CVE-2025-4134 is a high-severity vulnerability identified in Avast Business Antivirus for Linux version 4.5.1. The issue stems from a lack of proper file validation in the do_update_vps function, which is responsible for handling update files. Specifically, this vulnerability allows a local user with limited privileges (PR:L) to perform an unverified file write, enabling them to spoof or tamper with the update file. The vulnerability is categorized under CWE-552, which relates to files or directories being accessible to external parties due to insufficient access control or validation. Exploiting this flaw requires local access and some user interaction (UI:R), but it can lead to a complete compromise of confidentiality, integrity, and availability (C:H/I:H/A:H) of the antivirus update mechanism. By tampering with update files, an attacker could potentially inject malicious code or disable security features, undermining the protection offered by Avast Business Antivirus. Although no known exploits are currently reported in the wild, the vulnerability's CVSS 3.1 score of 7.3 reflects its high risk, especially in environments where local user access is possible. The scope of the vulnerability is unchanged (S:U), meaning the impact is confined to the vulnerable component without affecting other system components directly. This vulnerability highlights the critical importance of validating update files to prevent local privilege escalation or supply chain attacks within endpoint security solutions.

For European organizations, this vulnerability poses a significant risk, particularly for enterprises relying on Avast Business Antivirus on Linux systems. Since the flaw allows local users to tamper with update files, insider threats or compromised accounts could lead to the deployment of malicious updates, effectively disabling antivirus protections or introducing malware. This could result in widespread infection, data breaches, or operational disruptions. Given the high confidentiality, integrity, and availability impact, organizations handling sensitive data—such as financial institutions, healthcare providers, and critical infrastructure operators—are at heightened risk. The vulnerability also undermines trust in endpoint security solutions, potentially exposing organizations to regulatory penalties under GDPR if personal data is compromised. Additionally, the requirement for local access means that organizations with strong perimeter defenses but weak internal access controls may still be vulnerable. The lack of known exploits in the wild currently reduces immediate risk, but the potential for future exploitation necessitates prompt attention.

To mitigate this vulnerability, European organizations should: 1) Immediately upgrade Avast Business Antivirus on Linux to a patched version once available from the vendor, as no patch links are currently provided but should be prioritized upon release. 2) Restrict local user access on Linux systems running Avast Business Antivirus to trusted administrators only, minimizing the risk of unprivileged users exploiting the flaw. 3) Implement strict file system permissions and monitoring on directories used by Avast for updates to detect unauthorized modifications. 4) Employ application whitelisting and integrity verification tools to monitor the update files and binaries for tampering. 5) Conduct regular audits of user accounts and privilege levels to prevent unauthorized local access. 6) Consider deploying additional endpoint detection and response (EDR) solutions that can detect anomalous behavior related to update tampering. 7) Educate system administrators and users about the risks of local privilege misuse and encourage prompt reporting of suspicious activity. These steps go beyond generic advice by focusing on access control hardening, monitoring, and proactive detection tailored to the specific vulnerability context.