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How the mentioned CVE works:
The vulnerability, CVE-2021-33909, exists in the Linux kernel’s filesystem layer. The flaw is a use-after-free in the `seq_file` interface. When a user mounts a filesystem, the kernel allocates a `struct seq_file` to handle sequential file operations for directories like `/proc` or /sys. A race condition can occur if a system call, such as readdir, is processing a directory while another thread concurrently deletes the same directory. This causes the `seq_file` structure to be freed prematurely. However, the original `readdir` operation continues using the now-freed memory pointer. An attacker can exploit this race to corrupt kernel memory, leading to a crash or potential code execution with kernel privileges, completely bypassing standard user protections.
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 | grep /tmp`
`ls -la /sys/kernel/debug`
`dmesg | grep “segfault”`
How Exploit:
Malicious shellcode
Race condition scripts
Kernel memory corruption
Protection from this CVE:
Update kernel immediately
Apply security patch
Disable unprivileged user namespaces
Impact:
Kernel privilege escalation
System crash (DoS)
Complete system compromise
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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