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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a use-after-free flaw in the Linux kernel’s sequence file system (seq_file) interface. The vulnerability exists within the `seq_operations` structure, which holds function pointers for handling file operations on virtual files in the `/proc` and `/sys` pseudo-filesystems. An unprivileged local user can exploit this by creating a deep directory structure that exceeds the maximum allowed pathname length, causing a negative integer wrap in the `fs/seq_file.c` code. This triggers an error condition that leads to the kernel freeing the `seq_operations` structure. However, the file descriptor remains open and its associated kernel pointer is not cleared. Subsequent file operations, such as reading or closing the file descriptor, will then use this dangling pointer, leading to a use-after-free condition. A skilled attacker can manipulate the kernel’s memory allocator to place a malicious object in the freed memory, ultimately hijacking kernel control flow and achieving arbitrary code execution with elevated privileges.
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:
`mkdir -p $(python -c ‘print(“A”1000000)’)`
`cat /proc/self/mountinfo`
`dmesg | grep “segfault”`
How Exploit:
Local Privilege Escalation
Kernel Code Execution
Bypass Security Mechanisms
Protection from this CVE
Update Kernel Version
Apply Security Patches
Restrict User Access
Impact:
Full System Compromise
Data Breach Risk
Service Disruption
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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