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colmmacc's profile
Colm MacCárthaigh
Colm MacCárthaigh
Colm MacCárthaigh
@colmmacc

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Colm MacCárthaigh

@colmmacc

AWS, Apache, Crypto, Irish Music, Haiku, Photography

Seattle
notesfromthesound.com
Joined April 2008

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    1. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      Tracing through the code of things that were impacted showed that the problem only happened when the application called SSL_shutdown() twice, even if there is a protocol error.

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    2. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      Calling SSL_shutdown() twice is normal when there's no problem with a connection, and it should be harmless in the error case, so it's understandable that some applications do it ... but thankfully it's not common.

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    3. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      The actual leak of info, whether it was a padding or MAC error, would effectively show up as a timing or connection close difference between these calls. Impacted applications would either seem to time out, or close connections, differently, depending on the error. Subtle.

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    4. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      O.k. so next question: why don't existing padding oracle tests find this? Well it turns out only to happen to zero byte records. Records that have no data in them. And the scanning tool happens to send zero byte records.

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    5. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      Zero-byte records aren't common: browsers don't send them afaict, and packet dumps seem to show that they are exceedingly rare: which makes sense, if you have no data to send, why would bother? So that's very re-assuring.

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    6. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      Next weird thing: the problem also happened if OpenSSL wasn't using AES-NI hardware acceleration. In practice this means it impacted 3DES (which people should have turned off for other reasons!) and older hardware.

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    7. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      This also explained why FIPS software appeared in the list, because FIPS software generally can't use AES-NI.

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    8. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      At this point, a lot of factors have to be combined: TLS sw would have to be coded in an uncommon way, using OpenSSL, negotiating older cipher suites, on older HW, with clients that send 0-byte records, and can be made repeat the same data over and over, with an active MITM.

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    9. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      But that makes it more interesting! How do we find and prevent even these kind of rarefied cases? Automation, like the scanning tool, is clearly critical - but can we do more at the point of code?

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    10. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      One thing I'm grateful for is that in s2n we kill connections on any error, and we do it in a way where s2n will completely refuse to interact with the connection after the error has happened. Just with a closed flag ... https://github.com/awslabs/s2n/blob/master/tls/s2n_connection.c#L1031 …

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      Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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      s2n uses OpenSSL's libcrypto for the underlying cryptography, and the same issue in that code /could/ have caused impact within s2n were it not for that practice. Basically this check .... https://github.com/awslabs/s2n/blob/master/tls/s2n_send.c#L94 …

      8:29 AM - 26 Feb 2019
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        2. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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          Of course the impact still would have been small, because of the other factors, but I'm glad we have that check! Anyway, thanks again to the issue reporters, read their paper when it comes! and thanks for Andrew and Steven from the TLS team. That's it, unless AMA.

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        3. Colm MacCárthaigh‏ @colmmacc 26 Feb 2019
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          Colm MacCárthaigh Retweeted Robert Merget

          Quick addendum:https://twitter.com/ic0nz1/status/1100432621149003783 …

          Colm MacCárthaigh added,

          Robert Merget @ic0nz1
          Replying to @colmmacc
          actually you do not need clients which send 0-byte records. An Attacker can cut the record (adjust the iv) and adjust the length of the record header. This way a normal record appears to the server as zero length with an invalid mac, although the original record was way longer
          2 replies 1 retweet 3 likes
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        4. End of conversation

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