Linux Kernel af_packet.c Double Free Local Privilege Escalation Vulnerability (CVE-2021-22600) Security Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTLinux Kernel environments prior to specific patched versions
A local attacker can trigger a double free via crafted system calls, leading to kernel memory corruption, Denial of Service (DoS), or local privilege escalation to root.
Upgrade the Linux kernel to the latest distribution package or rebuild with upstream commit ec6af094ea28.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Update the kernel to the latest version using your distribution's package manager and reboot the system to apply changes.
# For Debian/Ubuntu-based systems
sudo apt-get update && sudo apt-get install --only-upgrade -y linux-image-generic
sudo reboot
# For RHEL/Rocky Linux-based systems
sudo dnf clean all && sudo dnf update -y kernel
sudo reboot2. Temporary Workaround & Hardening Configuration
If an immediate reboot is not feasible, mitigate the attack surface by disabling unprivileged user namespaces.
# Disable unprivileged user namespace cloning
sudo sysctl -w kernel.unprivileged_userns_clone=0
echo 'kernel.unprivileged_userns_clone=0' | sudo tee -a /etc/sysctl.d/99-disable-userns.conf3. Log Audit & IoC Verification Commands
Check the running kernel version and inspect system logs for any anomalies related to af_packet or kernel panics.
# Verify current running kernel version
uname -r
# Search kernel ring buffer and system logs for af_packet anomalies
sudo dmesg | grep -Ei 'double free|af_packet|packet_set_ring'
sudo journalctl -k | grep -Ei 'general protection fault|kernel panic'Block Kernel Privilege Escalation & Ransomware Payloads Before You Can Reboot
Protect unpatched Windows, Linux, and macOS workloads against in-memory exploitation and lateral movement with autonomous EDR.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2021-22600) is a double free bug located in the packet_set_ring() function within net/packet/af_packet.c of the Linux kernel. When reconfiguring packet ring buffers, a flaw in the error handling and resource deallocation logic allows an already freed memory pointer to be freed again without being properly nullified, corrupting the kernel heap memory structure.
2. Exploitation Mechanics & Attack Scenario
A local attacker with basic execution privileges can invoke the setsockopt() system call with specially crafted PACKET_RX_RING or PACKET_TX_RING options. By triggering specific error paths (such as memory allocation failures or validation errors), the attacker forces the kernel to call kfree() twice on the same memory block. This double free state in the SLUB/SLAB allocator can be leveraged via heap spraying to overwrite kernel structures, hijack control flow, and execute arbitrary code with elevated privileges.
3. Infrastructure & Business Impact
As this vulnerability is actively cataloged in the CISA KEV list, it represents a high-risk threat with known real-world exploitation. Successful exploitation allows an attacker with low-privilege local access (including from within containers or restricted shells) to compromise the host kernel and gain full root privileges, bypassing container isolation and threatening multi-tenant cloud infrastructures.
Official Vendor Advisories & References
A double free bug in packet_set_ring() in net/packet/af_packet.c can be exploited by a local user through crafted syscalls to escalate privileges or deny service. We recommend upgrading kernel past the effected versions or rebuilding past ec6af094ea28f0f2dda1a6a33b14cd57e36a9755
Frequently Asked SecOps Questions (CVE-2021-22600)
FAQQ1. How critical is CVE-2021-22600 and is it actively exploited in the wild?
CVE-2021-22600 is rated CVSS 7.0 (HIGH) with a 30-day EPSS exploitation probability of 6.6%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-22600?
Primary impacted vendors include linux across linux_kernel. Scope summary: Linux Kernel versions prior to upstream commit ec6af094ea28f0f2dda1a6a33b14cd57e36a9755
Q3. What temporary mitigations or workarounds exist for CVE-2021-22600 before patching?
[Mitigation] Upgrade the Linux kernel to the latest distribution package or rebuild with upstream commit ec6af094ea28. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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