CVE-2025-21357 is a vulnerability identified in Microsoft Office 2019, specifically affecting Microsoft Outlook. The vulnerability is categorized under CWE-908, which refers to the use of uninitialized resources. This type of flaw occurs when a program uses memory or resources that have not been properly initialized, potentially leading to unpredictable behavior, including memory corruption or leakage of sensitive information. In this case, the vulnerability allows for remote code execution (RCE), meaning an attacker could execute arbitrary code on a victim's machine by exploiting this flaw remotely. The CVSS 3.1 base score is 6.7, indicating a medium severity level. The vector string (CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C) reveals several key details: the attack vector is local (AV:L), requiring the attacker to have local access; the attack complexity is high (AC:H), meaning exploitation is not straightforward; privileges required are low (PR:L), so a low-privileged user can exploit it; user interaction is required (UI:R), so the victim must perform some action; the scope is unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). The exploitability is limited by the need for local access and user interaction, but the potential impact is significant due to the high impact on all three security properties. No known exploits are currently in the wild, and no patches have been linked yet. This vulnerability could be leveraged by attackers to gain control over affected systems, potentially leading to data breaches, system compromise, or further lateral movement within an organization.

For European organizations, the impact of CVE-2025-21357 could be substantial, especially for those heavily reliant on Microsoft Office 2019 and Outlook for daily communications and operations. Successful exploitation could lead to unauthorized access to sensitive emails, confidential business information, and internal communications, severely compromising confidentiality. The integrity of data could be undermined by unauthorized modifications, and availability could be affected if attackers disrupt Outlook functionality or broader system operations. Given the medium severity and the requirement for local access and user interaction, the threat is more likely to manifest in scenarios involving insider threats, compromised endpoints, or targeted attacks where attackers have some foothold within the network. Organizations in sectors such as finance, government, healthcare, and critical infrastructure in Europe could face increased risks due to the sensitive nature of their communications and regulatory requirements around data protection (e.g., GDPR). The lack of available patches at the time of disclosure further elevates risk, as organizations must rely on mitigations until official fixes are released.

To mitigate the risk posed by CVE-2025-21357, European organizations should implement the following specific measures: 1) Restrict local access to systems running Microsoft Office 2019, especially Outlook, by enforcing strict endpoint access controls and least privilege principles. 2) Educate users about the risks of interacting with suspicious content or performing unexpected actions in Outlook, as user interaction is required for exploitation. 3) Employ application whitelisting and endpoint detection and response (EDR) solutions to monitor and block suspicious activities related to Outlook processes. 4) Regularly audit and monitor user activities on endpoints to detect potential insider threats or compromised accounts. 5) Until patches are available, consider disabling or limiting features in Outlook that may be exploited or isolating vulnerable systems in network segments with restricted access. 6) Maintain up-to-date backups and incident response plans to quickly recover from potential compromises. 7) Stay informed via official Microsoft security advisories for patch releases and apply them promptly once available.