CVE-2025-66506 is a high-severity (CVSS 7.5) CWE-405 vulnerability in github.com/sigstore/fulcio. A fix is available for github.com/sigstore/fulcio — see the affected versions and patch details below.
Fulcio allocates excessive memory during token parsing
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-66506.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2025-66506 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/sigstore/fulcioReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/sigstore/fulcio | all versions | 1.8.3go get github.com/sigstore/fulcio@v1.8.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/sigstore/fulcio, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/sigstore/fulcio to 1.8.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-66506 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-66506 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-66506. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This vulnerability is rated Important for Red Hat as Fulcio, a certificate authority used for issuing code signing certificates, is susceptible to a denial of service when processing a specially crafted OpenID Connect (OIDC) token. This could lead to resource exhaustion and service unavailability in affected Red Hat…
| Product | Fixed in | Advisory |
|---|---|---|
| Network Observability (NETOBSERV) 1.11.0 | network-observability/network-observability-cli-rhel9:1771226060 | RHSA-2026:2900 |
| OpenShift API for Data Protection 1.3 | oadp/oadp-mustgather-rhel9:1785177359 | RHSA-2026:51033 |
| Red Hat Advanced Cluster Security 4.8 | advanced-cluster-security/rhacs-main-rhel8:1769615659 | RHSA-2026:1517 |
| Red Hat Advanced Cluster Security 4.9 | advanced-cluster-security/rhacs-main-rhel8:1770250889 | RHSA-2026:2350 |
| Red Hat Advanced Cluster Security 4.9 | advanced-cluster-security/rhacs-main-rhel8:1770074713 | RHSA-2026:2568 |
| Red Hat AI Inference Server 3.2 | rhaiis/model-opt-cuda-rhel9:3.2.5-1765361184 | RHSA-2025:23202 |
| Red Hat AI Inference Server 3.2 | rhaiis/vllm-cuda-rhel9:3.2.5-1765552580 | RHSA-2025:23204 |
| Red Hat AI Inference Server 3.2 | rhaiis/vllm-rocm-rhel9:3.2.5-1765361180 | RHSA-2025:23205 |
Frequently Asked Questions
Is CVE-2025-66506 in your dependencies?
O3 Security finds CVE-2025-66506 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.