CVE-2020-16119: Linux Kernel Use-After-Free Vulnerability

CVE-2020-16119 Overview

CVE-2020-16119 is a use-after-free vulnerability affecting the Linux kernel's DCCP (Datagram Congestion Control Protocol) implementation. The vulnerability allows a local attacker to exploit a race condition where a DCCP socket with an attached dccps_hc_tx_ccid object can be reused as a listener after the object has been released. This memory corruption flaw can lead to local privilege escalation, allowing unprivileged users to gain elevated system access.

Critical Impact

Local attackers can exploit this use-after-free condition to achieve privilege escalation, potentially gaining root access on affected Linux systems running vulnerable kernel versions.

Affected Products

• Linux Kernel (all versions prior to patches)
• Canonical Ubuntu Linux 12.04 LTS, 14.04 LTS, 16.04 LTS, 18.04 LTS, 20.04 LTS
• Debian Linux 9.0 and 11.0

Discovery Timeline

• 2021-01-14 - CVE-2020-16119 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-16119

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) exists within the Linux kernel's DCCP socket handling code. DCCP is a message-oriented transport layer protocol designed to provide congestion control for unreliable datagram flows. The vulnerability specifically affects the congestion control identifier (ccid) object management within DCCP socket operations.

When a DCCP socket transitions through various states, particularly when being converted to a listener socket, the kernel may fail to properly manage the lifecycle of the attached dccps_hc_tx_ccid object. If this object is freed but the socket structure retains a dangling pointer to it, subsequent operations on the socket can reference deallocated memory.

An attacker with local access can craft a sequence of socket operations that trigger this improper state transition, causing the kernel to access freed memory. This can be leveraged to corrupt kernel data structures, bypass security mechanisms, or execute arbitrary code with kernel privileges.

Root Cause

The root cause lies in improper object lifecycle management within the DCCP subsystem. Specifically, when a DCCP socket's congestion control handler (dccps_hc_tx_ccid) is released during certain socket state transitions, the reference to this object is not properly cleared. The kernel code fails to ensure that the socket cannot be reused as a listener while still holding references to the freed CCID object.

This represents a classic use-after-free pattern where:

1. A dccps_hc_tx_ccid object is allocated and attached to a DCCP socket
2. The object is freed during a state change or cleanup operation
3. The socket retains a stale pointer to the freed object
4. Subsequent use of the socket as a listener triggers access to freed memory

Attack Vector

The attack vector is local, requiring an attacker to have the ability to execute code on the target system. The exploitation process involves manipulating DCCP socket states through system calls to trigger the use-after-free condition. The attacker would typically create a DCCP socket, perform specific operations to trigger the premature freeing of the CCID object, and then force the socket into a listener state where the freed memory is accessed.

Successful exploitation requires low privileges and no user interaction. The vulnerability affects the confidentiality, integrity, and availability of the system, as kernel-level code execution can lead to complete system compromise.

Detection Methods for CVE-2020-16119

Indicators of Compromise

• Unexpected kernel crashes or panics related to DCCP socket operations
• Suspicious DCCP socket activity from unprivileged processes
• Memory corruption indicators in kernel logs referencing dccp or ccid subsystems
• Processes unexpectedly gaining elevated privileges without authorization

Detection Strategies

• Monitor kernel logs for DCCP-related warnings, errors, or crashes using dmesg or syslog analysis
• Implement kernel auditing to track DCCP socket creation and state transitions by unprivileged users
• Deploy endpoint detection solutions capable of monitoring for kernel exploit behaviors and privilege escalation attempts
• Use Linux Audit subsystem to log socket() syscalls with AF_DCCP protocol family

Monitoring Recommendations

• Configure alerting on kernel oops or panic events that reference DCCP subsystem components
• Monitor for unusual patterns of DCCP socket operations, especially from non-network service processes
• Implement behavioral analysis to detect privilege escalation attempts following DCCP socket manipulation
• Review system call patterns for sequences indicative of socket state manipulation attacks

How to Mitigate CVE-2020-16119

Immediate Actions Required

• Update affected Linux kernels to patched versions immediately
• Apply vendor-provided security updates for Ubuntu (kernel versions 5.4.0-51.56, 5.3.0-68.63, 4.15.0-121.123, 4.4.0-193.224, 3.13.0.182.191, 3.2.0-149.196)
• Apply Debian security updates as documented in DSA-4978 and related LTS announcements
• Restrict local system access to trusted users until patches can be applied

Patch Information

Security patches are available from multiple vendors. Ubuntu has released fixed kernel versions across multiple supported releases. The patch commit can be found in the Ubuntu Kernel Commit Update. Additional details are available in Launchpad Bug Report #1883840.

Debian users should apply updates as specified in Debian Security Advisory DSA-4978 and the Debian LTS announcements.

NetApp customers should review NetApp Security Advisory ntap-20210304-0006 for affected product information.

Workarounds

• Disable the DCCP kernel module if not required by running modprobe -r dccp and blacklisting the module
• Add blacklist dccp and blacklist dccp_ipv4 to /etc/modprobe.d/blacklist.conf to prevent module loading
• Implement strict user access controls to limit local shell access on vulnerable systems
• Use container isolation or virtualization to limit the impact of potential exploitation

# Disable and blacklist DCCP kernel module
sudo modprobe -r dccp dccp_ipv4 dccp_ipv6 2>/dev/null
echo "blacklist dccp" | sudo tee -a /etc/modprobe.d/blacklist-dccp.conf
echo "blacklist dccp_ipv4" | sudo tee -a /etc/modprobe.d/blacklist-dccp.conf
echo "blacklist dccp_ipv6" | sudo tee -a /etc/modprobe.d/blacklist-dccp.conf