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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 name that exceeds the maximum allowed path length. When the `ls` command or a similar utility attempts to traverse this path, the kernel’s `seq_file` interface mishandles the memory allocation. Specifically, the `seq_buf_alloc()` function fails to allocate a sufficiently large buffer for the pathname. This leads to an out-of-bounds write, which can corrupt adjacent kernel memory. Subsequently, when the kernel attempts to free the memory associated with the operation, a use-after-free condition occurs because the internal state is already corrupted. This flaw allows a local, unprivileged attacker to gain arbitrary read/write access to kernel memory, ultimately leading to privilege escalation and full system compromise.
Platform: Linux Kernel
Version: Versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch expected by 2021-07-27
What Undercode Say:
`ls -la /proc/$(pidof systemd)/root/…/very_long_path`
`dmesg | grep -i “seq_file”`
`cat /sys/kernel/debug/seq_file_stats`
`echo “useradd attacker” | crash /usr/lib/debug/boot/vmlinux-$(uname -r) /proc/kcore`
How Exploit:
An attacker exploits this by creating a deep directory structure with a single, extremely long directory name. By tricking a privileged process into listing the contents of this directory or by accessing it via the `/proc` filesystem, the attacker triggers the memory corruption. Crafted payloads are then used to overwrite kernel function pointers, leading to the execution of arbitrary code with root privileges.
Protection from this CVE:
Update kernel to version 5.13.4 or later. Apply vendor-specific security patches for supported long-term stable kernels. Restrict user access to filesystem mount operations. Use security modules like SELinux/AppArmor to limit process capabilities.
Impact:
Privilege escalation from an unprivileged user to root. Full system compromise, including confidentiality, integrity, and availability loss. Bypass of all security boundaries within the operating system.
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Sources:
Reported By: nvd.nist.gov
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