CVE-2026-31417 Overview
CVE-2026-31417 is a Linux kernel vulnerability in the net/x25 subsystem. The flaw involves an integer overflow when accumulating packets in the X.25 protocol implementation. Specifically, the x25_sock.fraglen field can overflow during packet fragmentation handling. Additionally, the fraglen value is not properly reset when x25_clear_queues() purges the fragment_queue. A remote attacker can exploit this condition over the network without authentication to cause a denial-of-service condition affecting kernel availability.
Critical Impact
Remote, unauthenticated attackers can trigger a kernel-level denial of service by sending crafted X.25 packets that overflow the fraglen accumulator.
Affected Products
- Linux Kernel (multiple stable branches referenced by upstream commits)
- Systems with the net/x25 protocol module loaded or available
- Distributions shipping unpatched Linux kernels prior to backport
Discovery Timeline
- 2026-04-13 - CVE-2026-31417 published to NVD
- 2026-04-27 - Last updated in NVD database
Technical Details for CVE-2026-31417
Vulnerability Analysis
The vulnerability resides in the X.25 protocol implementation within the Linux kernel networking stack. The x25_sock structure tracks fragmented packet state through a fraglen field that accumulates the total length of fragments queued for reassembly. The kernel fails to validate this accumulator against an upper bound, allowing the value to overflow when an attacker sends a sufficient quantity or size of fragments.
A second defect compounds the issue. When the kernel calls x25_clear_queues() to purge the fragment_queue, the routine does not reset fraglen to zero. The stale length value persists across socket state transitions, creating inconsistent internal accounting that can be leveraged to amplify the overflow condition.
The upstream fix introduces a bounds check on fraglen accumulation and resets the field during queue purges. Patches were committed to multiple stable branches, referenced by commits 1734bd8, 4e2d1bc, 6e56883, 798d613, 8c92969, 96fc163, a1822cb, and f953f11.
Root Cause
The root cause is missing arithmetic bounds validation on the x25_sock.fraglen counter and missing state reset logic in x25_clear_queues(). Both conditions together permit the integer overflow to corrupt fragment-reassembly accounting in the X.25 socket layer.
Attack Vector
An attacker reachable over the network can send crafted X.25 packets to a target socket. By chaining fragmented packets, the attacker grows fraglen until the integer wraps. The resulting inconsistent state can trigger kernel panics or hangs, producing a denial of service. No authentication or user interaction is required.
// No verified proof-of-concept code is publicly available.
// Refer to the upstream commits for the precise code paths in
// net/x25/x25_in.c and net/x25/x25_subr.c that introduce the
// fraglen bound check and the queue-purge reset.
Detection Methods for CVE-2026-31417
Indicators of Compromise
- Unexpected kernel oops, panic, or soft-lockup messages referencing x25_ symbols in dmesg or /var/log/kern.log.
- Abnormal traffic to TCP/UDP encapsulated X.25 endpoints or raw X.25 sockets on hosts that do not normally use the protocol.
- Sudden loss of network responsiveness on hosts where the x25 kernel module is loaded.
Detection Strategies
- Inventory hosts where the x25 module is loaded using lsmod | grep x25 and flag systems with no legitimate X.25 use case.
- Monitor kernel ring buffer telemetry for crash signatures originating in the X.25 packet input path.
- Correlate kernel crash events with inbound network flows targeting X.25-enabled hosts to identify exploitation attempts.
Monitoring Recommendations
- Forward kernel logs and crash dumps to a centralized logging platform for retention and correlation.
- Alert on repeated x25_ stack traces, which indicate active probing or exploitation.
- Track patch compliance against the upstream stable kernel commits listed in the references.
How to Mitigate CVE-2026-31417
Immediate Actions Required
- Apply the vendor-provided kernel update that includes the upstream fix for net/x25 fragment accumulation.
- Unload and blacklist the x25 kernel module on systems that do not require X.25 connectivity.
- Restrict network reachability to hosts running X.25 services using host or network firewalls.
Patch Information
The fix is available in the upstream Linux stable tree through the following commits: Linux Kernel Commit 1734bd8, Linux Kernel Commit 4e2d1bc, Linux Kernel Commit 6e56883, Linux Kernel Commit 798d613, Linux Kernel Commit 8c92969, Linux Kernel Commit 96fc163, Linux Kernel Commit a1822cb, and Linux Kernel Commit f953f11. Rebuild and deploy kernels from these branches or install distribution updates incorporating the backports.
Workarounds
- Blacklist the x25 module by adding blacklist x25 to a file under /etc/modprobe.d/.
- Block inbound X.25 protocol traffic at perimeter and host firewalls where the protocol is not in use.
- Restrict the creation of AF_X25 sockets to trusted users via seccomp or Linux Security Modules.
# Disable and prevent loading of the x25 kernel module
sudo modprobe -r x25
echo "blacklist x25" | sudo tee /etc/modprobe.d/disable-x25.conf
sudo update-initramfs -u
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
