CVE-2026-31512 Overview
A vulnerability has been identified in the Linux kernel's Bluetooth L2CAP (Logical Link Control and Adaptation Protocol) subsystem. The flaw exists in the l2cap_ecred_data_rcv() function, which reads the SDU (Service Data Unit) length field from skb->data using get_unaligned_le16() without first verifying that the socket buffer contains at least L2CAP_SDULEN_SIZE (2) bytes. When skb->len is less than 2 bytes, this operation reads past the valid data in the skb, resulting in an out-of-bounds read condition.
Critical Impact
An attacker could exploit this vulnerability to read kernel memory beyond buffer boundaries via malformed Bluetooth L2CAP packets, potentially leading to information disclosure or system instability.
Affected Products
- Linux Kernel (Bluetooth L2CAP subsystem)
- Systems with Enhanced Credit Based Flow Control enabled
- Devices with active Bluetooth connectivity
Discovery Timeline
- 2026-04-22 - CVE CVE-2026-31512 published to NVD
- 2026-04-23 - Last updated in NVD database
Technical Details for CVE-2026-31512
Vulnerability Analysis
The vulnerability resides in the Enhanced Credit Based Flow Control data path within the Linux kernel's Bluetooth L2CAP implementation. The l2cap_ecred_data_rcv() function is responsible for processing incoming Enhanced Credit Based Mode data frames. This function attempts to read a 2-byte SDU length field from the beginning of the socket buffer without first validating that sufficient data exists.
The ERTM (Enhanced Retransmission Mode) reassembly path correctly implements this validation by calling pskb_may_pull() before reading the SDU length in the l2cap_reassemble_sdu function's L2CAP_SAR_START case. However, this safety check was not consistently applied to the Enhanced Credit Based Flow Control data path, creating an inconsistency in input validation across the L2CAP subsystem.
When an attacker sends a specially crafted L2CAP PDU with less than 2 bytes of data, the vulnerable function reads beyond the allocated buffer boundaries, accessing adjacent kernel memory.
Root Cause
The root cause is missing input validation in l2cap_ecred_data_rcv(). The function uses get_unaligned_le16() to read the SDU length directly from skb->data without first calling pskb_may_pull() to ensure at least L2CAP_SDULEN_SIZE (2) bytes are available in the linear data portion of the socket buffer. This violates the principle of validating input length before accessing data.
Attack Vector
An attacker within Bluetooth range could exploit this vulnerability by sending malformed L2CAP Enhanced Credit Based Flow Control data frames with truncated or empty payloads. The attack requires:
- Bluetooth connectivity to the target system
- Establishing an L2CAP connection with Enhanced Credit Based Flow Control mode
- Sending a crafted PDU with less than 2 bytes of payload data
The vulnerable code path processes the malformed packet and attempts to read the SDU length field, causing an out-of-bounds read from kernel memory.
Detection Methods for CVE-2026-31512
Indicators of Compromise
- Kernel log messages indicating Bluetooth subsystem errors or crashes
- Unexpected system instability when processing Bluetooth connections
- Memory access violations in L2CAP-related kernel functions
- Bluetooth service disruptions or daemon crashes
Detection Strategies
- Monitor kernel logs for L2CAP-related error messages using dmesg or journalctl -k
- Deploy kernel tracing with ftrace to monitor l2cap_ecred_data_rcv() function calls
- Implement network-level Bluetooth traffic analysis for anomalous L2CAP frame sizes
- Use KASAN (Kernel Address Sanitizer) in development environments to detect out-of-bounds reads
Monitoring Recommendations
- Enable kernel auditing for Bluetooth subsystem events
- Configure alerting on repeated Bluetooth connection failures or resets
- Monitor for unusual Bluetooth device pairing attempts from unknown sources
- Implement host-based intrusion detection rules for kernel crash patterns
How to Mitigate CVE-2026-31512
Immediate Actions Required
- Apply the latest Linux kernel patches from your distribution vendor
- Disable Bluetooth if not required in your environment using rfkill block bluetooth
- Restrict Bluetooth visibility and disable automatic pairing
- Monitor systems for signs of exploitation attempts
Patch Information
The Linux kernel development team has addressed this vulnerability by adding proper length validation before reading the SDU length field. The fix applies the same pskb_may_pull() validation used in the ERTM reassembly path to the Enhanced Credit Based Flow Control data path.
Multiple kernel commits have been released to address this issue across different stable kernel branches:
- Kernel Git Commit 3340be2
- Kernel Git Commit 40c7f7e
- Kernel Git Commit 477ad49
- Kernel Git Commit 5ad9812
- Kernel Git Commit 8c96f3b
- Kernel Git Commit c65bd94
- Kernel Git Commit cef0969
- Kernel Git Commit e47315b
Workarounds
- Disable Bluetooth functionality at the kernel module level by blacklisting bluetooth and btusb modules
- Implement firewall rules to restrict Bluetooth protocol access where applicable
- Physical removal or disabling of Bluetooth hardware on critical systems
- Network segmentation to isolate systems with required Bluetooth functionality
# Configuration example
# Disable Bluetooth by blacklisting kernel modules
echo "blacklist bluetooth" >> /etc/modprobe.d/disable-bluetooth.conf
echo "blacklist btusb" >> /etc/modprobe.d/disable-bluetooth.conf
# Disable Bluetooth service immediately
rfkill block bluetooth
systemctl stop bluetooth
systemctl disable bluetooth
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
