# CVE-2025-66506: fulcio — Fixed in 1.8.3 🐹 🐹 Go Not in CISA KEV HIGH severity # CVE-2025-66506 — fulcio HIGH [Fix: sigstore/fulcio@765a0e5](https://github.com/sigstore/fulcio/commit/765a0e57608b9ef390e1eeeea8595b9054c63a5a) 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 Also known as[GHSA-f83f-xpx7-ffpw](/vulnerability/GHSA-f83f-xpx7-ffpw)GO-2025-4193 Published Dec 4, 2025 Updated Oct 28, 2026 Affected 1 pkg Patched 1 / 1 Exploits None indexed Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS) ## 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 [via FIRST.org ↗](https://www.first.org/epss/) 0.2%probability of exploitation in next 30 days Lower Risk+0.01% Lower risk than most CVEs13th percentile — riskier than 13% of all scored CVEsHighest risk 0.00%0.24%0.48%0.73%0.0%0.2%May 26Sep 26Nov 26 Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/). ## How urgent is this, really ## Where this sits among everything scored ## Real-World Exposure 1 pkg affected 🐹`github.com/sigstore/fulcio` Real-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 1 total 1 fixed Ecosystem Package Vulnerable range Fix 🐹Go `github.com/sigstore/fulcio` all versions 1.8.3`go get github.com/sigstore/fulcio@v1.8.3` ## Affected Products 1 product · 1 configurations Application `fulcio`linuxfoundation < 1.8.3 range ## Detection & mitigation playbook Open-source dependency 1. ### Detect 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. 2. ### 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. 3. ### Workarounds Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service. ## 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. Red HatImportant 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](https://access.redhat.com/errata/RHSA-2026:2900) OpenShift API for Data Protection 1.3 `oadp/oadp-mustgather-rhel9:1785177359` [RHSA-2026:51033](https://access.redhat.com/errata/RHSA-2026:51033) OpenShift Security Profiles Operator 1 `compliance/openshift-security-profiles-operator-bundle:1771367226` [RHSA-2026:2852](https://access.redhat.com/errata/RHSA-2026:2852) Red Hat Advanced Cluster Security 4.8 `advanced-cluster-security/rhacs-main-rhel8:1769615659` [RHSA-2026:1517](https://access.redhat.com/errata/RHSA-2026:1517) Red Hat Advanced Cluster Security 4.9 `advanced-cluster-security/rhacs-main-rhel8:1770250889` [RHSA-2026:2350](https://access.redhat.com/errata/RHSA-2026:2350) Red Hat Advanced Cluster Security 4.9 `advanced-cluster-security/rhacs-main-rhel8:1770074713` [RHSA-2026:2568](https://access.redhat.com/errata/RHSA-2026:2568) Red Hat AI Inference Server 3.2 `rhaiis/model-opt-cuda-rhel9:3.2.5-1765361184` [RHSA-2025:23202](https://access.redhat.com/errata/RHSA-2025:23202) Red Hat AI Inference Server 3.2 `rhaiis/vllm-cuda-rhel9:3.2.5-1765552580` [RHSA-2025:23204](https://access.redhat.com/errata/RHSA-2025:23204) ## Frequently Asked Questions 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 O3 Security · Impact-Aware SCA ### Is CVE-2025-66506 in your dependencies? Find it across Go, including transitive dependencies. [Scan my dependencies](/book-demo) [How O3 SCA works](/impact-aware-sca)