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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 writes a large size value (over 2GB) to the `/proc/self/attr/fscreate` file. This operation causes the kernel to allocate a buffer based on the user-supplied size. However, due to an integer overflow in the size calculation, a much smaller buffer is allocated than expected. When the kernel subsequently writes data into this undersized buffer, it leads to a heap buffer overflow. This overflow can corrupt critical kernel memory structures, potentially allowing an unprivileged local user to achieve arbitrary code execution with kernel privileges, completely compromising the system.
Platform: Linux Kernel
Version: 3.16 to 5.13.x
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch expected 2021-07-27
What Undercode Say:
`cat /proc/self/attr/fscreate`
`echo -n -e “AAAAAAAAAAAAAAAAAAAA” > /proc/self/attr/fscreate`
`dmesg | grep “segfault”`
How Exploit:
Local user crafts program to write oversized input to /proc/self/attr/fscreate, triggering integer overflow and heap corruption to gain root privileges.
Protection from this CVE
Update kernel to version 5.13.4 or later. Apply vendor security patches. Restrict access to proc filesystem.
Impact:
Full system compromise, privilege escalation from unprivileged user to kernel-level root access.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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