CVE-2026-25793 affects SlackHQ's Nebula, a scalable overlay networking tool widely used for secure network connectivity. The vulnerability exists in versions 1.7.0 to 1.10.2 when P256 elliptic curve certificates are used, which is a non-default configuration. The core issue is improper verification of cryptographic signatures (CWE-347), specifically related to ECDSA signature malleability. Signature malleability allows an attacker to modify a valid signature to produce a different signature that is still accepted as valid. In this case, an attacker can create a copy of a certificate with a different fingerprint by exploiting this malleability, thereby evading blocklist entries that rely on certificate fingerprints to block malicious or unauthorized certificates. This undermines the integrity of the blocklisting mechanism, potentially allowing unauthorized network access or persistence within Nebula overlay networks. The vulnerability has a CVSS 4.0 base score of 7.6 (high severity), reflecting network attack vector, low attack complexity, partial privileges required, no user interaction, and high impact on confidentiality and integrity. The scope is high, meaning the vulnerability affects components beyond the initially vulnerable one. The issue was addressed and patched in Nebula version 1.10.3. No public exploits have been reported yet, but the nature of the flaw suggests it could be leveraged in targeted attacks against organizations relying on Nebula for secure networking.

For European organizations, the impact of this vulnerability can be significant, particularly for those using Nebula with P256 certificates in critical infrastructure, telecommunications, finance, or government sectors. Exploitation could allow attackers to bypass certificate-based blocklists, potentially enabling unauthorized lateral movement, persistent access, or interception of sensitive network traffic within overlay networks. This could lead to confidentiality breaches, data exfiltration, or disruption of secure communications. Given the high scope and impact on integrity and confidentiality, organizations relying on Nebula for secure connectivity could face increased risk of compromise. The vulnerability's exploitation does not require user interaction but does require some level of privilege, which means insider threats or attackers who have gained limited access could escalate their capabilities. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits following public disclosure. European organizations with regulatory requirements for data protection and network security (e.g., GDPR, NIS Directive) must address this vulnerability promptly to avoid compliance issues and potential reputational damage.

The primary mitigation is to upgrade Nebula to version 1.10.3 or later, where the vulnerability has been patched. Organizations should audit their Nebula deployments to identify usage of P256 certificates and verify the version in use. If upgrading immediately is not feasible, organizations should consider disabling P256 certificate usage or implementing additional network-level controls to monitor and restrict unauthorized certificate usage. Enhancing logging and monitoring for anomalous certificate fingerprints or network behavior can help detect attempts to exploit this vulnerability. Additionally, reviewing and tightening access controls to limit privileges that could be leveraged for exploitation is recommended. Network segmentation and zero-trust principles can reduce the potential impact of a compromised certificate. Finally, organizations should stay informed about any emerging exploit reports and apply security advisories promptly.