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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a critical use-after-free flaw in the Linux kernel’s filesystem layer. The issue resides in the `seq_file` interface, which is used by the `/proc` filesystem to handle sequential files. The vulnerability is triggered when a user writes a large number of bytes to a filesystem’s `seq_file` interface, specifically exceeding the `size_t` data type’s maximum value (2^32 or 2^64) and causing an integer overflow. This overflow leads to an undersized buffer allocation. Subsequent operations write beyond the allocated buffer’s bounds, corrupting adjacent kernel memory. If this corrupted memory contains a reference to a `seq_operations` structure, a use-after-free condition occurs when the file is closed. An attacker can exploit this by spraying the kernel heap to place a malicious payload where the freed object resided, ultimately leading to arbitrary code execution with kernel privileges.
Platform: Linux Kernel
Version: 3.16 to 5.13
Vulnerability : Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch expected 2021-07-20
What Undercode Say:
`cat /proc/self/stat`
`dmesg | grep -i “seq_file”`
`echo -n “A” > /proc/self/stat`
`perf record -g -e kmem:kmalloc -e kmem:kfree`
How Exploit:
1. Trigger integer overflow via large write.
2. Corrupt kernel memory via buffer overflow.
3. Force `seq_operations` structure free.
4. Heap spray to control freed memory.
5. Gain root privileges.
Protection from this CVE
Update kernel to version 5.13.4 or apply vendor patches. Restrict user namespaces. Use kernel hardening (SLAB_FREELIST_RANDOM, SLAB_FREELIST_HARDENED). Enforce kernel module signing.
Impact:
Local Privilege Escalation to root. Full system compromise. Bypass all security mechanisms.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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