CVE-2025-66506: GO-2025-4193 - Fulcio allocates excessive memory during token parsing in github.com/sigstore/fulcio

CVE-2025-66506 · Severity: high · CVSS 7.5 · Published 2025-12-04

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.

Technologies: github.com/sigstore/fulcio (Go). Vendors: Go, Linuxfoundation.

Quick answers

Is CVE-2025-66506 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2025-66506 being exploited in the wild.
How severe is CVE-2025-66506?
High severity, with a CVSS score of 7.5, published 4 December 2025.
Which products and versions are affected?
github.com/sigstore/fulcio (Go) SEMVER:introduced=0,fixed=1.8.3.
Is there a fix for CVE-2025-66506?
Yes. github.com/sigstore/fulcio (Go): upgrade to 1.8.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2025-66506?
Check whether you run github.com/sigstore/fulcio (Go) or Linuxfoundation Fulcio at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • github.com/sigstore/fulcio (Go): upgrade to 1.8.3 or later
    go get github.com/sigstore/fulcio@v1.8.3

Affected products

  • github.com/sigstore/fulcio (Go) SEMVER:introduced=0,fixed=1.8.3
  • Linuxfoundation Fulcio

Identifiers

  • CVE-2025-66506
  • GHSA-f83f-xpx7-ffpw
  • GO-2025-4193

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