CVE-2026-25793 is a cryptographic signature verification vulnerability in SlackHQ's Nebula, a scalable overlay networking tool used to create secure peer-to-peer networks. The issue affects versions 1.7.0 through 1.10.2 when configured to use P256 elliptic curve certificates, which is not the default setting. The vulnerability stems from improper verification of ECDSA signatures, specifically allowing signature malleability to alter the signature without invalidating it cryptographically. This malleability enables an attacker to generate a different signature for the same certificate, resulting in a different fingerprint. Since Nebula uses certificate fingerprints to enforce blocklists, an attacker can bypass these blocklists by presenting a malleable signature variant of a blocked certificate. This undermines the integrity of access control mechanisms relying on fingerprint blocking. The vulnerability is classified under CWE-347 (Improper Verification of Cryptographic Signature) and has a CVSS 4.0 base score of 7.6 (high severity), reflecting network attack vector, low complexity, partial privileges required, no user interaction, and high impact on confidentiality and integrity. No known exploits are reported in the wild as of publication. The issue was addressed in Nebula version 1.10.3 by improving signature verification to prevent malleability-based fingerprint evasion.

For European organizations deploying Nebula with P256 certificates, this vulnerability poses a significant risk to network security. Attackers can circumvent fingerprint-based blocklists, potentially gaining unauthorized access to overlay networks. This can lead to unauthorized data access, lateral movement within networks, and compromise of sensitive communications. Organizations relying on Nebula for secure connectivity, especially in critical infrastructure, finance, or government sectors, may face confidentiality and integrity breaches. The vulnerability's exploitation does not require user interaction but does require some level of privilege, which may be achievable through insider threats or compromised credentials. The impact is heightened in environments where fingerprint blocklists are a primary defense mechanism. Given Nebula's use in scalable and distributed network environments, the scope of affected systems can be broad, increasing the risk of widespread unauthorized access if unpatched.

The primary mitigation is to upgrade all Nebula deployments to version 1.10.3 or later, where the signature verification flaw is fixed. Organizations should audit their current Nebula configurations to identify use of P256 certificates and assess reliance on fingerprint-based blocklists. Where possible, transition to default certificate configurations or alternative cryptographic algorithms less susceptible to signature malleability. Implement additional layers of access control beyond fingerprint blocking, such as certificate revocation lists (CRLs), OCSP checks, or multi-factor authentication for network access. Network monitoring should be enhanced to detect anomalous certificate usage or unexpected network connections. Security teams should review and update incident response plans to address potential exploitation scenarios. Finally, educating administrators about cryptographic signature malleability and its implications can help prevent misconfigurations that expose this vulnerability.