Linux kernel SMB client: Use-after-free in compound PDU handling
Summary
The Linux kernel SMB client mishandles encrypted compound responses from SMB servers, which leads to a use-after-free and insufficient bounds checks on NextCommand. A Linux host that connects to a malicious, compromised or man-in-the-middle SMB server could crash or corrupt kernel memory. The CVSS vector rates the impact on confidentiality, integrity and availability as high and requires user interaction, such as mounting or accessing a share.
Affected
- Linux kernel from commit b24df3e30cbf48255db866720fb71f14bf9d2f39 onward; the CNA lists affected from 4.19 until the fixed stable builds
- Azure Linux: affected before 6.6.158.1-1 (per tracker entries, not yet confirmed by Microsoft)
Fixed in
- Linux stable branches 5.10.273, 5.15.224, 6.1.189, 6.6.158, 6.12.112, 6.18.54 and 7.2.8 (per CNA and OSV)
- Azure Linux 6.6.158.1-1 (per tracker entries, to be confirmed with Microsoft)
- Distribution kernels: check the vendor tracker, as the fix status varies
Workaround
- Unknown. No vendor-documented workaround found.
- Interim measure: avoid mounting SMB shares from servers you do not control and avoid SMB traffic over untrusted networks until patched.
Exploitation
- No evidence of exploitation in the wild found.
- No public proof of concept found.
- Not listed in the CISA KEV catalog at the time of research.
Our recommendation
- Identify Linux hosts that use CIFS/SMB mounts, including workstations, NAS and partner or hosted storage.
- Confirm the kernel build against your distribution's security tracker.
- Apply the kernel update in your normal patch window, starting with systems that mount external shares.
- Restrict SMB access to trusted servers until the update is installed.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Linux kernel stable commit 05762c5b git.kernel.org
- Linux kernel stable commit 1ed29f7a git.kernel.org
- Linux kernel stable commit 491e3314 git.kernel.org
- Ubuntu security notice CVE-2026-98171 ubuntu.com
- OSV AZL-106215 (Azure Linux) osv.dev
- Debian CVE tracker entry vulners.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 31 Oct 2026, 22:24.
Original description (NVD)
In the Linux kernel, the following vulnerability has been resolved: smb: client: fix next_buffer UAF and NextCommand bounds in compound PDUs Fix several related bounds checking and pointer lifecycle issues in receive_encrypted_standard()'s handling of compound encrypted frames: - Clear next_buffer after assigning it to server->bigbuf. A stale next_buffer pointer can lead to a use-after-free on subsequent error paths. - Update pdu_length to the decrypted plaintext size (buf_size). Using the pre-decryption length allows NextCommand to point into stale ciphertext residue. - Reject next_cmd values smaller than MID_HEADER_SIZE(server). - Fix an integer overflow in the upper bound check by verifying pdu_length - next_cmd < MID_HEADER_SIZE(server), ensuring the trailing slice is large enough for a header.
NVD references (8)
- https://git.kernel.org/stable/c/05762c5bc1cfdcac36747994fde2c04387a457f1
- https://git.kernel.org/stable/c/1ed29f7a03417b405f38c09cfe04124883127b53
- https://git.kernel.org/stable/c/491e33144dee872cffda207f6fcb09260728f803
- https://git.kernel.org/stable/c/72eaef1f37a3b6bec11c342736834dc3707be3e4
- https://git.kernel.org/stable/c/7f022867ec9f0166a0b4400385bb601b1c6b2c75
- https://git.kernel.org/stable/c/858d5ac22cb889266993e7670f9f0c4f4aeedd78
- https://git.kernel.org/stable/c/8749946579708ea0d339034bb7f423a67dbe89cf
- https://git.kernel.org/stable/c/96c436e4b010711452b2872558938f4ef276492a
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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