CVE-2025-66506 Fulcio allocates excessive memory during token parsing
Attackers can cause excessive memory allocation and denial of service in Fulcio by sending OIDC tokens with many period characters.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-66506?
CVE-2025-66506 is a high-severity denial of service vulnerability in sigstore fulcio, published on 4 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-405
Which versions are affected by CVE-2025-66506?
3 open source packages ship an affected version, including github.com/sigstore/fulcio. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/sigstore/fulcio Go | All versions | 1.8.3 |
github.com/sigstore/fulcio Go | All versions | 1.8.3 |
github.com/sigstore/fulcio Go | All versions | 1.8.3 |
Products include Linuxfoundation Fulcio.
How do I fix CVE-2025-66506?
Upgrade github.com/sigstore/fulcio to 1.8.3 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-66506 being exploited?
There is no known exploitation of CVE-2025-66506 in the wild and no public exploit code.
Other vulnerabilities in sigstore fulcio
Sources
Also known as CLEANSTART-2026-KK01897, CLEANSTART-2026-TG34900, DEBIAN-CVE-2025-66506, GHSA-f83f-xpx7-ffpw, GO-2025-4193, MINI-22r3-82g9-3qq6 and 27 more.