Cryptsetup Flaw Exposes Linux Systems to Attacks

A vulnerability related to the disk encryption utility Cryptsetup can expose some Linux systems to local and remote attacks, but the developers of the affected distributions see it as a low-risk issue.

The flaw, discovered by researchers Hector Marco and Ismael Ripoll, can allow attackers to gain root access, and copy or alter data on the hard disk. The weakness, identified as CVE-2016-4484, can be exploited by an attacker with physical access to the targeted system simply by holding down the “Enter” key for roughly 70 seconds at boot.

The vulnerability exists when the system partition is encrypted using the Linux Unified Key Setup (LUKS) disk encryption standard. Due to incorrect handling of password checks, on x86 systems, users are allowed to attempt 93 LUKS passwords at boot. Once this limit is reached, the system opens a BusyBox shell.

Simply put, if the attacker enters a blank password 93 times – or holds down the “Enter” key for roughly 70 seconds – they gain access to a root shell.

According to Marco and Ripoll, a local attacker who has the ability to reboot the system can exploit this flaw for privilege escalation, to copy information from unencrypted partitions, or disrupt the system by deleting files. The experts believe the vulnerability is particularly dangerous for protected systems, such as Internet, airport, tourist, financial and other types of interactive kiosks that have a keyboard and/or a mouse.

“The vulnerability is very reliable because it doesn’t depend on specific systems or configurations,” the researchers said. “Attackers can copy, modify or destroy the hard disc as well as set up the network to exfiltrate data.”

While the most obvious attack scenario requires physical access to the targeted machine, the researchers believe the vulnerability can also be exploited remotely in cloud environments.

The experts have confirmed that the flaw affects Debian, Ubuntu and Fedora, but they believe other distributions could be impacted as well. The issue has been fixed in version 2:1.7.3-2 of the cryptsetup package, but the fix has not been rolled out to stable releases. Debian developers believe the flaw has “negligible security impact,” while Canonical has classified it as a low-priority brute-force attack issue. A patch and a workaround have also been made available by the researchers.

Last year, Marco and Ripoll reported finding a GRUB2 bootloader vulnerability that allowed attackers to bypass authentication.

UPDATE. CryptSetup developers said the vulnerability does not affect their project:

“This is problem in intramfs scripts only (these are not part of cryptsetup project), it is neiter bug in cryptsetup nor in LUKS. Some distributions could add these scripts to distributed package, please check your distro updates for more info.”