CVE-2024-26251 is a cross-site scripting (XSS) vulnerability classified under CWE-79, affecting Microsoft SharePoint Server 2019, specifically version 16.0.0. This vulnerability arises from improper neutralization of input during web page generation, allowing an attacker to inject malicious scripts into web pages viewed by other users. The vulnerability is remotely exploitable over the network (AV:N) but requires high attack complexity (AC:H), no privileges (PR:N), and user interaction (UI:R). The scope is unchanged (S:U), meaning the impact is limited to the vulnerable component. Successful exploitation can lead to high confidentiality and integrity impacts (C:H/I:H), but no impact on availability (A:N). The CVSS v3.1 base score is 6.8, categorized as medium severity. The vulnerability enables attackers to execute arbitrary scripts in the context of the SharePoint site, potentially stealing sensitive information such as authentication tokens, session cookies, or manipulating content displayed to users. Although no known exploits are currently reported in the wild, the presence of this vulnerability in a widely used enterprise collaboration platform like SharePoint Server 2019 poses a significant risk, especially in environments where SharePoint is used to manage sensitive or proprietary information. The vulnerability was reserved in February 2024 and published in April 2024, with no official patches currently linked, indicating that mitigation may rely on workarounds or configuration changes until a patch is released.

For European organizations, the impact of CVE-2024-26251 can be substantial due to the widespread use of Microsoft SharePoint Server 2019 in enterprise environments for document management, collaboration, and intranet portals. Exploitation of this XSS vulnerability could lead to unauthorized disclosure of confidential corporate data, including intellectual property, personal data protected under GDPR, and internal communications. The high confidentiality and integrity impact means attackers could impersonate users, manipulate content, or conduct phishing attacks within trusted SharePoint sites, undermining user trust and potentially leading to further compromise. Given the regulatory environment in Europe, data breaches resulting from such vulnerabilities could result in significant legal and financial penalties. Additionally, the requirement for user interaction means that social engineering or phishing campaigns could be used to trigger the exploit, increasing the risk in organizations with less mature security awareness programs. The absence of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits once the vulnerability details become widely known.

1. Implement strict input validation and output encoding on all user-supplied data within SharePoint pages, especially in custom web parts or extensions, to reduce the risk of XSS. 2. Apply available Microsoft security updates promptly once released; monitor Microsoft security advisories for patches addressing CVE-2024-26251. 3. Employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts within SharePoint sites, limiting the impact of potential XSS attacks. 4. Restrict permissions and access controls within SharePoint to minimize exposure, ensuring users have the least privilege necessary. 5. Educate users about the risks of interacting with suspicious links or content within SharePoint, enhancing awareness to reduce successful exploitation via social engineering. 6. Monitor SharePoint logs and network traffic for unusual activities indicative of attempted XSS exploitation, such as anomalous script injections or unexpected user behavior. 7. Consider isolating SharePoint environments or using web application firewalls (WAFs) with custom rules to detect and block XSS attack patterns targeting SharePoint. 8. Review and harden custom SharePoint solutions and third-party add-ons that may introduce additional XSS vectors.