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CVE-2026-89631— smb: client: reject a tree connect response whose byte count is too small

Quick assessment

Affected
Linux Linux
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Linux kernel是美国Linux基金会开源的一个操作系统内核。 Linux kernel存在安全漏洞，该漏洞源于CIFSTCon()对SMB树连接响应中ByteCount的边界处理不当，ByteCount为0或1时相减结果为负数并转换为巨大的size_t，导致越界读取，使数据通过/proc/fs/cifs/DebugData中的tcon->nativeFileSystem泄露至用户空间。

CVSS 9.1 · Critical EPSS 0.45% · P38

Possible ATT&CK Techniques 1 AI

T1059 · Command and Scripting Interpreter

Affected Version Matrix 8

VendorProduct Version RangeStatus
Linux Linux cc20c031bb067eb3280a1c4b5c42295093e24863< 3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d affected
cc20c031bb067eb3280a1c4b5c42295093e24863< 411e484fe71a7f1028de617447edad9cb6d9d68a affected
cc20c031bb067eb3280a1c4b5c42295093e24863< 65deb18359341141d37dc86fc7853511be3c87a7 affected
2.6.30 affected
< 2.6.30 unaffected
6.18.51≤ 6.18.* unaffected
7.2.4≤ 7.2.* unaffected
7.3-rc1≤ * unaffected
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I. Basic Information for CVE-2026-89631

Vulnerability Information
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Vulnerability Title
smb: client: reject a tree connect response whose byte count is too small
Source: CVE Program / CVE List V5
Vulnerability Description
In the Linux kernel, the following vulnerability has been resolved: smb: client: reject a tree connect response whose byte count is too small CIFSTCon() bounds its strnlen() over the byte area with the server's ByteCount minus two, which for ByteCount 0 or 1 goes negative as an int and converts to a huge size_t. The later subtraction wraps the __u16 bytes_left, and that is what bounds cifs_strndup_from_utf16(): a bound of up to 65535 against a ~16 KB cifs_req_poolp object runs off the end of the slab object, and the bytes reach userspace through tcon->nativeFileSystem in /proc/fs/cifs/DebugData. Reject a byte area too small for what the parser consumes. Two bytes is the least it can consume, and no conformant response carries fewer. The new trace point is the 129th smb_eio_trace entry, which __mode(byte) cannot represent, so the attribute goes with it.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
N/A
Source: CVE Program / CVE List V5
Vulnerability Title
Linux kernel 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Linux kernel是美国Linux基金会开源的一个操作系统内核。 Linux kernel存在安全漏洞，该漏洞源于CIFSTCon()对SMB树连接响应中ByteCount的边界处理不当，ByteCount为0或1时相减结果为负数并转换为巨大的size_t，导致越界读取，使数据通过/proc/fs/cifs/DebugData中的tcon->nativeFileSystem泄露至用户空间。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Linux Linux cc20c031bb067eb3280a1c4b5c42295093e24863 ~ 3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d -
Linux Linux 2.6.30 -

II. Public POCs for CVE-2026-89631

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III. Intelligence Information for CVE-2026-89631

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Patches & Fixes for CVE-2026-89631 (3)

Same Patch Batch · Linux · 2026-09-11 · 431 CVEs total

CVE-2026-89688 9.8 CRITICAL nfsd: drop the stateid, not the stateowner, on seqid_op replay retry
CVE-2026-89656 9.8 CRITICAL libceph: reject buckets with mismatched CRUSH ids
CVE-2026-89655 9.8 CRITICAL ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock
CVE-2026-89658 9.8 CRITICAL NFSD: Prevent client use-after-free during NFSv4.0 revoked-state cleanup
CVE-2026-89660 9.8 CRITICAL NFSD: Prevent client use-after-free during admin state revocation
CVE-2026-89659 9.8 CRITICAL NFSD: Prevent client use-after-free during delegation revoke
CVE-2026-89662 9.8 CRITICAL NFSD: Prevent lock owner use-after-free during client teardown
CVE-2026-89669 9.8 CRITICAL nfsd: initialize copy-notify stateid before publishing it
CVE-2026-89674 9.8 CRITICAL nfsd: fix XDR length calculation in nfsd4_ff_encode_layoutget
CVE-2026-89675 9.8 CRITICAL nfsd: fix UAF in async copy cancel and shutdown
CVE-2026-89676 9.8 CRITICAL nfsd: fix stale s2s_cp_stateids IDR entry for async COPY
CVE-2026-89677 9.8 CRITICAL nfsd: fix possible fh_compose of wrong dentry in nfsd4_create_file()
CVE-2026-89681 9.8 CRITICAL nfsd: fix layout fence worker double-reference race
CVE-2026-89686 9.8 CRITICAL nfsd: fix BUG_ON in nfsd4_alloc_layout_stateid on racing delegation revoke
CVE-2026-89703 9.8 CRITICAL nfsd: set SC_STATUS_FREED in nfsd4_drop_revoked_stid for delegations
CVE-2026-89479 9.8 CRITICAL sctp: stop processing a packet once its association is deleted
CVE-2026-89478 9.8 CRITICAL sctp: drop a chunk if its transport was removed
CVE-2026-89702 9.8 CRITICAL nfsd: size fh_verify server sockaddr slot by xpt_locallen
CVE-2026-89482 9.8 CRITICAL nvme-tcp: do not accept C2HData based on blk_rq_payload_bytes() alone
CVE-2026-89708 9.8 CRITICAL nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown

Showing top 20 of 431 CVEs. View all on vendor page &rarr; →

IV. Related Vulnerabilities

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Same vendor: Linux

V. Comments for CVE-2026-89631

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