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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, which is used for producing files for the `/proc` filesystem. The vulnerability is triggered when a user writes a large number of characters to the `/proc/self/mountinfo` file, which has the `O_WRONLY` flag set. This operation causes the `seq_buf_alloc()` function to allocate a buffer of a specific size. However, if the write operation is subsequently aborted, the allocated buffer is incorrectly freed, leaving a dangling pointer in the `seq_file` object. A local attacker can exploit this by manipulating the kernel’s memory allocator to reoccupy the freed memory space with controlled data before the dangling pointer is used again, leading to a use-after-free condition. This can be leveraged to achieve arbitrary code execution with kernel privileges, effectively compromising the entire 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:
`echo -ne ‘\\xde\\xad\\xbe\\xef’ > /proc/self/mountinfo`
`dmesg | grep “seq_file:”`
`cat /proc/sys/kernel/ptr_limit`
`grep -r “seq_buf_alloc” /usr/src/linux-headers-$(uname -r)/`
How Exploit:
Local Privilege Escalation
Arbitrary Code Execution
Kernel Memory Corruption
Protection from this CVE
Update Kernel Version
Apply Security Patch
Restrict User Access
Impact:
Complete System Compromise
Root Privilege Escalation
Bypass Security Mechanisms
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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