server-Side Request Forgery (SSRF) vulnerability identified as CVE-2026-27730 (CWE-918). The vulnerability resides in the /http(s) fetch route, which allows external users to request esm.sh to fetch arbitrary URLs. To prevent abuse, esm.sh attempts to block requests targeting localhost or internal network addresses by performing hostname string checks. However, this validation is insufficient and can be bypassed by using DNS alias domains that resolve to internal IP addresses, effectively tricking the service into fetching internal resources. This can lead to unauthorized access to internal services that are otherwise inaccessible externally, potentially exposing sensitive data or internal APIs. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. No patches or fixed versions have been published at the time of disclosure, and no known exploits have been reported in the wild. The CVSS v3.0 score of 8.6 reflects a high severity due to the ease of exploitation and the significant confidentiality impact. The vulnerability affects all esm.sh users running vulnerable versions, especially those relying on the /http(s) fetch route for dynamic content fetching.

The SSRF vulnerability in esm.sh can have severe consequences for organizations using this CDN service. Attackers can leverage the vulnerability to access internal network resources, including metadata services, internal APIs, databases, or other sensitive endpoints that are not exposed externally. This can lead to data leakage, unauthorized information gathering, and potentially facilitate further internal network compromise. Since esm.sh is a CDN used globally by web developers, the attack surface is broad. The confidentiality impact is high as internal services may contain sensitive configuration, credentials, or proprietary data. Although the vulnerability does not directly impact integrity or availability, the information gained through SSRF can be used in chained attacks to escalate privileges or disrupt services. Organizations relying on esm.sh for production or development environments are at risk, especially if internal services are accessible from the esm.sh server network. The lack of a patch increases the urgency for mitigation. The vulnerability could be exploited by remote attackers without authentication or user interaction, increasing the likelihood of exploitation once publicized.

Until an official patch is released, organizations should implement several practical mitigations: 1) Avoid using esm.sh versions up to 137 in production environments, or disable the /http(s) fetch route if possible. 2) Employ network-level controls such as firewall rules or egress filtering on servers running esm.sh to restrict outbound requests to trusted external IPs only, preventing internal network access via SSRF. 3) Use DNS filtering or monitoring to detect and block suspicious DNS alias domains that could be used to bypass hostname checks. 4) Monitor network traffic and logs for unusual or unexpected requests originating from esm.sh servers targeting internal IP ranges. 5) Consider isolating esm.sh usage in segmented network environments with limited access to sensitive internal services. 6) Engage with the esm-dev project or community to track patch releases and apply updates promptly once available. 7) Review internal services for exposure risk and implement authentication and authorization controls to limit damage if accessed via SSRF. These targeted mitigations go beyond generic advice by focusing on network controls, DNS monitoring, and service isolation specific to the nature of this SSRF vulnerability.