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Ubuntu Security Notice USN-6560-2

Ubuntu Security Notice 6560-2 - USN-6560-1 fixed several vulnerabilities in OpenSSH. This update provides the corresponding update for Ubuntu 16.04 LTS and Ubuntu 18.04 LTS. Fabian Bäumer, Marcus Brinkmann, Joerg Schwenk discovered that the SSH protocol was vulnerable to a prefix truncation attack. If a remote attacker was able to intercept SSH communications, extension negotiation messages could be truncated, possibly leading to certain algorithms and features being downgraded. This issue is known as the Terrapin attack. This update adds protocol extensions to mitigate this issue.

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Ubuntu Security Notice USN-6560-2
January 11, 2024

openssh vulnerabilities

A security issue affects these releases of Ubuntu and its derivatives:

  • Ubuntu 18.04 LTS (Available with Ubuntu Pro)
  • Ubuntu 16.04 LTS (Available with Ubuntu Pro)

Summary:

Several security issues were fixed in OpenSSH.

Software Description:

  • openssh: secure shell (SSH) for secure access to remote machines

Details:

USN-6560-1 fixed several vulnerabilities in OpenSSH. This update provides
the corresponding update for Ubuntu 16.04 LTS and Ubuntu 18.04 LTS.

Original advisory details:

Fabian Bäumer, Marcus Brinkmann, Jörg Schwenk discovered that the SSH
protocol was vulnerable to a prefix truncation attack. If a remote attacker
was able to intercept SSH communications, extension negotiation messages
could be truncated, possibly leading to certain algorithms and features
being downgraded. This issue is known as the Terrapin attack. This update
adds protocol extensions to mitigate this issue. (CVE-2023-48795)

It was discovered that OpenSSH incorrectly handled user names or host names
with shell metacharacters. An attacker could possibly use this issue to
perform OS command injection. This only affected Ubuntu 18.04 LTS. (CVE-2023-51385)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 18.04 LTS (Available with Ubuntu Pro):
openssh-client 1:7.6p1-4ubuntu0.7+esm3
openssh-server 1:7.6p1-4ubuntu0.7+esm3

Ubuntu 16.04 LTS (Available with Ubuntu Pro):
openssh-client 1:7.2p2-4ubuntu2.10+esm5
openssh-server 1:7.2p2-4ubuntu2.10+esm5

In general, a standard system update will make all the necessary changes.

References:
https://ubuntu.com/security/notices/USN-6560-2
https://ubuntu.com/security/notices/USN-6560-1
CVE-2023-48795, CVE-2023-51385

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### Summary Russh v0.40.1 and earlier is vulnerable to a novel prefix truncation attack (a.k.a. Terrapin attack), which allows a man-in-the-middle attacker to strip an arbitrary number of messages right after the initial key exchange, breaking SSH extension negotiation (RFC8308) in the process and thus downgrading connection security. ### Mitigations To mitigate this protocol vulnerability, OpenSSH suggested a so-called "strict kex" which alters the SSH handshake to ensure a Man-in-the-Middle attacker cannot introduce unauthenticated messages as well as convey sequence number manipulation across handshakes. Support for strict key exchange has been added to Russh in the patched version. **Warning: To take effect, both the client and server must support this countermeasure.** As a stop-gap measure, peers may also (temporarily) disable the affected algorithms and use unaffected alternatives like AES-GCM instead until patches are available. ### Details The SSH specifications of Ch...

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