[SSHD-1136] Diffie Hellmann group exchange falls back to insecure DHG1 if agreement on modulo size is not possible

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.6.0
• Fix Version/s: 2.7.0
• Assignee: Lyor Goldstein
• Reporter: Achim Hügen
• Created: 02/Mar/21 08:14
• Updated: 01/Apr/21 15:52
• Resolved: 01/Apr/21 15:52

Description

After implementation of SSHD-1107 we configured the minimum modulo size of diffie-hellman-group-exchange-sha256 to 2048 bit and expected clients that doesn't support this minimum to not be able to connect.
But what happens is that, those clients still can connect and this warning is logged:

chooseDH(DHGEXServer[diffie-hellman-group-exchange-sha256])[ShaftServerSession[null@/10.42.110.99:44222]][prf=1024, min=1024, max=1024] No suitable primes found, defaulting to DHG1

My understanding is, that this is a fallback to diffie-hellman-group1-sha1 which is week, see https://www.openssh.com/legacy.html

Mina should make this fallback configurable and not activate it by default.