The SSH server used in Junos allows an unauthenticated remote attacker to consume large amounts of CPU time prior to authentication, possibly leading to a partial Denial of Service attack. The attacker must be able to reach the device via SSH through any existing access lists or firewall filters, and the issue only occurs if SSH is enabled.
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This issue is assigned CVE-2015-7752.

A new configuration parameter, 'max-pre-authentication-packets', was added to allow specification of the maximum number of pre-auth SSH packets permitted per connection:

[system services ssh max-pre-authentication-packets]

The default is 128.

The following software releases have been updated to address this specific issue: Junos OS 12.1X44-D50, 12.1X46-D35, 12.1X47-D25, 12.3R10, 12.3X48-D10, 13.2R8, 13.2X51-D35, 13.3R6, 14.1R5, 14.1X53-D25, 14.2R3, 15.1R1, 15.1X49-D20, and all subsequent releases.

This issue was found during internal product security testing.

Juniper SIRT is not aware of any malicious exploitation of this vulnerability.

This issue is being tracked as PR 1058906 and is visible on the Customer Support website.
KB16765 [juniper.net] - "In which releases are vulnerabilities fixed?" describes which release vulnerabilities are fixed as per our End of Engineering and End of Life support policies.​​

Use access lists or firewall filters to limit access to the device via SSH only from trusted hosts.

2015-10-14: Initial publication
2015-10-30: Added more detail about the new configuration parameter.

• ​KB16613: Overview of the Juniper Networks SIRT Quarterly Security Bulletin Publication Process [juniper.net]
• KB16765: In which releases are vulnerabilities fixed? [juniper.net]
• KB16446: Common Vulnerability Scoring System (CVSS) and Juniper's Security Advisories [juniper.net]
• Report a Vulnerability - How to Contact the Juniper Networks Security Incident Response Team
• CVE-2015-7752: SSH allows unauthenticated remote user to consume large amounts of resources