CVE-2026-74670 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
ipvs: stop estimator after disabled calc phase
IPVS estimator kthread 0 starts with zeroed chain and tick limits until its initial calculation phase completes. If network namespace teardown clears ipvs->enable during that phase, ip_vs_est_calc_phase() can return without installing positive limits.
The kthread can then continue into its main loop and drain est_temp_list with zero chain_max, tick_max and est_max_count values. Each enqueue consumes one available tick row, but est_count never reaches the zero est_max_count value. After all rows are consumed, the row lookup returns IPVS_EST_NTICKS and ip_vs_enqueue_estimator() writes past the ticks and tick_len arrays.
Exit kthread 0 after the calculation phase if the kthread is stopping or IPVS has been disabled. That keeps temporary estimators from being drained after the limits failed to initialize.
Estimator kthreads can now self-exit before teardown or reload stops kd->task. Keep an extra task reference after creation and release it with kthread_stop_put(), so kd->task remains valid until the stop paths consume that reference.
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Analysis
by VulDB Data Team • 08/22/2026
The vulnerability resides within the IP Virtual Server (IPVS) subsystem of the Linux kernel, specifically affecting the estimator thread responsible for tracking network statistics. This component operates as a background kernel thread designed to periodically calculate and update connection rates for various virtual services. The core issue arises from a race condition during the initialization phase of this kthread when combined with rapid namespace teardown or IPVS disablement events. Specifically, the initial calculation phase requires time to populate necessary limits such as chain maximums, tick maximums, and estimator count thresholds. During this window, if the network namespace is being torn down or IPVS functionality is disabled via ipvs->enable flag changes, the initialization logic may fail to install these positive limit values correctly. Consequently, the kthread proceeds into its main operational loop with zeroed parameters for chain_max, tick_max, and est_max_count.
This state of uninitialized limits leads directly to a heap-based buffer overflow vulnerability classified under CWE-787: Out-of-bounds Write. As the estimator thread enters its primary processing cycle, it attempts to drain temporary estimation data from the est_temp_list. Because the maximum count threshold is zero, the condition that normally prevents excessive queue consumption fails immediately. The loop continues to dequeue items and consume available tick rows without ever reaching a stopping condition based on est_max_count. Since each enqueue operation consumes one row in the underlying ticks array, this uncontrolled iteration rapidly exhausts all allocated memory slots for tracking time intervals. Once these rows are fully consumed, subsequent lookups return IPVS_EST_NTICKS, indicating no valid tick data exists. However, instead of halting or handling this error gracefully, ip_vs_enqueue_estimator() proceeds to write past the boundaries of the ticks and tick_len arrays. This out-of-bounds memory access corrupts adjacent kernel heap structures, potentially leading to arbitrary code execution with kernel privileges if an attacker can influence the timing of namespace teardowns or IPVS state changes relative to estimator initialization.
From a defensive security perspective, this flaw aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, as it involves improper handling of shared resources during concurrent lifecycle events like thread creation and namespace destruction. Furthermore, the exploitation vector relates to ATT&CK technique T1059.004: Unix Shell Command Interpretation if the memory corruption allows for control flow hijacking leading to shell execution, or more broadly T1068: Exploitation for Privilege Escalation due to the kernel-level nature of the vulnerability. The operational impact is severe, as successful exploitation grants an attacker full control over the host system by escalating privileges from a potentially unprivileged user context within a network namespace to root level via kernel memory corruption.
The remediation strategy implemented in this resolution focuses on ensuring thread safety and proper lifecycle management for the estimator kthread. First, the logic is modified to explicitly check if IPVS has been disabled or if the kthread is stopping immediately after the initial calculation phase completes. If either condition is true, the thread exits cleanly before entering the main loop that drains temporary estimators. This prevents the execution of code paths with uninitialized limits. Second, reference counting for the kernel task structure is adjusted to prevent use-after-free scenarios during teardown. An extra reference count is retained after kthread creation and released using kthread_stop_put() only when stop paths have fully consumed it. This ensures that kd->task remains valid throughout its entire lifecycle, preventing dereferences of freed memory structures. Administrators should apply the corresponding kernel patch or update to their systems immediately to mitigate this risk, particularly in environments where dynamic network namespace creation and destruction are frequent or automated.