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

Source: https://junglewise.ai/threats/cve-2025-66506-fulcio-allocates-excessive-memory-during-token-parsing  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-12-04  
License: CC BY 4.0 with a link back

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)](https://junglewise.ai/threats/technologies/github-com-sigstore-fulcio). Vendors: [Go](https://junglewise.ai/threats/vendors/go), [Linuxfoundation](https://junglewise.ai/threats/vendors/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

  ```sh
  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

## Related threats

- [Sigstore Fulcio SSRF and Token Leakage in OIDC Discovery](https://junglewise.ai/threats/cve-2026-49478-sigstore-fulcio-ssrf-and-token-leakage-in-oidc-discovery)
- [Fulcio is vulnerable to Server-Side Request Forgery (SSRF) via MetaIssuer Regex Bypass](https://junglewise.ai/threats/cve-2026-22772-fulcio-is-vulnerable-to-server-side-request-forgery-ssrf-via)