CVE-2026-23011: Linux Kernel ipv4 ip_gre DoS Vulnerability

CVE-2026-23011 Overview

A buffer underflow vulnerability exists in the Linux kernel's IPv4 GRE (Generic Routing Encapsulation) implementation, specifically in the ipgre_header() function within net/ipv4/ip_gre.c. This vulnerability allows attackers to trigger a kernel panic through improper handling of socket buffer (skb) headroom allocation when team or bonding network drivers dynamically change their dev->needed_headroom and/or dev->hard_header_len values.

Critical Impact

Successful exploitation can cause kernel panics and system crashes, leading to denial of service conditions on affected Linux systems using GRE tunneling with team or bonding network configurations.

Affected Products

• Linux kernel (multiple versions with IPv4 GRE support)
• Systems using team or bonding network drivers with GRE tunnels
• Network infrastructure utilizing IPv4 GRE encapsulation

Discovery Timeline

• January 25, 2026 - CVE CVE-2026-23011 published to NVD
• January 26, 2026 - Last updated in NVD database

Technical Details for CVE-2026-23011

Vulnerability Analysis

This vulnerability stems from a race condition between skb buffer allocation and network device configuration changes. The issue is analogous to a previously resolved vulnerability in the IPv6 GRE implementation (commit db5b4e39c4e6 - "ip6_gre: make ip6gre_header() robust").

The kernel's syzbot fuzzing tool discovered multiple crash vectors in ipgre_header() over the years. The fundamental problem occurs when team or bonding drivers dynamically modify their dev->needed_headroom and/or dev->hard_header_len properties after an skb has already been allocated with insufficient headroom.

In the specific crash scenario identified, mld_newpack() allocates an skb with a reserve/headroom value that becomes insufficient by the time mld_sendpack() is called. During this window, syzbot managed to attach an ipgre device, causing a mismatch between the allocated buffer space and the required header size.

Root Cause

The root cause is improper validation of available headroom in the ipgre_header() function before performing skb_push() operations. When the network device's header requirements change dynamically (as can happen with team or bonding drivers), the previously allocated skb may not have sufficient space to accommodate the GRE header, resulting in a buffer underflow condition.

The kernel crash manifests as an skb_under_panic with the following characteristics:

• The skb_push() operation at ipgre_header+0x67/0x290 attempts to push header data beyond the allocated buffer boundaries
• This triggers skb_under_panic in net/core/skbuff.c:223
• The system generates a kernel BUG with invalid opcode 0000

Attack Vector

The attack leverages the ability to dynamically reconfigure network devices while multicast operations are in progress. The attack path follows this sequence:

1. The system initiates MLD (Multicast Listener Discovery) packet construction via mld_newpack()
2. An skb is allocated with headroom based on current device configuration
3. An attacker or automated fuzzer reconfigures team/bonding device properties
4. An ipgre device is attached with different header requirements
5. When mld_sendpack() attempts to send the packet through ipgre_header(), the skb_push() operation exceeds buffer boundaries

The call trace demonstrates the execution path from the MLD workqueue through the IPv6 output path to the vulnerable ipgre_header() function:

mld_ifc_work → mld_sendpack → ip6_output → neigh_connected_output → ipgre_header → skb_push → skb_under_panic

Detection Methods for CVE-2026-23011

Indicators of Compromise

• Kernel panic messages containing skb_under_panic in system logs
• Crash dumps referencing ipgre_header in the call trace
• System instability when using GRE tunnels with team or bonding interfaces
• Workqueue crashes in the mld subsystem with mld_ifc_work in the trace

Detection Strategies

• Monitor kernel logs for skb_under_panic events with ipgre_header in the stack trace
• Implement crash dump analysis to identify patterns matching this vulnerability signature
• Deploy kernel tracing tools to monitor skb_push() operations in the GRE code path
• Use SentinelOne's kernel-level monitoring to detect anomalous network stack behavior

Monitoring Recommendations

• Enable kernel crash dump collection and analysis on systems using GRE tunneling
• Monitor for unusual activity around team or bonding device reconfigurations
• Track MLD workqueue execution anomalies on affected systems
• Implement alerting on kernel BUG conditions in network-related code paths

How to Mitigate CVE-2026-23011

Immediate Actions Required

• Update the Linux kernel to a patched version containing the fix commits
• Temporarily avoid using GRE tunnels in combination with team or bonding interfaces until patched
• Review and restrict permissions for network device reconfiguration
• Enable crash dump collection for forensic analysis if exploitation is suspected

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix makes ipgre_header() robust against dynamic headroom changes by properly validating buffer space before performing push operations. The following commits contain the fix:

• Kernel Git Commit 554201e
• Kernel Git Commit aa57bfe
• Kernel Git Commit e67c577

Administrators should apply the appropriate patch based on their kernel version branch.

Workarounds

• Avoid using team or bonding drivers in conjunction with GRE tunnel interfaces until the patch is applied
• Limit network device reconfiguration capabilities to trusted administrators only
• Consider using alternative tunneling technologies (e.g., VXLAN, IPsec) that are not affected by this specific issue
• Implement network segmentation to reduce exposure of vulnerable systems

# Check if GRE modules are loaded
lsmod | grep gre

# Temporarily disable ipgre module if not required (requires root)
modprobe -r ip_gre

# Verify team/bonding configuration
ip link show type team
ip link show type bond