CVE-2025-66506: 
NixOS vulnerability analysis and mitigation

CVE-2025-66506 is a denial-of-service vulnerability in Fulcio, the free-to-use certificate authority from the Sigstore project used for issuing code signing certificates via OpenID Connect (OIDC) identities. The flaw affects all versions of github.com/sigstore/fulcio up to and including v1.8.2, and was disclosed and patched on December 4, 2025. It carries a CVSS v3.1 base score of 7.5 (High) (GitHub Advisory, Sigstore Advisory).

The root cause is classified as CWE-405 (Asymmetric Resource Consumption / Amplification). The vulnerable function identity.extractIssuerURL in pkg/identity/issuerpool.go used strings.Split(token, ".") to parse an untrusted OIDC token string, which allocates a new slice element for every period character found — resulting in O(n) memory allocation proportional to the input length, with a constant factor of approximately 16x. An attacker can craft a malformed OIDC token containing a very large number of period characters and submit it in a certificate signing request, triggering unbounded memory allocation before any validation occurs. The fix, applied in commit 765a0e5, replaces the unbounded strings.Split with a pre-check using strings.Count followed by a bounded strings.SplitN(token, ".", 3), ensuring at most three parts are allocated regardless of input length (Sigstore Advisory, Fix Commit).

Successful exploitation causes excessive memory allocation in the Fulcio certificate authority service, leading to resource exhaustion and potential service unavailability (denial of service). There is no impact on confidentiality or integrity — attackers cannot read data or forge certificates through this vulnerability. However, disrupting Fulcio availability could interrupt software supply chain signing workflows that depend on it for issuing short-lived code signing certificates (GitHub Advisory).

No public proof-of-concept exploit code or evidence of in-the-wild exploitation has been reported. The vulnerability requires no authentication, no privileges, and no user interaction, making it trivially exploitable by any network-accessible attacker who can send a crafted HTTP request to a vulnerable Fulcio instance. The EPSS score is approximately 0.043% (14th percentile), indicating a low current probability of active exploitation. It is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog (GitHub Advisory).

1. Identify target: Locate a publicly accessible or network-reachable Fulcio instance running version ≤ 1.8.2 (e.g., a self-hosted Sigstore deployment).
2. Craft malicious token: Construct an invalid OIDC JWT-like string consisting of a very large number of period characters (e.g., a string of tens of thousands of . characters) to maximize memory allocation upon parsing.
3. Submit request: Send an HTTP POST request to the Fulcio certificate signing endpoint (e.g., /api/v2/signingCert) with the malformed token included as the OIDC identity token in the request payload.
4. Trigger memory exhaustion: The extractIssuerURL function calls strings.Split on the malicious token, allocating O(n) memory with a ~16x amplification factor, potentially exhausting available memory on the server.
5. Achieve denial of service: Repeated or sufficiently large requests cause the Fulcio process to consume excessive memory, leading to service degradation or crash (Sigstore Advisory, Fix Commit).

• Network: Unusual volume of HTTP POST requests to Fulcio certificate signing endpoints (e.g., /api/v2/signingCert) from single or distributed sources; requests with abnormally large OIDC token payloads (many kilobytes or megabytes of period characters).
• Logs: Fulcio application logs showing repeated oidc: malformed jwt errors or token parsing failures in rapid succession; access logs with large request body sizes from unexpected clients.
• Process/System: Sudden spike in memory consumption by the Fulcio process (fulcio-server); OOM (out-of-memory) killer events in system logs (dmesg, /var/log/syslog) targeting the Fulcio process; service restarts or crashes correlating with high-volume malformed token submissions.

Upgrade Fulcio to version 1.8.3 or later, which replaces the unbounded strings.Split call with a safe strings.SplitN bounded to 3 parts, eliminating the amplification vector. No configuration-based workaround is available; patching is the only remediation. Organizations using Fulcio as part of a self-hosted Sigstore deployment should prioritize this upgrade, particularly for internet-facing instances. Red Hat has also issued errata (RHSA-2025:23205, RHSA-2025:23209, RHSA-2025:23449, RHSA-2026:2136, RHSA-2026:2144, RHSA-2026:2924, RHSA-2026:3462) and SUSE issued SUSE-SU-2025:4395-1 for affected downstream packages (Sigstore Advisory, Fix Commit).

The vulnerability was reported by researcher adeinega and patched by Fulcio maintainer Bob Callaway (bobcallaway) on the same day it was disclosed (December 4, 2025). The fix was straightforward and well-received by the community as a clean remediation. Multiple downstream vendors including Red Hat, SUSE, Amazon Linux, and Chainguard issued follow-on advisories and package updates, reflecting the broad adoption of Fulcio in supply chain security tooling (GitHub Advisory).