CVE-2024-42084 is a vulnerability identified in the Linux kernel's implementation of the ftruncate() system call when operating in compatibility (compat) mode on 64-bit architectures. The issue arises because the legacy ftruncate() syscall uses a 32-bit off_t type that does not properly sign-extend the offset parameter when called from a 32-bit compatibility layer on a 64-bit system. Specifically, when a negative length value is passed, the sign extension is missed, causing the kernel to interpret the negative offset as a large positive value between 2 GiB and 4 GiB. This incorrect interpretation allows the truncation of files to unintended sizes within this range. The native truncate() syscall and the newer loff_t-based variants are not affected, as they correctly handle signed offsets. The vulnerability is addressed by changing the compat syscall to use a signed compat_off_t type, which causes the kernel to reject negative length values with an -EINVAL error instead of performing an erroneous truncation. This flaw could potentially be exploited by a local attacker or malicious process to truncate files improperly, possibly leading to data corruption or loss. However, exploitation requires the ability to invoke the affected syscall in compat mode, which typically implies running 32-bit applications on a 64-bit Linux kernel. No known exploits are currently reported in the wild, and the issue was published on July 29, 2024.

For European organizations, this vulnerability primarily poses a risk to systems running 64-bit Linux kernels that support 32-bit compatibility mode and have legacy 32-bit applications or binaries in use. Improper truncation of files could lead to data corruption or loss, impacting critical files or databases if exploited. This could disrupt business operations, especially in sectors relying heavily on Linux servers for file storage, data processing, or application hosting. Although exploitation requires local access and the ability to execute 32-bit code, insider threats or compromised accounts could leverage this flaw to damage data integrity. The vulnerability does not directly enable privilege escalation or remote code execution, limiting its impact to file integrity and availability. Nonetheless, organizations with mixed-architecture environments or legacy software dependencies should be vigilant. The absence of known exploits reduces immediate risk, but the potential for accidental or malicious misuse remains, particularly in environments with high data sensitivity or regulatory requirements for data integrity.

European organizations should take the following specific actions to mitigate this vulnerability: 1) Apply the latest Linux kernel patches that address CVE-2024-42084 as soon as they become available from their distribution vendors. 2) Audit and inventory systems running 64-bit Linux kernels with 32-bit compatibility enabled, identifying any legacy 32-bit applications that might invoke the affected ftruncate() syscall. 3) Where possible, phase out or containerize legacy 32-bit applications to reduce reliance on compat mode. 4) Implement strict access controls and monitoring to limit local user capabilities, preventing unauthorized execution of potentially malicious 32-bit binaries. 5) Employ file integrity monitoring solutions to detect unexpected truncation or modification of critical files. 6) Educate system administrators and developers about the risks of using deprecated syscalls and encourage migration to updated APIs that correctly handle file operations. 7) Conduct regular backups and verify restore procedures to mitigate the impact of accidental or malicious file truncation. These targeted measures go beyond generic advice by focusing on the specific syscall and compatibility mode context of this vulnerability.