CVE-2026-31405: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31405 Overview

CVE-2026-31405 is an out-of-bounds read vulnerability in the Linux kernel's dvb-net driver. The flaw resides in the handle_one_ule_extension() function, which processes Unidirectional Lightweight Encapsulation (ULE) extension headers used in Digital Video Broadcasting (DVB) streams. The function uses two handler tables, ule_mandatory_ext_handlers[] and ule_optional_ext_handlers[], each declared with 255 elements. The index htype is derived from network-controlled data via (ule_sndu_type & 0x00FF), producing a range of 0-255. When htype equals 255, an out-of-bounds read occurs against the function pointer table, and the retrieved value may be invoked as a function pointer.

Critical Impact

Network-attacker-controlled input can trigger an out-of-bounds function pointer dereference in the Linux kernel, potentially enabling arbitrary code execution at the kernel level.

Affected Products

• Linux kernel branches containing the dvb-net ULE handling code prior to the fix
• Systems with DVB network interfaces enabled and processing ULE-encapsulated traffic
• Multiple stable kernel branches patched in commits 145e50c, 1a6da3d, 24d8771, 29ef43c, 8bde543, b2bd2ee, e512387, and f2b65dc

Discovery Timeline

• 2026-04-06 - CVE-2026-31405 published to NVD
• 2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31405

Vulnerability Analysis

The vulnerability resides in the Linux kernel media subsystem, specifically the DVB network driver (drivers/media/dvb-core/dvb_net.c). The ULE protocol encapsulates IP traffic over MPEG-2 transport streams, and extension headers within ULE Subnetwork Data Units (SNDUs) are dispatched through function pointer tables. Because htype is computed from an attacker-controlled byte and the tables are sized to 255 entries, the maximum legal index is 254. A value of 255 reads past the end of the table and treats the resulting memory contents as a function pointer.

Invoking an attacker-influenced or unpredictable function pointer in kernel context can lead to a kernel crash (denial of service) or arbitrary code execution depending on the memory layout adjacent to the handler tables. The upstream fix introduces a bounds check on htype against the array size, causing out-of-range values to be discarded along with the SNDU.

Root Cause

The root cause is an off-by-one mismatch between the value space of a network-derived index and the declared size of the dispatch tables. The handler arrays accommodate 255 entries (indices 0-254), while the index derivation masks to 8 bits (0-255). No bounds check was performed before indexing, classifying this as an Out-of-Bounds Read leading to control-flow hijack via an unchecked function pointer call.

Attack Vector

Exploitation requires the target system to process attacker-controlled DVB transport streams carrying ULE-encapsulated data with a crafted ule_sndu_type whose low byte equals 0xFF. On systems where DVB network reception is exposed or where stream sources are untrusted, an attacker can deliver a malicious SNDU to trigger the out-of-bounds read and subsequent indirect call.

No verified public exploit code is available. Refer to the upstream patch commits for the precise code paths and remediation logic.

Detection Methods for CVE-2026-31405

Indicators of Compromise

• Unexpected kernel oops or panic messages referencing handle_one_ule_extension, dvb_net, or invalid instruction pointers in kernel logs
• Crashes or instability on hosts receiving DVB transport streams shortly after malformed ULE traffic arrives
• Abnormal kernel module behavior on systems where the dvb_net module is loaded but no legitimate DVB workload is expected

Detection Strategies

• Inventory hosts where the dvb_net kernel module is loaded using lsmod | grep dvb_net and prioritize them for patching
• Compare running kernel versions against the fixed commit set (145e50c, 1a6da3d, 24d8771, 29ef43c, 8bde543, b2bd2ee, e512387, f2b65dc) to identify unpatched systems
• Monitor kernel ring buffer output via dmesg and centralized logging for ULE-related faults

Monitoring Recommendations

• Forward kernel logs to a centralized logging or SIEM platform and alert on panics, oops events, and indirect call faults from media drivers
• Track loaded kernel module inventories across the fleet to identify systems that should not have dvb_net enabled
• Baseline DVB traffic sources and alert on unexpected ULE stream ingestion paths

How to Mitigate CVE-2026-31405

Immediate Actions Required

• Apply the upstream Linux kernel patches referenced in the commit list to all affected stable branches
• Unload the dvb_net module on systems that do not require DVB networking using modprobe -r dvb_net
• Restrict physical and network access to DVB tuners and transport stream sources to trusted inputs only

Patch Information

The fix adds a bounds check on htype against the array size before either the mandatory or optional handler table is accessed. Out-of-range values cause the SNDU to be discarded. Patched commits are available in the upstream kernel tree, including Linux Kernel Commit 145e50c, Linux Kernel Commit 1a6da3d, Linux Kernel Commit 24d8771, Linux Kernel Commit 29ef43c, Linux Kernel Commit 8bde543, Linux Kernel Commit b2bd2ee, Linux Kernel Commit e512387, and Linux Kernel Commit f2b65dc.

Workarounds

• Blacklist the dvb_net module on hosts that do not need DVB networking by adding blacklist dvb_net to /etc/modprobe.d/
• Ensure DVB transport stream inputs originate exclusively from trusted broadcast sources
• Apply distribution vendor kernel updates as soon as they incorporate the upstream fix

# Prevent the vulnerable module from loading on systems that do not require DVB networking
echo "blacklist dvb_net" | sudo tee /etc/modprobe.d/disable-dvb-net.conf
sudo modprobe -r dvb_net