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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 vulnerability exists within the `seq_file` interface, which is used for producing output from kernel sequences to user space. The issue arises when a large number of `seq_buf` allocations are created, consuming all available virtual memory. When a subsequent memory allocation fails, the `seq_file` interface incorrectly releases a buffer but fails to update its internal pointer, leaving a dangling reference. A local attacker can exploit this flaw by invoking the `read()` system call on specific files in the `/proc` filesystem, such as /proc/self/mountinfo, which utilize the vulnerable interface. By carefully controlling the system’s memory state, the attacker can trigger the use-after-free condition to execute arbitrary code with elevated kernel privileges, leading to a full compromise of the system.
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:
grep -r "seq_buf" /usr/src/linux-headers-$(uname -r)/fs/ cat /proc/self/mountinfo Check for patches apt list --upgradable | grep linux-image Manual check for CVE in changelog zcat /usr/share/doc/linux-base/changelog.Debian.gz | grep -i "CVE-2021-33909"
How Exploit:
// Conceptual exploit code structure
int main() {
int fd = open("/proc/self/mountinfo", O_RDONLY);
// Exhaust memory to trigger allocation failure
exhaust_vmalloc();
// Trigger UAF by reading from the file
read(fd, buffer, size);
// Manipulate freed memory for code execution
// ...
return 0;
}
Protection from this CVE
Update kernel immediately.
Restrict user access.
Use kernel hardening.
Impact:
Local privilege escalation.
Full root compromise.
Bypass all security.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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