CVE-2025-66506VulnerabilityPublished 4 Dec 2025

CVE-2025-66506: sigstore fulcio · Asymmetric Resource Consumption (Amplification)

Affects sigstore fulcioSource CVE Database V5

Not known to be exploited · CVSS 7.5 High : upgrade to 1.8.3 soon.

Data as of 25 Oct 2026, 19:02 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 11% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.8.3

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 4 Dec 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-25.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade github.com/sigstore/fulcio to 1.8.3.

Affected software

Product	Affected	Fixed in
Gogithub.com/sigstore/fulcio`pkg:golang/github.com/sigstore/fulcio`Other threats	Affected <1.8.3	Fixed in 1.8.3

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

1. Upgrade fulcio to version 1.8.3 or later immediately to apply the official fix addressing the resource consumption vulnerability. 2. Implement strict input validation on OIDC identity tokens before processing, specifically limiting the number of period characters allowed to prevent excessive splitting and memory allocation. 3. Deploy rate limiting and throttling mechanisms on endpoints handling OIDC tokens to mitigate the risk of large-scale or repeated malicious requests. 4. Monitor fulcio service resource usage and logs for abnormal spikes in memory or CPU consumption indicative of exploitation attempts. 5. Consider deploying web application firewalls (WAFs) or API gateways with custom rules to detect and block malformed OIDC tokens with excessive delimiters. 6. Incorporate anomaly detection for unusual request patterns targeting the certificate issuance service. 7. Educate development and security teams about the vulnerability and ensure secure coding practices when handling untrusted input in similar contexts. 8. Review and harden CI/CD pipeline dependencies on fulcio to include fallback or redundancy measures in case of service disruption.

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.

Impact

For European organizations, this vulnerability poses a significant risk to the availability of the sigstore fulcio service, which is critical for issuing code signing certificates used in software supply chains. Disruption of fulcio can delay or prevent code signing operations, potentially halting software deployment pipelines and impacting development velocity and security assurance processes. Organizations relying on automated signing and verification workflows may face operational downtime or forced manual interventions. While confidentiality and integrity remain unaffected, the denial of service potential can indirectly affect trust in software authenticity if signing services become unreliable. The risk is heightened for cloud service providers, software vendors, and enterprises with continuous integration/continuous deployment (CI/CD) pipelines that integrate sigstore fulcio. Given the remote, unauthenticated exploit vector, attackers can launch large-scale amplification attacks to degrade service availability. This could be leveraged as part of broader supply chain attacks or to disrupt critical software infrastructure. European entities involved in open source projects or those adopting sigstore for secure software signing should prioritize mitigation to maintain operational resilience.

CWE-405Asymmetric Resource Consumption (Amplification) (opens in a new tab)
Class weakness
The product does not properly control situations in which an adversary can cause the product to consume or produce excessive resources without requiring the adversary to invest equivalent work or otherwise prove authorization, i.e., the adversary's influence is "asymmetric."
Architecture and Design
An application must make resources available to a client commensurate with the client's access level.
Architecture and Design
An application must, at all times, keep track of allocated resources and meter their usage appropriately.
System Configuration
Consider disabling resource-intensive algorithms on the server side, such as Diffie-Hellman key exchange.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 12 Dec 2025, 00:12 UTC

WeaknessesCWE-405

Technical details

References

Community reviews

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