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

7.5HIGH

Key Information:

Vendor

Sigstore

Status
Fulcio
Vendor
CVE Published:
4 December 2025

What is CVE-2025-66506?

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 Version(s)

fulcio < 1.8.3

References

https://github.com/sigstore/fulcio/security/advisories/GH...
x_refsource_CONFIRM
https://github.com/sigstore/fulcio/commit/765a0e57608b9ef...
x_refsource_MISC

CVSS V3.1

Score:
7.5
Severity:
HIGH
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

JSON
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CVE-2025-66506 : Parsing Vulnerability in Fulcio Certificate Authority from Sigstore