Samba up to 4.2.13/4.3.10/4.4.4 Client-Signing Protection smbXcli_base.c smb2cli_session_set_session_key access control

CVSS Meta Temp Score CVSS is a standardized scoring system to determine possibilities of attacks. The Temp Score considers temporal factors like disclosure, exploit and countermeasures. The unique Meta Score calculates the average score of different sources to provide a normalized scoring system.	Current Exploit Price (≈) Our analysts are monitoring exploit markets and are in contact with vulnerability brokers. The range indicates the observed or calculated exploit price to be seen on exploit markets. A good indicator to understand the monetary effort required for and the popularity of an attack.	CTI Interest Score Our Cyber Threat Intelligence team is monitoring different web sites, mailing lists, exploit markets and social media networks. The CTI Interest Score identifies the interest of attackers and the security community for this specific vulnerability in real-time. A high score indicates an elevated risk to be targeted for this vulnerability.
7.2	$0-$5k	0.00

Summaryinfo

A vulnerability was found in Samba up to 4.2.13/4.3.10/4.4.4 and classified as critical. The affected element is the function smb2cli_session_set_session_key of the file libcli/smb/smbXcli_base.c of the component Client-Signing Protection. Such manipulation of the argument SMB2_SESSION_FLAG_IS_GUEST/SMB2_SESSION_FLAG_IS_GUEST leads to access control. This vulnerability is referenced as CVE-2016-2119. It is possible to launch the attack remotely. No exploit is available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability was found in Samba up to 4.2.13/4.3.10/4.4.4 (File Transfer Software). It has been rated as critical. Affected by this issue is the function smb2cli_session_set_session_key of the file libcli/smb/smbXcli_base.c of the component Client-Signing Protection. The manipulation of the argument SMB2_SESSION_FLAG_IS_GUEST/SMB2_SESSION_FLAG_IS_GUEST with an unknown input leads to a access control vulnerability. Using CWE to declare the problem leads to CWE-284. The product does not restrict or incorrectly restricts access to a resource from an unauthorized actor. Impacted is confidentiality, integrity, and availability. CVE summarizes:

libcli/smb/smbXcli_base.c in Samba 4.x before 4.2.14, 4.3.x before 4.3.11, and 4.4.x before 4.4.5 allows man-in-the-middle attackers to bypass a client-signing protection mechanism, and consequently spoof SMB2 and SMB3 servers, via the (1) SMB2_SESSION_FLAG_IS_GUEST or (2) SMB2_SESSION_FLAG_IS_NULL flag.

The weakness was published 07/07/2016 by Stefan Metzmacher with SerNet as Client side SMB2/3 required signing can be downgraded as confirmed advisory (Website). The advisory is shared for download at samba.org. The public release has been coordinated with the vendor. This vulnerability is handled as CVE-2016-2119 since 01/29/2016. The attack may be launched remotely. No form of authentication is required for exploitation. Successful exploitation requires user interaction by the victim. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1068. The advisory points out:

This means that the attacker can impersonate a server being connected to by Samba, and return malicious results.

The vulnerability scanner Nessus provides a plugin with the ID 91976 (Slackware 14.0 / 14.1 / 14.2 / current : samba (SSA:2016-189-01)), which helps to determine the existence of the flaw in a target environment. It is assigned to the family Slackware Local Security Checks and running in the context l. The commercial vulnerability scanner Qualys is able to test this issue with plugin 175915 (Debian Security Update for samba (DSA 3740-1)). The advisory illustrates:

By default, other tools in Samba are unprotected, but rarely they are configured to use smb signing, via the "client signing" parameter (the default is "if_required"). Even more rarely the "client max protocol" is set to SMB2, rather than the NT1 default.

Upgrading to version 4.2.14, 4.3.11 or 4.4.5 eliminates this vulnerability. It is possible to mitigate the problem by applying the configuration setting client ipc max protocol = NT1. The best possible mitigation is suggested to be upgrading to the latest version. A possible mitigation has been published before and not just after the disclosure of the vulnerability. The vulnerability will be addressed with the following lines of code:

if (!conn->mandatory_signing) {
   /*
    * only allow guest sessions without
    * mandatory signing.
    *
    * If we try an authentication with username != ""
    * and the server let us in without verifying the
    * password we don't have a negotiated session key
    * for signing.
    */
   no_sign_flags = SMB2_SESSION_FLAG_IS_GUEST;
}

The vulnerability is also documented in the databases at Tenable (91976), SecurityFocus (BID 91700†) and SecurityTracker (ID 1036244†). bugzilla.samba.org is providing further details. Once again VulDB remains the best source for vulnerability data.

