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How the mentioned CVE works:
The CVE-2021-33909 vulnerability, also known as “Sequoia,” exists in the filesystem layer of the Linux kernel. The flaw is a use-after-free in the seq_file interface. The issue occurs when a user creates a large directory structure that leads to an integer overflow in the `fs/seq_file.c` code. Specifically, when the `lseek()` function is called on a file descriptor for a directory, the `seq_read_iter()` function mishandles the offset calculation. This integer overflow bypasses security checks, allowing a negative offset to be used. Subsequently, when the `show_mountinfo()` function is called, it accesses a `mnt_namespace` object that was previously freed due to a race condition, leading to a use-after-free. This corrupted memory can be leveraged by a local, unprivileged user to achieve arbitrary code execution and escalate privileges to root on the vulnerable system.
Platform: Linux Kernel
Version: 3.16 to 5.13.x
Vulnerability : Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`uname -r`
`cat /proc/mounts`
`ls -la /proc/self/mountinfo`
`dmesg | grep “seq_file”`
`fork() { while true; do ls /proc/self/mountinfo >/dev/null; done }`
`./sequoia_exploit`
How Exploit:
Local attackers trigger the bug by calling `lseek()` with a crafted offset on a `/proc/self/mountinfo` file descriptor, racing it with a concurrent operation to free the mnt_namespace. This corrupts kernel memory, allowing for the execution of a carefully crafted payload to gain root privileges.
Protection from this CVE
Update the Linux kernel to a patched version (e.g., 5.13.4, 5.12.15, 5.10.46). Apply vendor security updates. Restrict user access to `/proc` filesystem where possible. Use kernel hardening features like SELinux/AppArmor.
Impact:
Full local privilege escalation from an unprivileged user to root. Complete compromise of the system. Bypasses common security mechanisms.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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