CVE-2026-98857 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
ipvs: bound LBLCR and LBLC cache growth
ip_vs_lblcr_new() and ip_vs_lblc_new() create cache entries for every previously unseen destination address. The table max_size only tells the periodic collector to reclaim entries after the cache has already exceeded the limit. It does not reclaim entries that the attacker continues to use.
Reject new cache entries once either table reaches max_size * 3 / 2. The extra headroom lets the periodic collector catch up while the existing scheduler fallback continues to use the selected destination when cache creation fails. New traffic therefore stays serviceable without growing the tables further.
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Analysis
by VulDB Data Team • 10/21/2026
The Linux kernel’s IP Virtual Server (IPVS) implementation contains a resource exhaustion vulnerability within its Least Connection with Local Weight and Round Robin (LBLCR) and Least Connection (LBLC) scheduling algorithms. These components maintain dynamic caches to track destination server states, creating new cache entries for every previously unseen destination address encountered during load balancing operations. The core technical flaw lies in the enforcement mechanism of the table size limits. While a maximum size parameter exists, it is only utilized by a periodic collector process responsible for reclaiming stale or expired entries after the cache has already exceeded this limit. This design creates a window where an attacker can continuously inject traffic directed at unique destination addresses, causing the kernel to allocate new memory structures without immediate restriction until the threshold is breached and the background reclamation cycle triggers.
This lack of proactive enforcement allows for rapid memory consumption on the host system running the IPVS service. An unauthenticated or authenticated remote attacker capable of sending a high volume of packets with varying destination addresses can exploit this behavior to trigger a denial-of-service condition. By forcing the kernel to allocate cache entries faster than they are reclaimed, the attacker induces excessive memory usage which may lead to system instability, process termination due to out-of-memory errors, or complete unavailability of the load-balanced services hosted behind the IPVS instance. The vulnerability effectively bypasses intended resource controls by exploiting the asynchronous nature of the cleanup routine relative to the allocation logic.
The resolution addresses this issue by implementing a hard limit on cache growth that operates independently of the periodic reclamation cycle. Specifically, new cache entries are now rejected once either the LBLCR or LBLC table reaches one and a half times the configured maximum size. This additional headroom serves two critical functions: it prevents immediate memory exhaustion while providing sufficient time for the periodic collector to catch up and reclaim unused entries. Furthermore, when cache creation fails due to this limit being reached, the scheduler falls back to using existing destinations or default behaviors rather than crashing or hanging, ensuring that new traffic remains serviceable without further expanding the tables. This approach balances resource protection with operational continuity under attack conditions.
From a classification perspective, this vulnerability aligns with CWE-400, which describes uncontrolled resource consumption, as well as CWE-787, regarding out-of-bounds write scenarios if memory corruption were involved in more severe variants of such flaws, though here the primary impact is denial of service through allocation limits. In terms of adversarial tactics, this relates to ATT&CK technique T1496, Resource Hijacking, where attackers consume resources to degrade system performance or availability. Mitigation strategies involve applying the provided kernel patch that enforces stricter cache size boundaries and rejecting new entries proactively rather than reactively. Administrators should also monitor IPVS table sizes and configure appropriate max_size limits based on expected traffic patterns to minimize the window of opportunity for such exhaustion attacks, ensuring that the fallback mechanisms remain effective during peak load or malicious activity.