CVE-2026-27730
Published: 25 February 2026
Summary
CVE-2026-27730 is a high-severity SSRF (CWE-918) vulnerability in Esm Esm.Sh. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 25.7th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified are NIST 800-53 SC-7 (Boundary Protection) and SI-10 (Information Input Validation).
Deeper analysis
CVE-2026-27730 is a Server-Side Request Forgery (SSRF) vulnerability (CWE-918) affecting esm.sh, a no-build content delivery network (CDN) used for web development. The issue resides in esm.sh's `/http(s)` fetch route and impacts versions up to and including 137. The service attempts to block requests to localhost and internal targets through hostname string validation, but this can be bypassed using DNS alias domains, enabling external attackers to force the esm.sh server to fetch internal localhost services. The vulnerability carries a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N).
Any unauthenticated attacker with network access to esm.sh can exploit this vulnerability by crafting requests to the `/http(s)` fetch route that leverage DNS aliases to evade hostname checks. Successful exploitation allows the attacker to make the esm.sh server retrieve content from internal localhost services, potentially exposing sensitive internal data or resources without requiring privileges or user interaction.
The GitHub security advisory at https://github.com/esm-dev/esm.sh/security/advisories/GHSA-p2v6-84h2-5x4r details the vulnerability, and as of the CVE publication on 2026-02-25, no patched versions of esm.sh exist. Security practitioners should monitor for updates from the esm.sh maintainers and consider avoiding the affected `/http(s)` route or implementing client-side controls to restrict fetch targets until a fix is available.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-8685
Vulnerability details
esm.sh is a no-build content delivery network (CDN) for web development. Versions up to and including 137 have an SSRF vulnerability (CWE-918) in esm.sh’s `/http(s)` fetch route. The service tries to block localhost/internal targets, but the validation is based on…
more
hostname string checks and can be bypassed using DNS alias domains. This allows an external requester to make the esm.sh server fetch internal localhost services. As of time of publication, no known patched versions exist.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
SSRF in public esm.sh /http(s) fetch endpoint directly enables T1190 by allowing unauthenticated external attackers to exploit the Internet-facing application and reach internal localhost services.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
SI-10 requires validating user-supplied hostnames and URLs in the /http(s) fetch route to prevent SSRF by ensuring they do not resolve to or target localhost/internal services despite DNS aliases.
SC-7 implements boundary protection mechanisms like network ACLs or firewalls to block esm.sh server outbound connections to localhost or internal IP ranges, mitigating SSRF exploitation.
AC-4 enforces information flow control policies to restrict fetch requests from external inputs to only approved external destinations, limiting SSRF access to internal resources.