CVE-2026-24858 is an authentication bypass vulnerability categorized under CWE-288, affecting a broad range of Fortinet products including FortiProxy, FortiAnalyzer, FortiManager, FortiOS, and FortiWeb across multiple versions (7.0.0 through 7.6.x). The vulnerability arises from improper access control in the FortiCloud Single Sign-On (SSO) mechanism. Specifically, an attacker who possesses a valid FortiCloud account and has a device registered under that account can exploit an alternate authentication path to gain unauthorized access to other devices registered to different FortiCloud accounts. This bypass does not require any additional privileges or user interaction, making it highly exploitable remotely over the network. The flaw compromises the authentication process, allowing attackers to impersonate legitimate users and potentially gain administrative access to critical security infrastructure. Such access could enable attackers to manipulate firewall policies, intercept or redirect traffic, disable security controls, or deploy further malware. The vulnerability affects network security devices that are often deployed at the perimeter or core of enterprise networks, amplifying the potential impact. Although no public exploits have been reported yet, the high CVSS score of 9.4 reflects the critical nature of this vulnerability, with high impact on confidentiality, integrity, and availability. The vulnerability is particularly concerning because it leverages FortiCloud SSO, a convenience feature intended to simplify authentication, thus exposing a systemic risk in centralized authentication management. Fortinet has published advisories and is expected to release patches; however, immediate mitigation steps are necessary to reduce exposure.

For European organizations, the impact of CVE-2026-24858 is severe due to the widespread use of Fortinet products in enterprise and government networks across Europe. Successful exploitation can lead to unauthorized administrative access to critical network security devices, resulting in potential data breaches, network disruptions, and loss of control over security policies. Confidential information could be exfiltrated, and attackers could manipulate or disable security controls, leading to further compromise of internal systems. The vulnerability undermines trust in centralized authentication mechanisms, complicating incident response and forensic investigations. Given the critical role of Fortinet devices in protecting sensitive infrastructure, the breach could affect sectors such as finance, healthcare, telecommunications, and government services. The ease of exploitation without user interaction or elevated privileges increases the likelihood of attacks, especially in environments where FortiCloud SSO is enabled. This could also facilitate lateral movement within networks, escalating the scope of compromise. The absence of known exploits in the wild currently provides a window for proactive defense, but the critical severity demands urgent attention to prevent potential targeted attacks.

European organizations should immediately verify if FortiCloud SSO is enabled on their Fortinet devices and assess the scope of affected systems. They should prioritize applying official patches from Fortinet as soon as they become available. Until patches are deployed, organizations should consider disabling FortiCloud SSO or restricting its use to trusted networks and users only. Implement strict access controls and multi-factor authentication (MFA) for FortiCloud accounts to reduce the risk of account compromise. Monitor Fortinet device logs and FortiCloud account activities for unusual login attempts or access patterns indicative of exploitation attempts. Network segmentation should be enforced to limit the potential lateral movement from compromised devices. Regularly audit registered devices in FortiCloud to detect unauthorized registrations. Employ threat hunting and intrusion detection systems tuned to detect anomalies related to Fortinet device access. Finally, update incident response plans to include scenarios involving authentication bypass in Fortinet infrastructure and conduct staff training on this specific threat.