CVE-2023-3776: Linux Kernel Privilege Escalation Vulnerability

CVE-2023-3776 Overview

A use-after-free vulnerability exists in the Linux kernel's net/sched: cls_fw component that can be exploited to achieve local privilege escalation. This vulnerability occurs in the traffic control (TC) classifier firewall subsystem, a critical component used for packet classification and filtering.

If tcf_change_indev() fails, the fw_set_parms() function immediately returns an error after incrementing or decrementing the reference counter in tcf_bind_filter(). If an attacker can control the reference counter and set it to zero, they can cause the reference to be freed, leading to a use-after-free condition that can be leveraged for privilege escalation.

Critical Impact

Local attackers with low privileges can exploit this use-after-free vulnerability to escalate privileges and potentially gain root access on affected Linux systems.

Affected Products

• Linux Kernel (multiple versions up to the fix commit)
• Debian Linux 10.0 (Buster)
• Debian Linux 11.0 (Bullseye)
• Debian Linux 12.0 (Bookworm)

Discovery Timeline

• July 21, 2023 - CVE-2023-3776 published to NVD
• February 13, 2025 - Last updated in NVD database

Technical Details for CVE-2023-3776

Vulnerability Analysis

This vulnerability is a use-after-free (CWE-416) condition in the Linux kernel's network scheduler firewall classifier component (cls_fw). The flaw exists in the interaction between the fw_set_parms() and tcf_bind_filter() functions during error handling scenarios.

When tcf_change_indev() encounters a failure condition, the execution flow returns through fw_set_parms() after the reference counter has already been modified by tcf_bind_filter(). This creates a race condition where the reference counter can be manipulated to reach zero prematurely, triggering the deallocation of memory that is still referenced by other parts of the kernel.

The vulnerability requires local access but does not require user interaction, making it a viable target for attackers who have already gained initial foothold on a system. Successful exploitation allows an attacker to execute arbitrary code in kernel context, effectively bypassing all security boundaries and gaining complete system control.

Root Cause

The root cause lies in improper reference counter management within the cls_fw classifier's error handling path. When tcf_change_indev() fails, the function fw_set_parms() returns an error without properly handling the reference count changes made by tcf_bind_filter(). This creates an inconsistent state where:

1. The reference counter may have been incremented or decremented before the error
2. The error return path does not properly unwind these reference count changes
3. An attacker who can trigger specific failure conditions can manipulate the reference count to zero
4. When the count reaches zero, the kernel frees the associated memory while it remains in use

Attack Vector

The attack requires local access to the system and the ability to interact with the network traffic control subsystem. An attacker with low-level privileges can craft specific sequences of TC classifier operations designed to trigger the failure condition in tcf_change_indev() while manipulating the reference counter state.

The exploitation typically involves:

1. Creating multiple firewall classifier entries to establish references
2. Triggering the failure condition in tcf_change_indev() through malformed input or resource exhaustion
3. Carefully timing operations to cause the reference counter to decrement to zero
4. Accessing the freed memory through a dangling pointer to achieve code execution

The fix for this vulnerability is documented in commit 0323bce598eea038714f941ce2b22541c46d488f, which properly handles reference counter management during error conditions in the cls_fw component.

Detection Methods for CVE-2023-3776

Indicators of Compromise

• Unexpected kernel panics or crashes related to the net/sched subsystem
• Anomalous traffic control classifier operations from low-privileged users
• Suspicious modifications to network scheduling rules or firewall classifiers
• Unexpected privilege escalation events on Linux systems

Detection Strategies

• Monitor kernel logs for use-after-free errors or memory corruption warnings in net/sched components
• Implement audit rules to track access to traffic control interfaces (/dev/net/tun, netlink sockets)
• Deploy kernel-level integrity monitoring to detect unauthorized privilege escalation
• Use memory sanitizers (KASAN) in development/testing environments to catch use-after-free conditions

Monitoring Recommendations

• Enable kernel auditing for network namespace and traffic control operations
• Monitor for unusual patterns of tc command usage from non-root users
• Implement real-time alerting on kernel oops messages referencing cls_fw or related components
• Track process privilege changes to detect successful exploitation attempts

How to Mitigate CVE-2023-3776

Immediate Actions Required

• Apply the latest kernel security updates that include commit 0323bce598eea038714f941ce2b22541c46d488f
• For Debian systems, apply the security patches from DSA-5480 or DSA-5492
• Consider implementing kernel live patching for production systems using solutions like those referenced in LSN-0098-1
• Restrict access to the traffic control subsystem for non-privileged users where possible

Patch Information

The vulnerability is addressed in Linux kernel commit 0323bce598eea038714f941ce2b22541c46d488f. The fix properly handles reference counter management in the cls_fw component's error paths, preventing the use-after-free condition.

Debian has released patches through multiple security advisories:

• DSA-5480 and DSA-5492 for stable releases
• Debian LTS announcements for long-term support versions

NetApp users should consult the NetApp Security Advisory NTAP-20240202-0003 for affected products.

Workarounds

• Restrict CAP_NET_ADMIN capability to trusted users only to limit access to traffic control functions
• Use seccomp filters to block netlink socket operations for untrusted processes
• Implement network namespace isolation to contain potential exploitation attempts
• Consider disabling the cls_fw module if not required in your environment using module blacklisting

# Restrict access to traffic control by limiting CAP_NET_ADMIN
# Add to /etc/security/capability.conf
cap_net_admin root

# Blacklist cls_fw module if not required (add to /etc/modprobe.d/blacklist.conf)
blacklist cls_fw

# Monitor for suspicious tc operations
auditctl -w /sbin/tc -p x -k traffic_control_monitoring