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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a critical use-after-free flaw in the Linux kernel’s filesystem layer. The issue resides in the `seq_file` interface, which is used by the `/proc` filesystem to handle sequential files. The vulnerability is triggered when a user mounts a filesystem with a pathname exceeding the maximum allowed size (PAGE_SIZE 4, typically 16KB on x86 systems). When an application, such as systemd, traverses this overly long path via the `readdir` system call, it causes an integer overflow when calculating the buffer size for the `d_path()` function. This overflow leads to a heap-based buffer overflow, which can subsequently corrupt kernel memory and result in a use-after-free condition. An unprivileged local attacker can exploit this flaw to gain arbitrary code execution with kernel privileges, effectively compromising the entire system.
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:
Check kernel version uname -r Check for long path mounts (example) mount | grep -o 'path=[^,]' | awk 'length > 40964' Potential proof-of-concept trigger structure include <sys/mount.h> // ... Code to create a deep directory structure and mount a filesystem with a long path ...
// Conceptual code snippet showing the vulnerable d_path call
char d_path(const struct path path, char buf, int buflen) {
// ... Integer overflow if buflen is negative due to overflow ...
return __d_path(path, buf, buflen);
}
How Exploit:
Local attacker crafts long path. Mounts filesystem with it. Triggers `readdir` (e.g., via ls). Integer overflow in d_path. Heap buffer overflow. Memory corruption. Use-after-free. Privilege escalation to root.
Protection from this CVE
Update kernel to version 5.13.4, 5.12.15, 5.10.46, or later. Apply security patches from distribution vendor. Restrict user namespaces if not required.
Impact:
Full system compromise. Root privilege escalation from unprivileged user. Bypasses all security mechanisms (SELinux, AppArmor).
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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