CVE-2026-63836 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
batman-adv: tp_meter: avoid divide-by-zero for dec_cwnd
The cwnd is always MSS <= cwnd <= 0x20000000. But the calculation in batadv_tp_update_cwnd() assumes unsigned 32 bit arithmetics.
((mss * 8) ** 2) / (cwnd * 8)
In case cwnd is actually 0x20000000, it will be shifted by 3 bit to the left end up at 0x100000000 or U32_MAX + 1. It will therefore wrap around and be 0 - resulting in:
((mss * 8) ** 2) / 0
This is of course invalid and cannot be calculated. The calculation should must be simplified to avoid this overflow:
(mss ** 2) * 8 / cwnd
It will keep the precision enhancement from the scaling (by 8) but avoid the overflow in the divisor.
In theory, there could still be an overflow in the dividend. It is at the moment fixed to BATADV_TP_PLEN in batadv_tp_recv_ack() - so it is not an imminent problem. But allowing it to use the whole u32 bit range, would mean that it can still use up to 67 bits. To keep this calculation safe for 32 bit arithmetic, mss must never use more than floor((32 - 3) / 2) bits - or in other words: must never be larger than 16383.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability identified in the Linux kernel's batman-adv module represents a critical divide-by-zero condition that could lead to system instability and potential denial of service attacks. This issue specifically affects the tp_meter functionality within the batman-adv subsystem, which is responsible for traffic policing and congestion control in wireless mesh networks. The flaw arises from improper handling of unsigned 32-bit arithmetic during congestion window calculations, creating a scenario where mathematical overflow can result in invalid division operations that crash kernel processes or render network functionality inoperable.
The technical root cause stems from the batadv_tp_update_cwnd() function's implementation of the formula ((mss 8) 2) / (cwnd 8) where the calculation assumes standard unsigned 32-bit arithmetic without proper overflow handling. When cwnd reaches its maximum value of 0x20000000, bit shifting operations cause the value to overflow beyond the 32-bit boundary, wrapping around to zero and creating a division by zero error that fundamentally breaks the mathematical operation. This particular vulnerability maps directly to CWE-369: Divide by Zero and CWE-191: Integer Underflow/Overflow, as both conditions are present in the flawed implementation.
The operational impact of this vulnerability extends beyond simple system crashes to potentially compromise network reliability in mesh networking environments where batman-adv is deployed. Attackers could exploit this condition to cause persistent denial of service by triggering the overflow scenario, effectively disrupting wireless communication pathways within mesh networks. The vulnerability affects systems utilizing the batman-adv kernel module for wireless mesh networking, particularly those with high bandwidth utilization or heavy traffic loads that might push cwnd values toward the maximum threshold.
The recommended mitigation strategy involves implementing the simplified calculation (mss * 2) 8 / cwnd which maintains precision through the scaling factor while eliminating the overflow condition in the divisor. This approach aligns with defensive programming principles from the ATT&CK framework's defense evasion techniques, specifically targeting the prevention of arithmetic overflow conditions that could be exploited. Additionally, the fix requires ensuring that mss values never exceed 16383 to prevent potential overflow in the dividend calculation, establishing bounds checking that prevents the 67-bit arithmetic overflow scenario mentioned in the vulnerability description.
The resolution addresses both immediate and potential future risks by establishing proper integer boundary handling and mathematical precision preservation. This mitigation approach ensures that network traffic policing remains functional even under extreme load conditions while maintaining the original intent of improving throughput calculations through scaling operations. The fix also demonstrates adherence to secure coding practices that prevent similar vulnerabilities in kernel-level networking code, reducing the attack surface for sophisticated exploitation attempts targeting wireless mesh network infrastructure.