A recent security advisory has brought attention to a critical vulnerability (CVE-2023-4273) found in the exFAT driver of the Linux kernel. The flaw specifically exists in the implementation of the file name reconstruction function. This could potentially allow a local privileged attacker to overflow the kernel stack, leading to severe consequences for the system's stability and security.

Background

exFAT (Extended File Allocation Table) is a proprietary Microsoft file system used for flash memory devices like SD cards and USB drives. It has been integrated into the Linux kernel since version 5.4 to provide better support for these devices. However, the newly discovered vulnerability CVE-2023-4273 makes the kernel prone to attacks by exploiting the improper handling of file name characters.

Vulnerability Details

The CVE-2023-4273 vulnerability exists in the implementation of the file name reconstruction function, which is responsible for reading file name entries from a directory index and merging file name parts belonging to one file into a single long file name. The issue arises when the characters from the file names are copied into a stack variable without proper bounds checking.

Imagine a scenario where an attacker has local privileged access to the system, and they create a specially crafted exFAT file system with malicious file names. When the corrupt file system is mounted on a Linux system, it triggers the vulnerability. The attacker could then exploit this flaw to overflow the kernel stack, thereby causing a system crash, privilege escalation, or even arbitrary code execution.

The problematic code section in the exFAT driver looks something like this

int exfat_parse_long_name(struct exfat_sb_info *sbi, u16 *lfn,
			   struct file_id_dirent *fid) {
	char namebuf[EXFAT_MAX_FILE_LEN * EXFAT_UCS2_CHAR_LEN];
	char *buf = namebuf;
	int ret;

        // ... omitted for brevity ...

	while (fid) {
                // ... omitted for brevity ...

		memcpy(buf, fid->name, len);
		buf += len;
		*buf = '\';

                // ... omitted for brevity ...
	}
}

Notice that the memcpy operation directly copies the file name data to the namebuf variable while incrementing its buffer pointer, but does not perform any bounds checking, which leads to the potential for a buffer overflow vulnerability.

Original References

1. Official CVE Record - CVE-2023-4273
2. Linux Kernel Mailing List (LKML) Security Advisory

Exploit Details

Currently, there are no known public exploits for this vulnerability. However, users and administrators are strongly advised to apply the available patches and security updates to mitigate the risk associated with this flaw.

Mitigation

The Linux kernel development team has acknowledged the vulnerability and released a patch to address it. The patch introduces proper bounds checking to prevent buffer overflow, thus mitigating the risk associated with CVE-2023-4273.

You can find the patch for the kernel source code here

- Patch for Linux Kernel - Addressing CVE-2023-4273

Administrators and users are advised to update their Linux kernel to the latest patched version to protect their systems against this vulnerability. If updating immediately is not possible or advisable due to various reasons (such as compatibility risks or production environment constraints), be sure to restrict privileged access to the systems and monitor any suspicious activities.

Conclusion

The CVE-2023-4273 vulnerability in the exFAT driver of the Linux kernel serves as a reminder to always be vigilant in scrutinizing open-source software for security flaws and providing timely patches and mitigations. It also highlights the importance of keeping systems and software updated to protect against potential cyber threats.

Timeline

Published on: 08/09/2023 15:15:00 UTC
Last modified on: 09/10/2023 12:16:00 UTC