How severe is CVE-2025-66506?

CVE-2025-66506 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2025-66506 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (9th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-66506?

CVE-2025-66506 affects Linuxfoundation Fulcio. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-66506?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2025-66506 have an EU (EUVD) identifier?

Yes. CVE-2025-66506 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-201293.

When was CVE-2025-66506 published?

CVE-2025-66506 was published on 2025-12-04 and last updated on 2026-06-17.

CVE-2025-66506

CVE-2025-66506 is a high-severity vulnerability in Linuxfoundation Fulcio with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-405.

Key facts

Severity: High (CVSS 3.x base score 7.5)
EPSS exploit prediction: 0% (9th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-201293
Weakness: CWE-405
Affected product: Linuxfoundation Fulcio
Published: 2025-12-04
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2025-66506?: 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.
How severe is CVE-2025-66506?: CVE-2025-66506 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2025-66506 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (9th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-66506?: CVE-2025-66506 affects Linuxfoundation Fulcio. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-66506?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2025-66506 have an EU (EUVD) identifier?: Yes. CVE-2025-66506 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-201293.
When was CVE-2025-66506 published?: CVE-2025-66506 was published on 2025-12-04 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:linuxfoundation:fulcio:*:*:*:*:*:*:*:*

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