Terrapin Attack for prefix truncation in SSH

Re:Terrapin Attack for prefix truncation in SSH by petecooper on Monday December 18, 2023 @02:23PM (#38686961)

>I am an admin, should I drop everything and fix this?

>Probably not.

(I will preface this by saying I'm likely missing something obvious.)

I am an admin, and it's not clear _how_ I might fix this at the server end.

Edit: thanks for the replies, I'll sit tight for now.

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Re:Terrapin Attack for prefix truncation in SSH by gorkish on Monday December 18, 2023 @02:57PM (#38687356)

> I am an admin, and it's not clear _how_ I might fix this at the server end.

You're just hearing about this early as it's the first hours of public disclosure. There are 3 CVE's assigned. You'll track those with your vendors or upstream and patch as fixes become available, which they already should be for at least some implementations that had earlier disclosure. The main issue is CVE-2023-48795. It does not appear there is any immediate need to implement compensating controls or workarounds at this time, but that assessment really depends on your own environment, security policy, and risk tolerance.

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Re:Terrapin Attack for prefix truncation in SSH by justinclift on Monday December 18, 2023 @02:45PM (#38687209)

Seems like it'll be a case of "wait for updated ssh packages", potentially with some mitigation advice appearing in the next day or so.

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Re:Terrapin Attack for prefix truncation in SSH by 1letterunixname on Tuesday December 19, 2023 @04:37AM (#38693621)

No. Mitigations were available for openssh before you wrote that comment. Follow the recommendations from ssh-audit (master version). [0]

For packages, openssh and openssh-portable 9.6 include the fix, but every vendor will need to backport if they maintain their own pile of patches against a particular version of -portable.

0. https://github.com/jtesta/ssh-audit

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Re:Terrapin Attack for prefix truncation in SSH by justinclift on Tuesday December 19, 2023 @12:06PM (#38698232)

Thanks, hadn't come across ssh-audit before. Looks like a good tool. :)

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Re:Terrapin Attack for prefix truncation in SSH by Skrillor on Monday December 18, 2023 @02:58PM (#38687365)

We updated the FAQ question to cover this. For most users, fixing can be done by installing patched versions of their SSH implementations as they come available. If you feel uncomfortable waiting for an update, you may disable the affected cipher modes temporarily and use other modes like AES-GCM instead. But keep in mind that a faulty configuration can cause you to loose access to the server.

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Re:Terrapin Attack for prefix truncation in SSH by colmmacc on Monday December 18, 2023 @03:03PM (#38687435)

Fwiw, I found a bunch of clients and libraries that didn't support AES-GCM at all, or only very recently. libssh2 is one example. That's made me nervous to make AES-GCM the only supported cipher as a mitigation.

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Re:Terrapin Attack for prefix truncation in SSH by Aloisius on Monday December 18, 2023 @03:31PM (#38687773)

What about aes-ctr?

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Re:Terrapin Attack for prefix truncation in SSH by lambdafu on Monday December 18, 2023 @04:58PM (#38688941)

Depends a bit on the MAC. CTR-EtM is technically vulnerable (i.e. cryptographically broken), but due to key stream desynchronization the attack will quickly lead to application errors, defeating the attacker. See Sect. 4.3.3.

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Re:Terrapin Attack for prefix truncation in SSH by shaicoleman on Monday December 18, 2023 @08:01PM (#38690503)

FYI: libssh2 supports AES-GCM since 1.11.0 (released 2023-05-30)

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