CVE-2023-41763 is a Server-Side Request Forgery (SSRF) vulnerability classified under CWE-918, found in Microsoft Skype for Business Server 2015 CU13 (build 9319.0). SSRF vulnerabilities occur when an attacker can abuse a server's functionality to send HTTP requests to arbitrary domains or IP addresses, often internal to the network, which the attacker cannot directly access. In this case, the vulnerability allows an unauthenticated remote attacker to craft requests that the Skype for Business Server processes, potentially accessing internal services or sensitive data not normally exposed externally. This can lead to an elevation of privilege scenario where the attacker gains access to information that could facilitate further attacks or lateral movement within the network. The CVSS 3.1 base score is 5.3 (medium), reflecting that the attack vector is network-based, requires no privileges or user interaction, and impacts confidentiality only, without affecting integrity or availability. No public exploits or patches have been reported at the time of publication, but the vulnerability is officially recognized and published by Microsoft. The lack of authentication requirement and the ability to leverage the server as a proxy to internal resources make this a significant concern for organizations relying on Skype for Business Server 2015 CU13, especially in environments with sensitive internal services.

For European organizations, this vulnerability poses a risk primarily to confidentiality, as attackers can potentially access internal resources or sensitive information by exploiting the SSRF flaw. Organizations using Skype for Business Server 2015 CU13 may face unauthorized data disclosure, which could lead to compliance violations under GDPR if personal or sensitive data is exposed. Although the vulnerability does not directly affect system integrity or availability, the information gained through SSRF could facilitate further attacks, including privilege escalation or lateral movement within the network. Enterprises with critical internal services accessible only via the Skype for Business Server could see increased risk. The absence of known exploits reduces immediate risk, but the ease of exploitation without authentication means attackers could quickly develop weaponized exploits. This is particularly concerning for sectors with high regulatory and security requirements such as finance, healthcare, and government institutions across Europe.

Since no official patches are currently linked, European organizations should implement immediate compensating controls. These include restricting outbound HTTP requests from the Skype for Business Server to only trusted internal and external endpoints via firewall rules or network segmentation. Monitoring and logging all outgoing requests from the server can help detect suspicious activity indicative of SSRF exploitation attempts. Organizations should also review and harden internal services accessible through the Skype for Business Server to minimize sensitive data exposure. Applying the latest cumulative updates for Skype for Business Server 2015 as they become available is critical. Additionally, consider isolating the Skype for Business Server in a dedicated network segment with strict access controls. Conduct regular vulnerability scanning and penetration testing focused on SSRF vectors. Finally, prepare incident response plans to quickly address any exploitation attempts.