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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 excessively long command-line arguments and environment strings. When the `seq_file` interface is used to output these strings for a process (via `/proc/self/cmdline` or /proc/self/environ), it does not properly handle scenarios where a single argument exceeds the internal buffer size (kmalloc-4k). This triggers an integer overflow in the `seq_buf_alloc()` function, leading to a buffer allocation of insufficient size. A subsequent buffer overflow occurs when writing the argument, corrupting adjacent kernel memory. This corruption can be carefully engineered to overwrite a `struct seq_operations` pointer. When the file is closed, the kernel attempts to call a function from this now-controlled structure, leading to a use-after-free condition that can be leveraged to achieve arbitrary code execution with kernel privileges.
Platform: Linux Kernel
Version: 3.16 to 5.13
Vulnerability : Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`gcc exploit.c -o exploit`
`./exploit`
`id`
` uid=0(root) gid=0(root)`
How Exploit:
Exploit manipulates `/proc/self/cmdline` via a execve call with a single, meticulously crafted argument of ~2GB in size. This triggers the integer overflow and subsequent buffer overflow, allowing for the hijacking of control flow and elevation of privileges.
Protection from this CVE
Update kernel to versions 5.13.4, 5.12.15, 5.10.46, 5.4.128, 4.19.199, 4.14.242, 4.9.276, or 4.4.276 and later. Apply security patches from your distribution vendor. Restrict access to untrusted users.
Impact:
Local Privilege Escalation from unprivileged user to root (kernel) compromising the entire system. Full control over the operating system.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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