Date: Wed, 17 Jan 2018 13:59:45 +0100 From: Tomas Hoger <thoger@...hat.com> To: OSS Security <oss-security@...ts.openwall.com> Subject: MySQL sha256_password authentication plugin DoS issues Hi! As Oracle does not share any information about the CVEs they assign, here's info about two CVEs fixed in MySQL 5.6.39 and 5.7.21 and listed in Oracle CPU Jan 2018. Both flaws affect sha256_password authentication plugin, which uses SHA256 crypt algorithm to hash passwords, and was affected by the known algorithm issues. MySQL did not set any explicit limit on the length of the password that can be provided during the authentication phase. Long passwords trigger DoS - high CPU usage and even server crash (because of use of alloca()). This was assigned CVE-2018-2696 and fixed by enforcing maximum password length limit: https://github.com/mysql/mysql-server/commit/475dcde2c7856dd0050b967099a86c087d94f32f SHA256 crypt makes it possible to adjust the cost of computing password hash by changing the number of "rounds". This is only a problem if user can directly specify their password hash and hence manipulate the rounds setting. That is possible in MySQL 5.6 (via SET PASSWORD = 'hash_string';), but no longer possible in MySQL 5.7. This isn't very exciting issue, as SQL access is a precondition. Impact is that sha256_password can be made to use excessive amount of CPU even if short password is provided. This was assigned CVE-2018-2703 and fixed by limiting the maximum rounds value: https://github.com/mysql/mysql-server/commit/efb4087cfe12134e1541b39ee9a4305f7cd225f5 References: http://www.oracle.com/technetwork/security-advisory/cpujan2018-3236628.html#AppendixMSQL https://dev.mysql.com/doc/relnotes/mysql/5.6/en/news-5-6-39.html https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-21.html Note that these issues were found while researching a related fix from Oct 2017 CPU: https://github.com/mysql/mysql-server/commit/f4e4405ebe319a840eb720db52c0e28b4fef5062 I believe that's CVE-2017-10155, wonder if any Oracle subscribers on this list are going to speak up to confirm. -- Tomas Hoger / Red Hat Product Security
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