CVE-2016-2119

What it actually does

The flaw in libcli/smb/smbXcli_base.c permits man-in-the-middle attackers to bypass SMB2/SMB3 client-signing checks by supplying SMB2_SESSION_FLAG_IS_GUEST or SMB2_SESSION_FLAG_IS_NULL, enabling server impersonation and traffic manipulation.

Why it matters to the business

Organizations relying on Samba for file and print services face risk of intercepted or altered network traffic, potential data exposure, and unauthorized access within internal networks; the issue has been public for years with low but stable exploit probability and no known ransomware use.

How an attack unfolds

An attacker on the same network segment performs a man-in-the-middle position, waits for an SMB2/SMB3 client connection from an unpatched Samba client, injects the guest or null session flags to disable signing verification, and then impersonates the legitimate server to receive or alter subsequent requests.

How to fix it

Upgrade Samba to version 4.2.14 or later, 4.3.11 or later, or 4.4.5 or later; the referenced fix commit fb3e6296bb09534f882ea35e7f7ca4e5871d3222 resolves the flag-handling logic; enforce mandatory SMB signing on clients and servers where possible.

How to confirm the fix worked

Confirm Samba version is at or above the fixed releases listed for each branch; test that client connections reject unsigned or improperly flagged sessions; review smb.conf for client signing = mandatory.

Typical attack path

1. Establish network position between SMB client and server
2. Observe initiation of SMB2/SMB3 session
3. Inject SMB2_SESSION_FLAG_IS_GUEST or SMB2_SESSION_FLAG_IS_NULL during session setup
4. Bypass client signing validation
5. Impersonate server and supply malicious responses or capture credentials/data

How to detect it

Inspect SMB logs and network captures for sessions established with guest or null flags that bypass expected signing; correlate with unusual server responses or data flows after session setup.

• SMB2_SESSION_FLAG_IS_GUEST
• SMB2_SESSION_FLAG_IS_NULL

What your security team should do

• Apply the fixed Samba versions immediately on all affected systems
• Enable mandatory SMB signing across the environment
• Monitor for unsigned SMB2/SMB3 traffic until patches are deployed
• Review network segmentation to reduce man-in-the-middle exposure