The vulnerability lies in Fulcio’s metaRegex() function, which uses unanchored regular expression matching to validate MetaIssuer URLs. An attacker can craft a regex that bypasses this check, allowing the server to perform a GET request to any internal address. Because the HTTP method is limited to GET and the response is discarded, the attacker cannot alter state or exfiltrate data directly. However, blind SSRF can still be used to probe internal services and discover their presence, increasing the risk of future attacks. The weakness is identified as CWE‑918.

The flaw affects all installations of sigstore:fulcio running any version before 1.8.5. No approved workaround is listed, and the official fix is to upgrade to version 1.8.5 or later.

The CVSS score of 5.8 indicates moderate severity, and the EPSS score of less than 1% suggests exploitation is currently unlikely. The vulnerability is not listed in the CISA KEV catalogue, reducing the urgency for immediate action. Nevertheless, because it permits internal network discovery, administrators should treat the flaw as a potential precursor to more serious attacks and consider both patching and network containment measures.