Listen to this Post
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’s handling of specially crafted paths. An attacker can exploit this by creating a deep directory structure that exceeds the maximum allowed path length. When a privileged process attempts to traverse this path using functions like `readdir` or sys_getdents, it triggers an integer overflow in the `fs/seq_file.c` code. This overflow leads to an out-of-bounds write, corrupting kernel memory and ultimately freeing a memory object while a reference to it (a “seq_file” buffer) is still held. This dangling pointer is later used, allowing for 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: 2021-07-27
What Undercode Say:
`mkdir -p $(python -c ‘print(“A”1000 + “/”1000)’)`
`ls -la /path/to/deep/directory`
`dmesg | grep -i “segfault\|oops”`
How Exploit:
Craft deep directory tree.
Trigger via syscall (getdents).
Control freed object memory.
Execute arbitrary kernel code.
Protection from this CVE
Update kernel version.
Apply security patch.
Restrict user namespaces.
Impact:
Full system compromise.
Root privilege escalation.
Kernel memory corruption.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
Undercode
