CVE-2026-68169 in Linux
Summary
by MITRE • 08/10/2026
In the Linux kernel, the following vulnerability has been resolved:
mptcp: pm: userspace: fix use-after-free in get_local_id
In mptcp_pm_userspace_get_local_id(), the address entry is looked up under spinlock, but its id is read after dropping the lock. A concurrent deletion can free the entry between the unlock and the read, leading to UAF.
The race window is narrow. It was reproduced only with a locally constructed stress test that repeatedly overlaps an MP_JOIN SYN with a MPTCP_PM_CMD_SUBFLOW_DESTROY request.
However, the KASAN report below confirms that the race is reachable:
[ 666.319376] BUG: KASAN: slab-use-after-free in mptcp_userspace_pm_get_local_id+0x1dc/0x1f0
[ 666.319386] Read of size 1 at addr ffff888124845610 by task swapper/0/0
... [ 666.319401] Call Trace:
[ 666.319405] <IRQ>
[ 666.319408] dump_stack_lvl+0x53/0x70
[ 666.319412] print_address_description.constprop.0+0x2c/0x3b0
[ 666.319418] print_report+0xbe/0x2b0
[ 666.319421] ? mptcp_userspace_pm_get_local_id+0x1dc/0x1f0
[ 666.319423] kasan_report+0xce/0x100
[ 666.319426] ? mptcp_userspace_pm_get_local_id+0x1dc/0x1f0
[ 666.319429] mptcp_userspace_pm_get_local_id+0x1dc/0x1f0
[ 666.319433] mptcp_pm_get_local_id+0x371/0x440
... [ 666.319821] Allocated by task 45539:
[ 666.319844] kasan_save_stack+0x33/0x60
[ 666.319855] kasan_save_track+0x14/0x30
[ 666.319858] __kasan_kmalloc+0x8f/0xa0
[ 666.319863] __kmalloc_noprof+0x1e7/0x520
[ 666.319867] sock_kmalloc+0xdf/0x130
[ 666.319885] sock_kmemdup+0x1b/0x40
[ 666.319888] mptcp_userspace_pm_append_new_local_addr+0x261/0x500
[ 666.319910] mptcp_pm_nl_announce_doit+0x16a/0x610
... [ 666.319967] Freed by task 45560:
[ 666.319988] kasan_save_stack+0x33/0x60
[ 666.319991] kasan_save_track+0x14/0x30
[ 666.319994] kasan_save_free_info+0x3b/0x60
[ 666.319998] __kasan_slab_free+0x43/0x70
[ 666.320000] kfree+0x166/0x440
[ 666.320003] sock_kfree_s+0x1d/0x50
[ 666.320007] mptcp_userspace_pm_delete_local_addr.isra.0+0x157/0x200
[ 666.320011] mptcp_pm_nl_subflow_destroy_doit+0x51d/0xea0
Fix by copying the id into a local variable while still holding the lock, and use -1 as a "not found" sentinel.
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Analysis
by VulDB Data Team • 08/10/2026
The vulnerability exists within the Linux kernel's Multipath TCP (MPTCP) implementation, specifically in the userspace path manager component where a use-after-free condition can occur during local identifier retrieval. This flaw manifests in the mptcp_pm_userspace_get_local_id() function which performs a lookup operation under a spinlock but reads the identifier value after releasing the lock. The race condition allows for a concurrent deletion of the address entry between the unlock and read operations, resulting in access to freed memory. The vulnerability is classified as a classic use-after-free scenario that can lead to system instability or potential privilege escalation depending on the execution context.
The technical nature of this flaw aligns with CWE-416, which describes the use of freed memory condition where memory is accessed after it has been freed by virtue of a double free or similar error. The race window, while narrow, demonstrates that the vulnerability is theoretically reachable under specific concurrent conditions involving overlapping MP_JOIN SYN packets and MPTCP_PM_CMD_SUBFLOW_DESTROY requests. The KASAN (Kernel Address Sanitizer) report provides concrete evidence of this race condition occurring in practice, showing a read operation accessing freed memory at address ffff888124845610 by the swapper/0 task. The stack trace confirms that the memory was allocated through mptcp_userspace_pm_append_new_local_addr and subsequently freed by mptcp_userspace_pm_delete_local_addr during subflow destruction processing.
The operational impact of this vulnerability extends beyond simple system crashes, as it represents a potential attack vector for malicious actors seeking to exploit kernel memory management flaws. The MPTCP protocol's implementation in the Linux kernel makes this particularly concerning since it affects network communication stability and could be leveraged for denial-of-service attacks against systems using multipath TCP connections. Attackers might construct specific network traffic patterns to trigger the race condition, potentially leading to system crashes or unauthorized privilege escalation if proper memory management controls are bypassed. The vulnerability affects systems running Linux kernel versions where MPTCP userspace path management is enabled and actively utilized.
The fix implemented addresses this issue by copying the identifier value into a local variable while still holding the spinlock, ensuring that no concurrent modifications can occur between lookup and access. This approach employs -1 as a sentinel value to indicate "not found" conditions, preventing the use of freed memory references. The solution follows established best practices for concurrent programming in kernel space by maintaining lock scope around critical data access operations. This mitigation directly addresses the root cause identified in the ATT&CK framework's technique T1068, which covers privilege escalation through local vulnerabilities, and aligns with defensive coding practices recommended in security standards such as those outlined in the Linux Kernel Security Best Practices documentation. The fix ensures that memory access patterns remain consistent with kernel safety requirements while maintaining the functional integrity of the MPTCP path management system.