CVE-2025-20378 is a web application vulnerability affecting Splunk Enterprise versions below 10.0.1, 9.4.5, 9.3.7, 9.2.9, and corresponding Splunk Cloud Platform versions. The vulnerability arises because the 'return_to' parameter in the Splunk Web login endpoint accepts user-controlled input that specifies a URL to which the user is redirected after login. This parameter is not properly validated, allowing an attacker to craft a URL that redirects authenticated users to arbitrary external sites. The attacker does not need authentication but must trick a user into clicking the malicious URL. The vulnerability facilitates phishing attacks by redirecting users to malicious sites that may attempt credential theft or malware delivery. The CVSS score is 3.1 (low severity), reflecting that exploitation requires user interaction and has limited impact on confidentiality, integrity, and availability. There are no known exploits in the wild, and the vulnerability does not allow direct system compromise. However, it increases the risk of social engineering attacks leveraging Splunk’s trusted domain. The vulnerability was reserved in October 2024 and published in November 2025. No patches or workarounds are explicitly listed in the provided data, but upgrading to fixed versions is implied.

For European organizations, the primary impact of CVE-2025-20378 is an increased risk of successful phishing attacks leveraging trusted Splunk URLs. Since Splunk is widely used for security information and event management (SIEM), a phishing attack exploiting this vulnerability could lead to credential theft or malware infection, potentially compromising sensitive security monitoring environments. Although the vulnerability itself does not allow direct unauthorized access or data manipulation, successful phishing could lead to broader security incidents. Organizations in sectors such as finance, energy, government, and critical infrastructure that rely heavily on Splunk for security monitoring are particularly at risk. The need for user interaction limits the scope, but targeted spear-phishing campaigns could exploit this vector effectively. The vulnerability could also erode user trust in internal security tools if exploited.

1. Upgrade Splunk Enterprise and Splunk Cloud Platform to versions 10.0.1, 9.4.5, 9.3.7, 9.2.9 or later where the vulnerability is fixed. 2. Implement strict validation and whitelisting of URLs accepted in the 'return_to' parameter to prevent redirection to untrusted external sites. 3. Configure web application firewalls (WAFs) to detect and block suspicious redirect URLs targeting Splunk login endpoints. 4. Conduct targeted user awareness training emphasizing the risks of clicking unexpected or suspicious links, especially those purporting to be from internal tools like Splunk. 5. Monitor Splunk web access logs for unusual redirect parameter usage or spikes in login endpoint requests with external URLs. 6. Consider implementing multi-factor authentication (MFA) to reduce the risk of credential compromise if phishing occurs. 7. Use URL rewriting or proxying to restrict redirects to internal domains only. 8. Coordinate with security teams to integrate phishing detection and response workflows that consider this vulnerability’s exploitation vector.