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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 for the `/proc` filesystem. The vulnerability is triggered when a user mounts a filesystem with a pathname that is exceptionally long, exceeding the maximum allowed limit. When the `seq_file` interface attempts to handle this oversized pathname during a `readdir` operation, it fails to allocate a buffer large enough, leading to a buffer overflow. This overflow corrupts adjacent kernel memory, including the `struct seq_operations` which contains function pointers. The corruption results in a use-after-free condition where the kernel attempts to call a function via a pointer that has been freed and potentially overwritten with attacker-controlled data. This grants an unprivileged local user the ability to execute arbitrary code with elevated kernel privileges, leading to a full system compromise.
Platform: Linux Kernel
Version: 3.16 to 5.13
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch expected 2021-07-27
What Undercode Say:
Check kernel version
uname -r
Craft a long path to trigger the vulnerability
mkdir -p $(python3 -c "print('a'5000000)")
Attempt to list the directory
ls /path/to/long/directory
// Conceptual code snippet showing the vulnerable pattern
int seq_printf(struct seq_file m, const char f, ...) {
// ... Inadequate buffer size calculation for long paths ...
// Buffer overflow corrupts adjacent memory
}
How Exploit:
An attacker exploits this by creating a directory structure with an extremely long pathname. By then performing a filesystem operation, such as listing the directory contents, they trigger the buffer overflow. This corrupts kernel memory and hijacks the control flow, allowing for privilege escalation from an unprivileged user account to root.
Protection from this CVE:
Update the Linux kernel to version 5.13.4 or later. Apply security patches from your distribution vendor. Restrict user access to filesystem mounting capabilities where possible.
Impact:
Full system compromise. Arbitrary code execution with kernel privileges. Complete bypass of all security mechanisms.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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