CVE-2025-62820 identifies a vulnerability in Slack Nebula, a networking tool used to create secure overlay networks, where versions prior to 1.9.7 mishandle CIDR (Classless Inter-Domain Routing) configurations. Specifically, the vulnerability arises because Nebula improperly validates source IP addresses within its network, allowing acceptance of arbitrary source IPs. This is classified under CWE-420, which refers to unprotected alternate channels that bypass intended security controls. The flaw can be exploited by an attacker with limited privileges (PR:L) remotely (AV:N) but requires high attack complexity (AC:H), and no user interaction (UI:N) is necessary. The vulnerability affects the integrity and availability of the network by allowing spoofed source IPs, potentially enabling unauthorized network traffic or denial of service conditions within the Nebula overlay. The scope is changed (S:C), meaning the vulnerability can affect resources beyond the initially vulnerable component. The CVSS v3.1 base score is 4.9, indicating medium severity. No known exploits have been reported in the wild, and no patches are explicitly linked in the provided data, but upgrading to version 1.9.7 or later is implied as a fix. This vulnerability is significant because Nebula is used to secure internal communications, and improper source IP validation undermines trust boundaries within the network.

For European organizations, the vulnerability could lead to unauthorized network traffic injection or disruption within Nebula-based VPNs or overlay networks, impacting internal communications and potentially critical services. While confidentiality is not directly impacted, integrity and availability could be compromised, leading to data manipulation or denial of service scenarios. Organizations relying on Nebula for secure remote access or inter-site connectivity may experience degraded network reliability or be exposed to lateral movement by attackers spoofing IP addresses. This could affect sectors such as finance, healthcare, and government, where secure internal networking is critical. The medium severity suggests a moderate risk, but the potential for cascading effects in complex network environments elevates concern. The lack of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits post-disclosure.

European organizations should immediately verify their Slack Nebula version and upgrade to version 1.9.7 or later where the vulnerability is addressed. In addition, network administrators should audit CIDR configurations within Nebula to ensure strict validation of source IP addresses and restrict network segments to trusted ranges. Implement network segmentation and monitoring to detect anomalous traffic patterns indicative of IP spoofing. Employ strict access controls and limit privileges to reduce the risk of exploitation by low-privilege attackers. Regularly review Nebula logs for unusual source IP activity and consider deploying intrusion detection systems tailored to overlay network traffic. Coordinate with Slack support or security advisories for any patches or workarounds. Finally, integrate this vulnerability into incident response plans to quickly address potential exploitation attempts.