CVE-2025-66506: Fulcio allocates excessive memory during token parsing

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-12-04 · updated 2026-10-18
Weakness: CWE-405

Description

Fulcio is a free-to-use certificate authority for issuing code signing certificates for an OpenID Connect (OIDC) identity. Prior to 1.8.3, function identity.extractIssuerURL splits (via a call to strings.Split) its argument (which is untrusted data) on periods. As a result, in the face of a malicious request with an (invalid) OIDC identity token in the payload containing many period characters, a call to extractIssuerURL incurs allocations to the tune of O(n) bytes (where n stands for the length of the function's argument), with a constant factor of about 16. This vulnerability is fixed in 1.8.3.

Affected products

sigstore fulcio: before 1.8.3 — fixed in 1.8.3

How to fix

Upgrade to a fixed version: sigstore fulcio 1.8.3. Check the vendor advisory for mitigations.

References

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