CVE-2023-38427

What it actually does

Integer underflow in ksmbd's deassemble_neg_contexts function leads to out-of-bounds read when processing malformed SMB2 negotiation contexts, enabling memory corruption or information disclosure.

Why it matters to the business

Remote attackers can potentially compromise Linux servers running the in-kernel SMB server, leading to data theft, system takeover, or service disruption on systems exposing SMB shares.

How an attack unfolds

An unauthenticated remote attacker sends a crafted SMB2 NEGOTIATE request containing specially malformed negotiation contexts that trigger the integer underflow during deassembly, resulting in an out-of-bounds read that can be leveraged for further memory corruption or code execution.

How to fix it

Upgrade the Linux kernel to version 6.3.8 or later, or apply the fix from commit d93fa2c78854d25ed4b67ac87f1c3c264d8b27fb. Disable ksmbd if SMB server functionality is not required.

How to confirm the fix worked

Verify the running kernel version is 6.3.8 or newer; confirm the patch commit is present in the source tree; test SMB2 negotiation handling with malformed contexts using kernel fuzzing tools.

Typical attack path

1. Attacker discovers a target exposing ksmbd on TCP/445
2. Attacker crafts SMB2 negotiation packet with manipulated context lengths
3. Packet triggers integer underflow in deassemble_neg_contexts
4. Out-of-bounds read occurs, potentially allowing memory disclosure or corruption
5. Attacker chains the primitive to achieve remote code execution or denial of service

How to detect it

Review kernel logs for oops or panic messages related to smb2pdu or ksmbd during SMB negotiation; inspect network traffic for anomalous SMB2 NEGOTIATE requests with unusual context structures; correlate with failed or malformed session setups.

What your security team should do

• Prioritize patching systems exposing ksmbd to untrusted networks due to the critical CVSS score
• Audit network exposure of TCP/445 and restrict access to trusted clients only
• Monitor for kernel updates in distributions and apply within standard change windows
• Consider disabling ksmbd module if SMB server capability is unused