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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. It resides within the `fs/seq_file.c` file. The vulnerability is triggered when a user mounts a filesystem with a pathname that is excessively long. The `seq_escape()` function, which is responsible for escaping special characters in a string for output, fails to properly handle a string that does not have a null-terminating byte within its allocated buffer. This leads to an out-of-bounds read. When combined with the `CONFIG_USER_NS` (user namespace) feature, an unprivileged local user can exploit this flaw. By crafting a very long directory structure and calling `getdents()` on it, they can cause a use-after-free condition. This ultimately allows the attacker to gain arbitrary code execution with elevated kernel privileges, bypassing standard security mechanisms and compromising the entire system.
Platform: Linux Kernel
Version: 3.16 to 5.13
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`mkdir -p $(python3 -c ‘print(“A”5000000)’)`
`ls -la /proc/self/mountinfo`
`dmesg | grep “segfault”`
How Exploit:
Unprivileged user creates massive directory tree. User traverses structure via getdents(). Kernel `seq_escape()` reads out-of-bounds. Use-after-free triggers privilege escalation.
Protection from this CVE
Update kernel to versions 5.13.4, 5.12.14, or 5.10.46. Disable unprivileged user namespaces (echo 0 > /proc/sys/user/max_user_namespaces). Restrict filesystem mounting capabilities.
Impact:
Local Privilege Escalation. Full System Compromise. Bypasses Security Modules.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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