CVE-2025-61661 is a vulnerability identified in the GNU GRUB2 bootloader, specifically related to how it calculates buffer sizes during string conversion when reading information from USB devices during the boot process. The flaw arises because GRUB2 mishandles length values, leading to inconsistent or incorrect buffer size calculations. This vulnerability can be exploited by a local attacker who has physical access to the machine and can connect a specially crafted malicious USB device during the boot sequence. When exploited, this causes GRUB2 to crash, resulting in a denial of service (DoS) condition that prevents the system from booting properly. Additionally, there is a possibility of data corruption, although the complexity of triggering this makes it less likely. The vulnerability does not require any privileges or user interaction beyond physical access. The CVSS 3.1 score is 4.8 (medium), reflecting the attack vector as physical (local), high attack complexity, no privileges required, no user interaction, unchanged scope, no confidentiality impact, low integrity impact, and high availability impact. GRUB2 is widely used as the default bootloader in many Linux distributions, making this vulnerability relevant to a broad range of systems. Currently, no known exploits are reported in the wild, and no official patches have been linked, though vendors are expected to address this issue. The vulnerability highlights the risks associated with physical access and boot-time device handling in critical system components.

The primary impact of CVE-2025-61661 is denial of service, where affected systems fail to boot due to GRUB2 crashing when processing malicious USB device data during startup. This can cause operational downtime, especially in environments where physical access is not tightly controlled or where unattended systems are rebooted frequently. The potential for data corruption, while less likely, could lead to system instability or loss of critical boot configuration data, complicating recovery efforts. Since GRUB2 is a fundamental component in the boot process of many Linux-based systems, this vulnerability affects a wide range of servers, desktops, and embedded devices. Organizations relying on Linux infrastructure may face service interruptions and increased support costs. The requirement for physical access limits remote exploitation, but insider threats or attackers with temporary physical access could leverage this vulnerability. No confidentiality impact is expected, so data breaches are unlikely. The medium severity reflects the balance between the limited attack vector and the significant availability impact.

To mitigate CVE-2025-61661, organizations should implement strict physical security controls to prevent unauthorized access to systems, especially during boot sequences. Disable or restrict booting from external USB devices in BIOS/UEFI settings where possible, or enable secure boot features to prevent unauthorized bootloaders or devices from being used. Monitor and audit physical access logs and consider using tamper-evident seals on critical hardware. Until patches are available, avoid rebooting critical systems unnecessarily and ensure that only trusted personnel have physical access. Vendors and system administrators should prioritize applying patches or updates once released by GNU or Linux distribution maintainers. Additionally, consider using hardware-based boot security features such as TPM (Trusted Platform Module) and secure boot chains to reduce the risk of bootloader manipulation. Document and test recovery procedures to quickly restore systems affected by a DoS condition caused by this vulnerability.