CVE-2026-31531 Overview
CVE-2026-31531 is a vulnerability in the Linux kernel's IPv4 nexthop handling that affects the rtm_get_nexthop() function. When querying a nexthop object via RTM_GETNEXTHOP, the kernel allocates a fixed-size socket buffer (skb) using NLMSG_GOODSIZE. While this allocation is sufficient for single nexthops and small Equal-Cost Multi-Path (ECMP) groups, it fails when handling large nexthop groups containing 512 or more nexthops, triggering a kernel warning and potential denial of service conditions.
Critical Impact
This vulnerability can cause kernel warnings and potential system instability when processing large nexthop group queries via the netlink interface, affecting network routing operations on Linux systems.
Affected Products
- Linux Kernel (IPv4 nexthop subsystem)
- Systems using large ECMP nexthop groups (512+ nexthops)
- Linux-based networking appliances and routers
Discovery Timeline
- April 23, 2026 - CVE-2026-31531 published to NVD
- April 23, 2026 - Last updated in NVD database
Technical Details for CVE-2026-31531
Vulnerability Analysis
The vulnerability exists in the rtm_get_nexthop() function within net/ipv4/nexthop.c. The function is responsible for handling RTM_GETNEXTHOP netlink requests, which query nexthop objects from the kernel's routing subsystem. The core issue stems from the use of a static buffer size (NLMSG_GOODSIZE) that cannot accommodate the response data for large nexthop groups.
When a user-space application queries a nexthop group with a large number of entries (such as 512 nexthops in an ECMP configuration), the allocated buffer is insufficient to hold the complete response. This triggers a warning at line 3395 of nexthop.c and causes the operation to fail. The call trace shows the path from do_syscall_64 through the netlink message handling subsystem to the vulnerable function.
Root Cause
The root cause is the static allocation strategy using NLMSG_GOODSIZE for the socket buffer in rtm_get_nexthop(). The function did not account for variable-sized nexthop groups, and the nh_nlmsg_size_grp() function was not correctly calculating the required buffer size based on the actual nexthop group configuration. Additionally, the size calculation was missing the NHA_FDB attribute size, further contributing to potential buffer insufficiency.
Attack Vector
An attacker or user with the ability to create large nexthop groups and query them via the RTM_GETNEXTHOP netlink interface could trigger this vulnerability. The attack requires local access or the ability to send netlink messages to the kernel, typically requiring appropriate capabilities or root-level access. While iproute2 currently limits group sizes and would fail before triggering this issue, custom tools or modified utilities could bypass these user-space limitations.
The vulnerability manifests when the kernel attempts to serialize a large nexthop group response into the undersized buffer. The fix involves dynamically allocating the skb using nh_nlmsg_size() and nlmsg_new(), consistent with the behavior in nexthop_notify(). This ensures the buffer is appropriately sized for the actual nexthop group being queried.
Detection Methods for CVE-2026-31531
Indicators of Compromise
- Kernel warning messages containing rtm_get_nexthop in system logs
- Stack traces in dmesg referencing net/ipv4/nexthop.c:3395
- Failed netlink queries when retrieving large nexthop groups
- Unexpected routing subsystem errors or instability
Detection Strategies
- Monitor kernel logs (dmesg, journalctl) for WARNING messages from rtm_get_nexthop
- Implement monitoring for netlink socket errors related to routing queries
- Deploy kernel tracing (ftrace, eBPF) to track rtm_get_nexthop() failures
- Use SentinelOne's Singularity platform to detect kernel anomalies and warning patterns
Monitoring Recommendations
- Enable kernel logging at appropriate verbosity levels to capture warning splats
- Monitor system stability metrics on systems with large ECMP configurations
- Track netlink message processing statistics for anomalies
- Implement alerting on repeated routing subsystem failures
How to Mitigate CVE-2026-31531
Immediate Actions Required
- Update the Linux kernel to a patched version containing the fix
- Avoid configuring nexthop groups with 512 or more entries until patched
- Review network configurations for large ECMP group usage
- Monitor affected systems for warning messages and instability
Patch Information
The Linux kernel development team has released patches to address this vulnerability. The fix modifies rtm_get_nexthop() to dynamically allocate the socket buffer using nh_nlmsg_size() and nlmsg_new(), matching the existing nexthop_notify() behavior. Additionally, nh_nlmsg_size_grp() has been updated to correctly calculate buffer sizes based on passed flags, and NHA_FDB is now included in nexthop group size calculations.
Patches are available through the following kernel commits:
- Kernel Git Commit 14cf0cd
- Kernel Git Commit 40bd39e
- Kernel Git Commit 615517f
- Kernel Git Commit 635038fe
Workarounds
- Limit nexthop group sizes to fewer than 512 entries until the patch is applied
- Implement network topology changes to reduce ECMP group sizes
- Use multiple smaller nexthop groups instead of single large groups
- Restrict access to netlink interfaces on sensitive systems
# Check current kernel version
uname -r
# View applied patches (if using git-based kernel)
git log --oneline net/ipv4/nexthop.c | head -20
# Monitor for vulnerability-related warnings
dmesg | grep -i "rtm_get_nexthop"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
