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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 number of bytes to a `/proc` file, specifically by exploiting the `lseek()` operation. A successful exploit manipulates the kernel’s memory management. When a process performs an lseek() operation beyond the maximum allowed offset for a seq_file, the `m_start()` callback function returns an error pointer (ERR_PTR). However, the subsequent `m_stop()` callback function incorrectly interprets this error pointer as a valid sequence object and attempts to free its associated memory. This creates a “use-after-free” condition where freed kernel memory can later be manipulated by an attacker, potentially leading to arbitrary code execution with kernel-level privileges, 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:
`cat /proc/self/maps`
`lseek(fd, 0xFFFFFFFF, SEEK_CUR)`
`echo “1” | dd of=/proc/self/mem […]`
How Exploit:
Craft program calling lseek() on proc file with large offset.
Protection from this CVE
Update kernel to version 5.13.4 or apply security patch.
Impact:
Local Privilege Escalation to root.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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