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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a use-after-free flaw in the Linux kernel’s sequence file system operation. The issue resides in the `fs/seq_file.c` file. When a program uses the `seq_operations` interface, the kernel allocates a structure to handle file operations. A vulnerability exists in the handling of the `single_start` function. If an attacker can trigger a fault during a specific point in the file operation sequence, the kernel does not properly release the allocated memory pointer. This results in a “use-after-free” condition where the memory is freed but a stale pointer to it is retained. An attacker can then manipulate the freed memory space by writing a custom payload. When the kernel later uses the stale pointer, it executes code from the attacker-controlled memory region, leading to a privilege escalation from an unprivileged user to root (kernel privilege).
Platform: Linux Kernel
Version: All versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch available
What Undercode Say:
`cat /proc/self/mountinfo`
`dmesg | grep -i “seq_file”`
`echo ‘module seq_file +p’ > /sys/kernel/debug/dynamic_debug/control`
How Exploit:
Craft a filesystem operation that induces a fault after `single_start` allocation.
Spray kernel heap to control freed memory.
Execute payload with kernel privileges.
Protection from this CVE
Update kernel to version 5.13.4 or later.
Apply vendor-specific security patches.
Restrict user namespace creation.
Impact:
Local Privilege Escalation to root.
Complete system compromise.
Bypass of all security mechanisms.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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