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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’s handling of specially crafted `lseek()` operations on a file. When a user triggers a specific sequence of `lseek()` calls, it can manipulate the internal state of the `seq_file` object. A subsequent `read()` operation on the file will then access a `struct seq_operations` pointer that has been freed and potentially overwritten with attacker-controlled data. This corrupted pointer is later dereferenced by the kernel when handling file operations, leading to a direct jump to a user-controlled address. This grants an unprivileged local user the ability to execute arbitrary code with kernel privileges, resulting in a full system compromise.
Platform: Linux Kernel
Version: All versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`lseek(fd, huge_offset, SEEK_CUR);`
`read(fd, buf, size);`
`dmesg | grep “segfault”`
How Exploit:
Craft sequence of lseek() calls. Trigger read() on seq_file. Control freed memory. Redirect kernel execution flow.
Protection from this CVE:
Update kernel to >=5.13.4. Apply vendor security patches. Restrict user access.
Impact:
Local Privilege Escalation. Arbitrary Code Execution. Full System Compromise.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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