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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a use-after-free flaw in the Linux kernel’s filesystem layer. The issue resides in the `seq_file` interface’s handling of specially crafted `lseek()` operations on files in the `/proc` directory. An unprivileged local attacker can trigger a race condition by performing a specific sequence of `lseek()` and `write()` syscalls. This sequence can force the kernel to release a `seq_file` buffer while it still holds a reference to it. Subsequently, by writing user-controlled data into the same memory location before the kernel attempts to use the freed object, an attacker can achieve a write-what-where primitive. This memory corruption ultimately leads to privilege escalation, granting root access from an unprivileged user account.
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:
`$ cd /proc/self`
`$ lseek(fd, 0xFFFFFFFF, SEEK_CUR)`
`char buff[bash];`
`write(fd, buff, 0xff);`
How Exploit:
Local user triggers UAF via `lseek()` on proc file.
Exploit code overwrites freed memory.
Gains root shell via privilege escalation.
Protection from this CVE
Update kernel to version 5.13.4 or later.
Apply security patches for older LTS branches.
Restrict user access if patching is delayed.
Impact:
Local Privilege Escalation to root.
Complete system compromise.
Bypass of all security restrictions.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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