CVE-2023-2156: Linux Kernel RPL Protocol DoS Vulnerability

CVE-2023-2156 Overview

CVE-2023-2156 is a denial of service vulnerability in the Linux kernel networking subsystem. The flaw exists within the handling of the Routing Protocol for Low-Power and Lossy Networks (RPL) protocol. The kernel fails to properly validate user-supplied data, leading to an assertion failure. An unauthenticated remote attacker can trigger this condition over the network to crash affected systems. The vulnerability is tracked under [CWE-617] Reachable Assertion and impacts multiple major Linux distributions including Red Hat Enterprise Linux, Fedora, and Debian.

Critical Impact

Remote attackers can crash Linux systems by sending crafted RPL protocol packets, with no authentication or user interaction required.

Affected Products

• Linux kernel (multiple versions implementing the RPL protocol stack)
• Red Hat Enterprise Linux 9.0
• Fedora 38
• Debian Linux 10.0

Discovery Timeline

• 2023-05-09 - CVE-2023-2156 published to NVD
• 2023-05-17 - Disclosure published via Openwall OSS Security mailing list
• 2023-06-22 - NetApp security advisory NTAP-20230622-0001 issued
• 2023-08 - Debian LTS security announcement released
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-2156

Vulnerability Analysis

The vulnerability resides in the Linux kernel's implementation of the RPL protocol, a routing protocol designed for low-power and lossy networks defined in RFC 6550. The kernel code processes incoming RPL packets without adequately validating fields supplied by remote peers. When the parser encounters specific malformed values, it reaches an internal assertion that the developers assumed could never trigger from external input. The assertion failure causes a kernel panic or BUG condition, halting normal system operation.

The issue is classified as a reachable assertion [CWE-617], where attacker-controlled input drives execution to a BUG_ON() or equivalent macro. Because the RPL protocol runs in kernel space and is reachable from the network stack, exploitation requires only the ability to deliver a packet to the target interface.

Root Cause

The root cause is missing input validation on user-supplied data within the RPL packet handling code. The kernel relies on an assertion to enforce an invariant about packet structure, but no upstream sanity check prevents that invariant from being violated by remote input. When the assertion fails, the kernel terminates the offending code path through a panic or BUG condition rather than gracefully discarding the malformed packet.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker delivers a specially crafted RPL packet to a vulnerable Linux host that has the RPL protocol module loaded and reachable. Processing the packet triggers the assertion failure, producing a denial of service. The vulnerability impacts availability only — confidentiality and integrity are not affected. See the Zero Day Initiative Advisory ZDI-23-547 for additional technical context.

No public proof-of-concept exploit code has been verified in the enriched data. Technical details are described in the Red Hat Bugzilla Report and accompanying Openwall OSS Security Notification.

Detection Methods for CVE-2023-2156

Indicators of Compromise

• Unexpected kernel panics or BUG: messages in dmesg and /var/log/kern.log referencing the RPL or IPv6 routing code paths.
• System crashes or reboots correlated with inbound IPv6 traffic containing RPL routing headers.
• Loss of network connectivity on hosts exposed to untrusted networks where RPL is not used operationally.

Detection Strategies

• Inventory Linux systems by kernel version and cross-reference against vendor advisories from Red Hat, Debian, Fedora, and NetApp to identify unpatched hosts.
• Inspect loaded kernel modules with lsmod | grep rpl to determine whether the RPL handler is active on production systems.
• Monitor network telemetry for IPv6 packets carrying RPL routing headers (Routing Type 3) arriving from untrusted sources.

Monitoring Recommendations

• Forward kernel logs to a centralized logging platform and alert on assertion failures, BUG conditions, and unexpected reboots.
• Track host availability metrics to detect repeated short-interval crashes consistent with active exploitation attempts.
• Correlate crash events with upstream packet captures to identify the source addresses delivering malformed RPL traffic.

How to Mitigate CVE-2023-2156

Immediate Actions Required

• Apply the kernel updates published in Debian Security Advisory DSA-5448, DSA-5453, and the Debian LTS Security Announcement.
• Red Hat Enterprise Linux 9.0 administrators should track the fix referenced in the Red Hat Bugzilla Report and install the corresponding errata kernel.
• Fedora 38 users should run dnf update kernel to obtain patched kernel builds.
• NetApp customers should review the NetApp Security Advisory for affected appliance firmware.

Patch Information

Patches were coordinated across distributions following the May 2023 disclosure. Debian shipped fixes through DSA-5448 and DSA-5453, while Red Hat and Fedora delivered patched kernel packages through their standard update channels. The upstream Linux kernel fix adds input validation in the RPL packet handling code to reject malformed input before the assertion is reached. Reboot is required after kernel installation for the fix to take effect.

Workarounds

• Blacklist the RPL kernel module on systems that do not require it by adding blacklist rpl to a file under /etc/modprobe.d/ and rebuilding the initramfs.
• Use iptables or nftables rules to drop IPv6 packets containing Routing Type 3 headers at the host or network perimeter.
• Restrict exposure of Linux hosts to untrusted networks until patched kernels are deployed across the fleet.

# Disable the RPL kernel module until patched kernel is deployed
echo 'blacklist rpl' | sudo tee /etc/modprobe.d/disable-rpl.conf
sudo rmmod rpl 2>/dev/null || true

# Drop IPv6 RPL routing headers with nftables
sudo nft add rule inet filter input meta nfproto ipv6 \
  exthdr rt type 3 drop

# Verify patched kernel is installed and rebooted into
uname -r