CVE-2026-63834 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
batman-adv: tp_meter: restrict number of unacked list entries
When the unacked_list is unbound, an attacker could send messages with small lengths and appropriated seqno + gaps to force the receiver to allocate more and more unacked_list entries. And the end either causing an out-of-memory situation or increase the management overhead for the (large) list that significant portions of CPU cycles are wasted in searching through the list.
When limiting the list to a specific number, it is important to still correctly add a new entry to the list. But if the list became larger than the limit, the last entry of the list (with the highest seqno) must be dropped to still allow the earlier seqnos to finish and therefore to continue the process. Otherwise, the process might get stuck with too high seqnos which are not handled by batadv_tp_ack_unordered().
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described represents a critical resource exhaustion flaw in the batman-adv kernel module that implements the Batman-Advanced mesh routing protocol. This issue stems from inadequate bounds checking on the unacked_list data structure, which is used to track transmitted packets awaiting acknowledgment within the mesh network. The problem manifests when an attacker crafts malicious messages with specifically sized payloads and sequence number patterns that exploit the lack of proper list size limitations, creating a denial-of-service condition that can either consume all available memory or severely degrade system performance through excessive CPU overhead.
The technical implementation flaw resides in the tp_meter component of batman-adv where the unacked_list grows without bound under certain conditions. When an attacker sends packets with small lengths and carefully constructed sequence numbers that create gaps in the acknowledgment sequence, the system continues to add new entries to the list indefinitely. This behavior directly violates the principle of resource limitation and proper memory management that should be enforced in kernel space code. The vulnerability aligns with CWE-770, which addresses allocation of resources without limits or proper checks, and demonstrates poor input validation practices that allow unbounded growth of data structures.
From an operational perspective, this vulnerability exposes systems running Batman-Advanced mesh networks to significant risks including complete system hangs due to memory exhaustion or severe performance degradation caused by excessive CPU cycles spent traversing the massive unacked_list. The impact extends beyond individual hosts to potentially disrupt entire mesh network operations, as the affected nodes may become unresponsive or fail to properly forward traffic. This vulnerability particularly affects wireless mesh networks where Batman-Advanced is commonly deployed for autonomous routing in scenarios such as community networks, IoT deployments, and enterprise wireless infrastructure.
The mitigation strategy requires implementing proper bounds checking on the unacked_list with a reasonable maximum size limit while maintaining correct sequence number handling logic. When the list exceeds its configured limit, the system must properly drop the entry with the highest sequence number to ensure that lower sequence numbers can still progress through the acknowledgment process. This approach prevents the system from becoming stuck with high sequence numbers that cannot be processed by batadv_tp_ack_unordered() function. The solution should follow ATT&CK framework domain T1499.004, which addresses network denial of service attacks, by implementing proper resource management and input validation to prevent exploitation through crafted packet sequences.
This vulnerability highlights the importance of defensive programming practices in kernel modules where untrusted network input must be carefully validated and bounded. The fix demonstrates the critical need for proper resource management in distributed systems, particularly in mesh networking protocols where nodes must handle potentially malicious traffic from other network participants. The implementation should include monitoring and logging capabilities to detect unusual patterns that might indicate exploitation attempts, while ensuring that legitimate network operations continue to function normally under normal conditions.