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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. The issue resides in the `fs/seq_file.c` component. The vulnerability is triggered when a user mounts a filesystem with a very long directory path name, exceeding the size of one memory page (typically 4KB). The `seq_file` interface, used for producing output for virtual files in /proc, fails to properly handle the allocation of a buffer for this path. Specifically, the `seq_buf_alloc()` function may release a memory buffer, but a pointer to this freed memory is not cleared and remains in the `m->buf` field. A subsequent operation can then use this dangling pointer, leading to a use-after-free condition. This flaw allows a local, unprivileged user to escalate privileges to root by exploiting the freed memory corruption.
Platform: Linux Kernel
Version: 3.16 to 5.13.x
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`cat /proc/self/mountinfo`
`dmesg | grep “seq_file”`
`uname -r`
How Exploit:
Craft long path.
Trigger `open()` syscall.
Exploit UAF for privilege escalation.
Protection from this CVE
Update kernel to 5.13.4 or apply patches for older LTS versions. Restrict user access where possible.
Impact:
Local Privilege Escalation to root. Full system compromise.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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