Detections
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OpenSSH < 9.6 Multiple Vulnerabilities

medium Nessus Plugin ID 187201

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Synopsis

The SSH server running on the remote host is affected by multiple vulnerabilities.

Description

The version of OpenSSH installed on the remote host is prior to 9.6. It is, therefore, affected by multiple vulnerabilities as referenced in the release-9.6 advisory.

 - ssh(1), sshd(8): implement protocol extensions to thwart the so-called Terrapin attack discovered by Fabian Bumer, Marcus Brinkmann and Jrg Schwenk. This attack allows a MITM to effect a limited break of the integrity of the early encrypted SSH transport protocol by sending extra messages prior to the commencement of encryption, and deleting an equal number of consecutive messages immediately after encryption starts. A peer SSH client/server would not be able to detect that messages were deleted. While cryptographically novel, the security impact of this attack is fortunately very limited as it only allows deletion of consecutive messages, and deleting most messages at this stage of the protocol prevents user user authentication from proceeding and results in a stuck connection. The most serious identified impact is that it lets a MITM to delete the SSH2_MSG_EXT_INFO message sent before authentication starts, allowing the attacker to disable a subset of the keystroke timing obfuscation features introduced in OpenSSH 9.5.
 There is no other discernable impact to session secrecy or session integrity. OpenSSH 9.6 addresses this protocol weakness through a new strict KEX protocol extension that will be automatically enabled when both the client and server support it. This extension makes two changes to the SSH transport protocol to improve the integrity of the initial key exchange. Firstly, it requires endpoints to terminate the connection if any unnecessary or unexpected message is received during key exchange (including messages that were previously legal but not strictly required like SSH2_MSG_DEBUG). This removes most malleability from the early protocol. Secondly, it resets the Message Authentication Code counter at the conclusion of each key exchange, preventing previously inserted messages from being able to make persistent changes to the sequence number across completion of a key exchange. Either of these changes should be sufficient to thwart the Terrapin Attack. More details of these changes are in the PROTOCOL file in the OpenSSH source distribition. (CVE-2023-48795)

 - ssh-agent(1): when adding PKCS#11-hosted private keys while specifying destination constraints, if the PKCS#11 token returned multiple keys then only the first key had the constraints applied. Use of regular private keys, FIDO tokens and unconstrained keys are unaffected. (CVE-2023-51384)

 - ssh(1): if an invalid user or hostname that contained shell metacharacters was passed to ssh(1), and a ProxyCommand, LocalCommand directive or match exec predicate referenced the user or hostname via %u, %h or similar expansion token, then an attacker who could supply arbitrary user/hostnames to ssh(1) could potentially perform command injection depending on what quoting was present in the user-supplied ssh_config(5) directive. This situation could arise in the case of git submodules, where a repository could contain a submodule with shell characters in its user/hostname. Git does not ban shell metacharacters in user or host names when checking out repositories from untrusted sources. Although we believe it is the user's responsibility to ensure validity of arguments passed to ssh(1), especially across a security boundary such as the git example above, OpenSSH 9.6 now bans most shell metacharacters from user and hostnames supplied via the command-line. This countermeasure is not guaranteed to be effective in all situations, as it is infeasible for ssh(1) to universally filter shell metacharacters potentially relevant to user-supplied commands. User/hostnames provided via ssh_config(5) are not subject to these restrictions, allowing configurations that use strange names to continue to be used, under the assumption that the user knows what they are doing in their own configuration files. (CVE-2023-51385)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

Solution

Upgrade to OpenSSH version 9.6 or later.

See Also

https://www.openssh.com/txt/release-9.6

Plugin Details

Severity: Medium

ID: 187201

File Name: openssh_96.nasl

Version: 1.5

Type: remote

Family: Misc.

Published: 12/22/2023

Updated: 3/27/2024

Supported Sensors: Nessus

Risk Information

VPR

Risk Factor: Medium

Score: 6.7

CVSS v2

Risk Factor: Medium

Base Score: 6.4

Temporal Score: 5

Vector: CVSS2#AV:N/AC:L/Au:N/C:P/I:P/A:N

CVSS Score Source: CVE-2023-51385

CVSS v3

Risk Factor: Medium

Base Score: 6.5

Temporal Score: 5.9

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Temporal Vector: CVSS:3.0/E:P/RL:O/RC:C

Vulnerability Information

CPE: cpe:/a:openbsd:openssh

Required KB Items: installed_sw/OpenSSH

Exploit Available: true

Exploit Ease: Exploits are available

Patch Publication Date: 12/18/2023

Vulnerability Publication Date: 12/18/2023

Reference Information

CVE: CVE-2023-48795, CVE-2023-51384, CVE-2023-51385

IAVA: 2023-A-0701