Productinfo

Type

• File Transfer Software

Name

• Samba

Version

• 4.2.0
• 4.2.1
• 4.2.2
• 4.2.3
• 4.2.4
• 4.2.5
• 4.2.6
• 4.2.7
• 4.2.8
• 4.2.9
• 4.2.10
• 4.2.11
• 4.2.12
• 4.2.13
• 4.3.0
• 4.3.1
• 4.3.2
• 4.3.3
• 4.3.4
• 4.3.5
• 4.3.6
• 4.3.7
• 4.3.8
• 4.3.9
• 4.3.10
• 4.4.0
• 4.4.1
• 4.4.2
• 4.4.3
• 4.4.4

License

• open-source

Website

• Product: https://www.samba.org/

CPE 2.3info

• 🔍
• 🔍
• 🔍

CPE 2.2info

• 🔍
• 🔍
• 🔍

CVSSv4info

VulDB Vector: 🔍
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 7.5
VulDB Meta Temp Score: 7.3

VulDB Base Score: 7.5
VulDB Temp Score: 7.2
VulDB Vector: 🔍
VulDB Reliability: 🔍

NVD Base Score: 7.5
NVD Vector: 🔍

CVSSv2info

AV	AC	Au	C	I	A
💳	💳	💳	💳	💳	💳
💳	💳	💳	💳	💳	💳
💳	💳	💳	💳	💳	💳

Vector	Complexity	Authentication	Confidentiality	Integrity	Availability
Unlock	Unlock	Unlock	Unlock	Unlock	Unlock
Unlock	Unlock	Unlock	Unlock	Unlock	Unlock
Unlock	Unlock	Unlock	Unlock	Unlock	Unlock

VulDB Base Score: 🔍
VulDB Temp Score: 🔍
VulDB Reliability: 🔍

NVD Base Score: 🔍

Exploitinginfo

Class: Access control
CWE: CWE-284 / CWE-266
CAPEC: 🔍
ATT&CK: 🔍

Physical: No
Local: No
Remote: Yes

Availability: 🔍
Status: Not defined

EPSS Score: 🔍
EPSS Percentile: 🔍

Price Prediction: 🔍
Current Price Estimation: 🔍

0-Day	Unlock	Unlock	Unlock	Unlock
Today	Unlock	Unlock	Unlock	Unlock

Nessus ID: 91976
Nessus Name: Slackware 14.0 / 14.1 / 14.2 / current : samba (SSA:2016-189-01)
Nessus File: 🔍
Nessus Risk: 🔍
Nessus Family: 🔍
Nessus Context: 🔍

OpenVAS ID: 20377
OpenVAS Name: RedHat Update for samba RHSA-2016:1486-01
OpenVAS File: 🔍
OpenVAS Family: 🔍

Qualys ID: 🔍
Qualys Name: 🔍

Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔍

Upgrade: Samba 4.2.14/4.3.11/4.4.5
Config: client ipc max protocol = NT1

Timelineinfo

01/29/2016 🔍
04/20/2016 +82 days 🔍
07/07/2016 +78 days 🔍
07/07/2016 +0 days 🔍
07/07/2016 +0 days 🔍
07/08/2016 +1 days 🔍
07/08/2016 +0 days 🔍
02/19/2019 +956 days 🔍

Sourcesinfo

Product: samba.org

Advisory: Client side SMB2/3 required signing can be downgraded
Researcher: Stefan Metzmacher
Organization: SerNet
Status: Confirmed
Confirmation: 🔍
Coordinated: 🔍

CVE: CVE-2016-2119 (🔍)
GCVE (CVE): GCVE-0-2016-2119
GCVE (VulDB): GCVE-100-88820

OVAL: 🔍

SecurityFocus: 91700
SecurityTracker: 1036244

Misc.: 🔍

Entryinfo

Created: 07/08/2016 09:51
Updated: 02/19/2019 19:54
Changes: 07/08/2016 09:51 (85), 02/19/2019 19:54 (10)
Complete: 🔍
Cache ID: 216:337:103

Once again VulDB remains the best source for vulnerability data.

Discussion

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