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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, which is used for producing files in the `/proc` filesystem. The vulnerability is triggered when a user mounts a filesystem with a very long directory path that exceeds the maximum allowed length. Specifically, the `seq_file` interface’s `single_start()` function fails to properly handle an error condition when the `kmalloc` call for a path buffer fails. Instead of safely aborting, it returns an error, but the associated `seq_file` structure is not properly cleaned up. This leaves a dangling pointer to the freed `seq_file` buffer. A local attacker can exploit this by repeatedly triggering the error condition, leading to a use-after-free situation. This dangling pointer can then be manipulated to achieve arbitrary code execution with kernel privileges, effectively granting full control over the system.
Platform: Linux Kernel
Version: All versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch available (2021-07-20)
What Undercode Say:
grep -r "seq_file" /usr/src/linux-kernel-version/fs/seq_file.c
cat /proc/self/mountinfo
python3 -c "print('A' 10000)"
//Conceptual code snippet showing the error path
if (!buf) {
return -ENOMEM; // Vulnerability: struct cleanup missing
}
How Exploit:
Local attacker crafts long path. Mounts filesystem with path. Triggers `kmalloc` failure. Manipulates freed memory. Gains kernel privileges.
Protection from this CVE:
Update kernel to version 5.13.4 or later. Apply security patches from distribution vendor. Restrict user access to filesystem mounting.
Impact:
Full system compromise. Elevation of privileges. Bypass of all security mechanisms.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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