CVE-2025-66506 | Fulcio allocates excessive memory during token parsing

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.

Published: 2025-12-04 Last update: 2026-03-10 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2025-66506 is rated Low Risk (31.8/100): CVSS High severity, with low exploitation likelihood (EPSS 0.02%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2025-66506

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2025-12-05 — 0.02% —

Full EPSS history (1 record total)

Common vulnerability scoring system (CVSS) metrics for CVE-2025-66506

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2025-66506

GitHub Security Advisory for CVE-2025-66506

GHSA-f83f-xpx7-ffpw · Severity: high · Ecosystem: go — Fulcio allocates excessive memory during token parsing

OS Trackers for CVE-2025-66506

vendor priority summary link
debian not yet assigned CVE-2025-66506 not yet assigned priority: Debian including 1 source packages (golang-github-sigstore-fulcio), 3 status rows across 3 suites (forky, sid, trixie): resolved 2, open 1. https://security-tracker.debian.org/tracker/CVE-2025-66506
redhat high — https://access.redhat.com/security/cve/CVE-2025-66506
ubuntu medium CVE-2025-66506 medium priority: Ubuntu including 1 source packages (golang-github-sigstore-fulcio), 5 status rows across 5 suites (jammy, noble, plucky, questing, upstream): DNE 2, needs-triage 2, ignored 1. https://ubuntu.com/security/CVE-2025-66506

Affected software / configurations for CVE-2025-66506

Vendor Product Version Raw CPE
linuxfoundation fulcio < 1.8.3 cpe:2.3:a:linuxfoundation:fulcio:*:*:*:*:*:*:*:*

References for CVE-2025-66506

cvelogic Threat Intelligence