The maintainers of shim have recently rolled out version 15.8, targeting six security vulnerabilities, with one deemed critical due to its potential for remote code execution. Discovered as CVE-2023-40547, this flaw allows a Secure Boot bypass, posing significant risks to major Linux distributions like Debian, Red Hat, SUSE, and Ubuntu, all of which have issued advisories. Oracle’s Alan Coopersmith highlights an out-of-bounds write issue in shim’s HTTP boot support, underscoring the gravity of the vulnerability. Additionally, firmware security firm Eclypsium emphasizes the severity of CVE-2023-40547, which stems from mishandling HTTP protocol, potentially leading to complete system compromise.
Moreover, the vulnerability allows threat actors on the same network to exploit it, loading a vulnerable shim boot loader. Additionally, local adversaries with adequate privileges can manipulate data on the EFI partition, exacerbating the threat landscape. Eclypsium warns of the potential for Man-in-the-Middle attacks, where attackers intercept HTTP traffic between victims and servers, exploiting the flaw to execute code during the boot process. Such an exploit grants attackers privileged access, enabling the deployment of stealthy bootkits for near-total control over compromised hosts.
Furthermore, aside from CVE-2023-40547, shim version 15.8 addresses five other vulnerabilities. These include out-of-bounds read issues (CVE-2023-40546, CVE-2023-40549, CVE-2023-40550, and CVE-2023-40551) and a buffer overflow (CVE-2023-40548). These flaws, though not as critical, collectively underline the importance of prompt updates to ensure system security. In response to the critical bug fixed in shim 15.8, major Linux distributions are swiftly updating their systems, emphasizing the urgency of implementing the security patch to mitigate potential risks and ensure robust protection against exploitation.