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The article is so low on details. I wish there was a more in-depth explanation. Isn't the first connection (with unknown host key) always a target for MitM atta
by Kenji 6y ago
The article is so low on details. I wish there was a more in-depth explanation. Isn't the first connection (with unknown host key) always a target for MitM attacks and thus insecure, unless you preload the host key?
- Xylakant 6y agoThe leak allows an attacker to distinguish first connections that are made without a cached host key, allowing them to target those connections only and reduce their risk of detection. OpenSSH seems affected, too, but have decided to not fix this. As far as I understand the paper, any fix may have undesirable side effects. The linked paper here https://www.fzi.de/en/news/news/detail-en/artikel/fsa-2020-2-ausnutzung-eines-informationslecks-fuer-gezielte-mitm-angriffe-auf-ssh-clients/ https://www.fzi.de/en/news/news/detail-en/artikel/fsa-2020-2... goes into detail. The PDF is in English, despite the German URL.
- Randor 6y agoYes, That is exactly the reported issue here. PuTTY 0.68 through 0.73 allows the remote attacker to accurately determine whether or not the client has cached the host key. It looks like this works because PuTTY sends a different algorithm list depending on whether or not the key has been cached. If the MiTM attacker sees the 'default algorithm list' it can be assumed that this is the first connection attempt and the attacker can substitute the server key with a compromised key.
- 8organicbits 6y agoAh, that's cool. I pushed AWS to give better ways of getting the host key for new EC2 instances a few years back. The whole start an EC2 instance, SSH in and blinkly trust the host key on first use thing seemed terrible! EC2 docs now include that detail, although its labeled as optional. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connection-prereqs.html#connection-prereqs-fingerprint https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connecti...