CVE-2026-25961 is a vulnerability in the SumatraPDF reader's update mechanism affecting versions 3.5.0 through 3.5.2 on Windows. The core issue lies in improper certificate validation (CWE-295) where the update process disables TLS hostname verification by using the INTERNET_FLAG_IGNORE_CERT_CN_INVALID flag. This means the update client does not verify that the TLS certificate's Common Name (CN) matches the update server's hostname, allowing man-in-the-middle (MITM) attackers to intercept and manipulate update requests if they possess any valid TLS certificate, including those issued by widely trusted authorities like Let's Encrypt. Additionally, the update mechanism executes downloaded installers without verifying their digital signatures (CWE-494), removing a critical layer of trust validation. An attacker can exploit this by injecting a malicious installer URL during the update check, causing the victim's system to download and execute arbitrary code with the user's privileges. The CVSS 3.1 score of 7.5 reflects high severity, with network attack vector, high impact on confidentiality, integrity, and availability, no privileges required, but requiring user interaction (to trigger the update). No patches or exploits in the wild are currently reported, but the vulnerability poses a significant risk due to the ease of exploitation via network interception and the widespread use of SumatraPDF in Windows environments.

For European organizations, this vulnerability presents a significant risk of compromise through supply chain attacks on the update mechanism of SumatraPDF. Successful exploitation can lead to arbitrary code execution, potentially allowing attackers to install malware, steal sensitive data, or disrupt operations. Organizations with users on unsecured or public networks are particularly vulnerable to MITM attacks. The lack of signature verification increases the risk of persistent compromise. Confidentiality is at risk as attackers can execute code to exfiltrate data; integrity is compromised as attackers can alter or replace software components; availability may be affected if attackers deploy ransomware or destructive payloads. Given SumatraPDF's popularity as a lightweight PDF reader in many European enterprises and public institutions, the vulnerability could be leveraged for targeted attacks or widespread campaigns. The threat is exacerbated in environments where endpoint protection or network segmentation is weak, and where users have administrative privileges.

Immediate mitigation steps include disabling automatic updates in SumatraPDF versions 3.5.0 to 3.5.2 until a patched version is released. Organizations should implement network-level protections such as enforcing TLS inspection and certificate pinning where feasible to detect and block MITM attempts. Endpoint detection and response (EDR) solutions should be configured to monitor for suspicious installer executions and anomalous network activity related to SumatraPDF update processes. Administrators should educate users to avoid manual update triggers from untrusted networks. Once available, promptly apply patches that restore proper TLS hostname verification and enforce digital signature checks on installers. Additionally, organizations should consider application whitelisting to prevent unauthorized executables from running and employ network segmentation to limit exposure of vulnerable endpoints. Regular audits of installed software versions and update mechanisms will help identify and remediate vulnerable instances.