{"id":"CVE-2026-64448","summary":"smb: client: restrict implied bcc[0] exemption to responses without data area","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: client: restrict implied bcc[0] exemption to responses without data area\n\nsmb2_check_message() has a long-standing quirk that accepts a response\nwhose calculated length is one byte larger than the bytes actually\nreceived (\"server can return one byte more due to implied bcc[0]\").\nThis was introduced to accommodate servers that omit the trailing bcc[0]\noverlap byte when no data area is present.\n\nHowever, the exemption is applied unconditionally, regardless of whether\nthe command actually carries a data area (has_smb2_data_area[]).  When a\nresponse with a data area is subject to the +1 exemption, the reported\ndata can extend one byte beyond the bytes actually received, yet\nsmb2_check_message() still accepts it.  The subsequent decoder then reads\npast the end of the receive buffer.  This is reachable during NEGOTIATE\nand SESSION_SETUP, before the session is established.\n\nThe resulting out-of-bounds reads are visible under KASAN when mounting\nagainst a non-conforming server; both the SPNEGO/negTokenInit and the\nNTLMSSP challenge decoders are affected:\n\n  BUG: KASAN: slab-out-of-bounds in asn1_ber_decoder+0x16a7/0x1b00\n  Read of size 1 at addr ffff8880084d67c0 by task mount.cifs/81\n  CPU: 1 UID: 0 PID: 81 Comm: mount.cifs Not tainted 7.1.0-rc6 #1\n  Call Trace:\n   \u003cTASK\u003e\n   dump_stack_lvl+0x4e/0x70\n   print_report+0x157/0x4c9\n   kasan_report+0xce/0x100\n   asn1_ber_decoder+0x16a7/0x1b00\n   decode_negTokenInit+0x19/0x30\n   SMB2_negotiate+0x31d9/0x4c90\n   cifs_negotiate_protocol+0x1f2/0x3f0\n   cifs_get_smb_ses+0x93f/0x17e0\n   cifs_mount_get_session+0x7f/0x3a0\n   cifs_mount+0xb4/0xcf0\n   cifs_smb3_do_mount+0x23a/0x1500\n   smb3_get_tree+0x3b0/0x630\n   vfs_get_tree+0x82/0x2d0\n   fc_mount+0x10/0x1b0\n   path_mount+0x50d/0x1de0\n   __x64_sys_mount+0x20b/0x270\n   do_syscall_64+0xee/0x590\n   entry_SYSCALL_64_after_hwframe+0x77/0x7f\n   \u003c/TASK\u003e\n  Allocated by task 85:\n   kmem_cache_alloc_noprof+0x106/0x380\n   mempool_alloc_noprof+0x116/0x1e0\n   cifs_small_buf_get+0x31/0x80\n   allocate_buffers+0x10d/0x2b0\n   cifs_demultiplex_thread+0x1d5/0x1d50\n   kthread+0x2c6/0x390\n   ret_from_fork+0x36e/0x5a0\n   ret_from_fork_asm+0x1a/0x30\n  The buggy address is located 0 bytes to the right of\n   allocated 448-byte region [ffff8880084d6600, ffff8880084d67c0)\n   which belongs to the cache cifs_small_rq of size 448\n\n  BUG: KASAN: slab-out-of-bounds in kmemdup_noprof+0x36/0x50\n  Read of size 329 at addr ffff88800726c678 by task mount.cifs/89\n  CPU: 0 UID: 0 PID: 89 Comm: mount.cifs Tainted: G    B      7.1.0-rc6 #1\n  Call Trace:\n   \u003cTASK\u003e\n   dump_stack_lvl+0x4e/0x70\n   print_report+0x157/0x4c9\n   kasan_report+0xce/0x100\n   kasan_check_range+0x10f/0x1e0\n   __asan_memcpy+0x23/0x60\n   kmemdup_noprof+0x36/0x50\n   decode_ntlmssp_challenge+0x457/0x680\n   SMB2_sess_auth_rawntlmssp_negotiate+0x6f0/0xcb0\n   SMB2_sess_setup+0x219/0x4f0\n   cifs_setup_session+0x248/0xaf0\n   cifs_get_smb_ses+0xf79/0x17e0\n   cifs_mount_get_session+0x7f/0x3a0\n   cifs_mount+0xb4/0xcf0\n   