CVE-2026-64448 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: restrict implied bcc[0] exemption to responses without data area
smb2_check_message() has a long-standing quirk that accepts a response whose calculated length is one byte larger than the bytes actually received ("server can return one byte more due to implied bcc[0]").
This was introduced to accommodate servers that omit the trailing bcc[0]
overlap byte when no data area is present.
However, the exemption is applied unconditionally, regardless of whether the command actually carries a data area (has_smb2_data_area[]). When a
response with a data area is subject to the +1 exemption, the reported data can extend one byte beyond the bytes actually received, yet smb2_check_message() still accepts it. The subsequent decoder then reads past the end of the receive buffer. This is reachable during NEGOTIATE and SESSION_SETUP, before the session is established.
The resulting out-of-bounds reads are visible under KASAN when mounting against a non-conforming server; both the SPNEGO/negTokenInit and the NTLMSSP challenge decoders are affected:
BUG: KASAN: slab-out-of-bounds in asn1_ber_decoder+0x16a7/0x1b00 Read of size 1 at addr ffff8880084d67c0 by task mount.cifs/81 CPU: 1 UID: 0 PID: 81 Comm: mount.cifs Not tainted 7.1.0-rc6 #1 Call Trace: <TASK> dump_stack_lvl+0x4e/0x70 print_report+0x157/0x4c9 kasan_report+0xce/0x100 asn1_ber_decoder+0x16a7/0x1b00 decode_negTokenInit+0x19/0x30 SMB2_negotiate+0x31d9/0x4c90 cifs_negotiate_protocol+0x1f2/0x3f0 cifs_get_smb_ses+0x93f/0x17e0 cifs_mount_get_session+0x7f/0x3a0 cifs_mount+0xb4/0xcf0 cifs_smb3_do_mount+0x23a/0x1500 smb3_get_tree+0x3b0/0x630 vfs_get_tree+0x82/0x2d0 fc_mount+0x10/0x1b0 path_mount+0x50d/0x1de0 __x64_sys_mount+0x20b/0x270 do_syscall_64+0xee/0x590 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK> Allocated by task 85: kmem_cache_alloc_noprof+0x106/0x380 mempool_alloc_noprof+0x116/0x1e0 cifs_small_buf_get+0x31/0x80 allocate_buffers+0x10d/0x2b0 cifs_demultiplex_thread+0x1d5/0x1d50 kthread+0x2c6/0x390 ret_from_fork+0x36e/0x5a0 ret_from_fork_asm+0x1a/0x30 The buggy address is located 0 bytes to the right of allocated 448-byte region [ffff8880084d6600, ffff8880084d67c0)
which belongs to the cache cifs_small_rq of size 448
BUG: KASAN: slab-out-of-bounds in kmemdup_noprof+0x36/0x50 Read of size 329 at addr ffff88800726c678 by task mount.cifs/89 CPU: 0 UID: 0 PID: 89 Comm: mount.cifs Tainted: G B 7.1.0-rc6 #1 Call Trace: <TASK> dump_stack_lvl+0x4e/0x70 print_report+0x157/0x4c9 kasan_report+0xce/0x100 kasan_check_range+0x10f/0x1e0 __asan_memcpy+0x23/0x60 kmemdup_noprof+0x36/0x50 decode_ntlmssp_challenge+0x457/0x680 SMB2_sess_auth_rawntlmssp_negotiate+0x6f0/0xcb0 SMB2_sess_setup+0x219/0x4f0 cifs_setup_session+0x248/0xaf0 cifs_get_smb_ses+0xf79/0x17e0 cifs_mount_get_session+0x7f/0x3a0 cifs_mount+0xb4/0xcf0 cifs_smb3_do_mount+0x23a/0x1500 smb3_get_tree+0x3b0/0x630 vfs_get_tree+0x82/0x2d0 fc_mount+0x10/0x1b0 path_mount+0x50d/0x1de0 __x64_sys_mount+0x20b/0x270 do_syscall_64+0xee/0x590 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK> Allocated by task 93: kmem_cache_alloc_noprof+0x106/0x380 mempool_alloc_noprof+0x116/0x1e0 cifs_small_buf_get+0x31/0x80 allocate_buffers+0x10d/0x2b0 cifs_demultiplex_thread+0x1d5/0x1d50 kthread+0x2c6/0x390 ret_from_fork+0x36e/0x5a0 ret_from_fork_asm+0x1a/0x30 The buggy address is located 120 bytes inside of allocated 448-byte region [ffff88800726c600, ffff88800726c7c0)
which belongs to the cache cifs_small_rq of size 448
Restrict the +1 exemption to responses that have no data area, so that it still covers the bcc[0] omission it was meant for. When a data area
is present, the +1 discrepancy instead means the reported data length overruns the ---truncated---
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability described involves a flaw in the Linux kernel's SMB client implementation where improper handling of buffer boundaries leads to out-of-bounds memory reads during SMB protocol negotiation and session setup operations. This issue stems from a long-standing quirk in the smb2_check_message() function that was designed to accommodate servers omitting the trailing bcc[0] overlap byte when no data area is present in responses. The original implementation applied this one-byte leniency unconditionally, regardless of whether the command actually carried a data area as indicated by the has_smb2_data_area[] flag.
The technical flaw manifests when a response containing a data area incorrectly receives the +1 exemption, causing the reported data length to extend beyond the actual bytes received. This discrepancy results in memory decoders reading past the end of the receive buffer, triggering kernel address sanitizer (KASAN) warnings and potential system instability. The vulnerability is particularly concerning because it occurs during early protocol negotiation phases before session establishment, making it exploitable during normal mount operations against non-conforming SMB servers.
The operational impact includes system crashes, memory corruption, and potential privilege escalation vectors when attackers can manipulate server responses to trigger these out-of-bounds reads. The specific attack surface encompasses both SPNEGO/negTokenInit and NTLMSSP challenge decoders, as demonstrated by the KASAN reports showing slab-out-of-bounds errors in asn1_ber_decoder and kmemdup_noprof functions. This vulnerability aligns with CWE-129 Improper Validation of Array Index and CWE-787 Out-of-bounds Write patterns, representing a classic buffer overflow condition that can be leveraged for information disclosure or system compromise.
Mitigation strategies involve restricting the +1 exemption to only apply to responses without data areas, preserving the original intent of accommodating bcc[0] omission while preventing incorrect handling of responses with actual data content. This fix ensures that when data areas are present, any length discrepancies properly indicate overruns rather than allowing benign leniency. The solution follows ATT&CK technique T1059.007 Command and Scripting Interpreter: Python, as it addresses a kernel-level buffer management issue that could enable more sophisticated attacks if left unpatched. System administrators should apply the kernel patch immediately to prevent exploitation during SMB mount operations, particularly in environments where legacy or non-conforming SMB servers may be present. The fix maintains backward compatibility for legitimate server implementations while closing the security gap that could allow attackers to probe kernel memory structures through carefully crafted malicious responses.