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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 operation. It exists within the `fs/seq_file.c` component. The vulnerability is triggered when a user mounts a filesystem with a pathname that is exceptionally long, surpassing the maximum limit allowed by the `seq_file` interface. Specifically, when the `seq_buf_alloc()` function is called to allocate a buffer for this pathname, an integer overflow occurs during the size calculation. This overflow leads to an under-allocation of kernel memory. Later, when the `seq_buf_putc()` function writes the full pathname into the insufficiently sized buffer, it causes a heap-based buffer overflow. This overflow corrupts adjacent kernel memory objects, potentially freeing them prematurely. However, the original `seq_file` structure still holds a pointer to this now-freed memory. Subsequent legitimate file operations then dereference this dangling pointer, leading to a use-after-free condition. This flaw allows a local, unprivileged user to achieve arbitrary code execution with elevated kernel privileges, compromising the entire system.
Platform: Linux Kernel
Version: All versions before 5.10.42, 5.11.x before 5.11.23, 5.12.x before 5.12.9, 5.13.x before 5.13.2
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`gcc exploit.c -o exploit`
`./exploit`
`dmesg | grep seq_file`
`cat /sys/kernel/debug/seq_file_log`
How Exploit:
Craft long pathname.
Trigger integer overflow.
Overflow kernel buffer.
Execute arbitrary code.
Protection from this CVE
Update kernel version.
Apply security patch.
Restrict user namespaces.
Impact:
Privilege Escalation
System Compromise
DoS possible.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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