CVE-2026-26747 is a Host Header Poisoning vulnerability identified in Monica version 4.1.2, specifically within the app/Providers/AppServiceProvider.php file. The root cause is the application's reliance on the HTTP Host header to generate absolute URLs without proper validation or sanitization. When the configuration parameter app.force_url is unset or set to false (the default), the application uses the user-supplied Host header value directly to build URLs, including sensitive links such as password reset URLs sent via email. An attacker can exploit this by sending HTTP requests with a manipulated Host header, causing the application to embed attacker-controlled domains in these URLs. This can lead to phishing attacks where victims receive password reset emails containing links that appear legitimate but redirect to malicious sites controlled by the attacker. The vulnerability does not require authentication, making it accessible to remote attackers. Although no public exploits have been reported yet, the flaw is significant due to the sensitive nature of password reset functionality and the ease of exploitation. The vulnerability highlights the importance of enforcing fixed base URLs or validating Host headers to prevent injection of malicious domains into generated links.

The primary impact of CVE-2026-26747 is on the confidentiality and integrity of user accounts managed by Monica 4.1.2. Attackers can poison password reset links, potentially tricking users into divulging credentials or resetting passwords via attacker-controlled endpoints, facilitating account takeover. This can lead to unauthorized access to personal or sensitive information stored within Monica, which is a personal relationship management tool. The attack can also damage organizational reputation and user trust. Since the vulnerability is remotely exploitable without authentication, it increases the attack surface significantly. Organizations relying on Monica for managing personal or customer data are at risk of targeted phishing campaigns leveraging this flaw. The availability of the application is not directly affected, but the indirect consequences of compromised accounts can lead to broader security incidents.

To mitigate CVE-2026-26747, organizations should immediately configure the app.force_url parameter to a fixed, trusted base URL to prevent the application from using user-supplied Host headers when generating absolute URLs. This ensures all generated links use a consistent and verified domain. Additionally, input validation should be implemented to strictly verify the Host header against an allowlist of expected domains. If possible, upgrade Monica to a patched version that addresses this vulnerability once released by the vendor. In the interim, monitor outgoing emails for suspicious URLs and educate users to verify password reset links before clicking. Employing web application firewalls (WAFs) to detect and block requests with suspicious Host headers can provide additional protection. Regular security audits and penetration testing focusing on HTTP header manipulation can help identify similar issues proactively.