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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 excessively long sequences. When an application uses the `readdir` function or similar to list directory contents, the kernel employs this interface. A local attacker can exploit this by creating a directory with a pathname longer than 1GB. This colossal pathname causes an integer overflow when the kernel calculates buffer sizes, leading to a heap-based buffer overflow during the `seq_buf_alloc()` call. This overflow corrupts heap memory and subsequently causes a use-after-free condition when the corrupted memory is later accessed, allowing for 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 Available
What Undercode Say:
`mkdir -p $(python -c ‘print(“A”1000000)’)`
`ls -l /proc/self/map_files`
`dmesg | grep “seq_file”`
How Exploit:
Craft long directory path.
Trigger `readdir` via `ls` or custom code.
Overflow heap buffer.
Execute privileged code.
Protection from this CVE
Update kernel to 5.13.4 or apply security patches from your distribution vendor. Restrict access to privileged users for creating deep directory structures where possible.
Impact:
Local Privilege Escalation to root. Full system compromise from an unprivileged user account.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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