cifs_smb3_do_mount+0x23a/0x1500\n   smb3_get_tree+0x3b0/0x630\n   vfs_get_tree+0x82/0x2d0\n   fc_mount+0x10/0x1b0\n   path_mount+0x50d/0x1de0\n   __x64_sys_mount+0x20b/0x270\n   do_syscall_64+0xee/0x590\n   entry_SYSCALL_64_after_hwframe+0x77/0x7f\n   \u003c/TASK\u003e\n  Allocated by task 93:\n   kmem_cache_alloc_noprof+0x106/0x380\n   mempool_alloc_noprof+0x116/0x1e0\n   cifs_small_buf_get+0x31/0x80\n   allocate_buffers+0x10d/0x2b0\n   cifs_demultiplex_thread+0x1d5/0x1d50\n   kthread+0x2c6/0x390\n   ret_from_fork+0x36e/0x5a0\n   ret_from_fork_asm+0x1a/0x30\n  The buggy address is located 120 bytes inside of\n   allocated 448-byte region [ffff88800726c600, ffff88800726c7c0)\n   which belongs to the cache cifs_small_rq of size 448\n\nRestrict the +1 exemption to responses that have no data area, so that\nit still covers the bcc[0] omission it was meant for.  When a data area\nis present, the +1 discrepancy instead means the reported data length\noverruns the\n---truncated---","modified":"2026-07-28T04:03:08.139723196Z","published":"2026-07-25T08:51:18.796Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/64xxx/CVE-2026-64448.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/31c6312608c60b72a1feb99a5afb680645a3e8a3"},{"type":"WEB","url":"https://git.kernel.org/stable/c/419ec1b604d7fb60c10aec2dc062371f9fcd4940"},{"type":"WEB","url":"https://git.kernel.org/stable/c/53b7c271f06be4dd5cfc8c6ef552a8355c891a7f"},{"type":"WEB","url":"https://git.kernel.org/stable/c/573e502d14714d2947e22e7eff40ec20a6a44a42"},{"type":"WEB","url":"https://git.kernel.org/stable/c/6e9d10f62773b99bd927940fd9cbdfe7207e23ff"},{"type":"WEB","url":"https://git.kernel.org/stable/c/8d0bbc78046d264bbf6a574ea6f9072258a43e35"},{"type":"WEB","url":"https://git.kernel.org/stable/c/b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0"},{"type":"WEB","url":"https://git.kernel.org/stable/c/ceb875a375dedbf51c9425c1d13a2d7a8435c08c"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/64xxx/CVE-2026-64448.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-64448"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"093b2bdad3221e3fae3c26d89387e7297a157664"},{"fixed":"8d0bbc78046d264bbf6a574ea6f9072258a43e35"},{"fixed":"b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0"},{"fixed":"31c6312608c60b72a1feb99a5afb680645a3e8a3"},{"fixed":"573e502d14714d2947e22e7eff40ec20a6a44a42"},{"fixed":"419ec1b604d7fb60c10aec2dc062371f9fcd4940"},{"fixed":"ceb875a375dedbf51c9425c1d13a2d7a8435c08c"},{"fixed":"6e9d10f62773b99bd927940fd9cbdfe7207e23ff"},{"fixed":"53b7c271f06be4dd5cfc8c6ef552a8355c891a7f"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-64448.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"3.6.0"},{"fixed":"5.10.261"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.11.0"},{"fixed":"5.15.212"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.178"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.145"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.96"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.39"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.1.4"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-64448.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H"}]}