CVE-2016-7055: Montgomery multiplication may produce incorrect results

Quoting form the OpenSSL upstream advisory:

Montgomery multiplication may produce incorrect results (CVE-2016-7055) =======================================================================

Severity: Low

There is a carry propagating bug in the Broadwell-specific Montgomery multiplication procedure that handles input lengths divisible by, but longer than 256 bits. Analysis suggests that attacks against RSA, DSA and DH private keys are impossible. This is because the subroutine in question is not used in operations with the private key itself and an input of the attacker's direct choice. Otherwise the bug can manifest itself as transient authentication and key negotiation failures or reproducible erroneous outcome of public-key operations with specially crafted input. Among EC algorithms only Brainpool P-512 curves are affected and one presumably can attack ECDH key negotiation. Impact was not analyzed in detail, because pre-requisites for attack are considered unlikely. Namely multiple clients have to choose the curve in question and the server has to share the private key among them, neither of which is default behaviour. Even then only clients that chose the curve will be affected.

OpenSSL 1.1.0 users should upgrade to 1.1.0c

This issue does not affect OpenSSL versions prior to 1.0.2. Due to the low severity of this defect we are not issuing a new 1.0.2 release at this time. We recommend that 1.0.2 users wait for the next 1.0.2 release for the fix to become available. The fix is also available in the OpenSSL git repository in commit 57c4b9f6a2.

This issue was publicly reported as transient failures and was not initially recognized as a security issue. Thanks to Richard Morgan for providing reproducible case. The fix was developed by Andy Polyakov of the OpenSSL development team.

https://www.openssl.org/news/secadv/20161110.txt

— Red Hat

Some calculations, when run on an Intel Broadwell or later CPU, can produce in erroneous results. This flaw has been previously discussed by the Node.js team on GitHub. It is not believed that practical attacks can be crafted to exploit this vulnerability except in very specific circumstances. Therefore this is a low severity flaw.

All active versions of Node.js are affected, but the severity is very low for Node.js users.

— Node.Js

There is a carry propagating bug in the Broadwell-specific Montgomery multiplication procedure in OpenSSL 1.0.2 and 1.1.0 before 1.1.0c that handles input lengths divisible by, but longer than 256 bits. Analysis suggests that attacks against RSA, DSA and DH private keys are impossible. This is because the subroutine in question is not used in operations with the private key itself and an input of the attacker's direct choice. Otherwise the bug can manifest itself as transient authentication and key negotiation failures or reproducible erroneous outcome of public-key operations with specially crafted input. Among EC algorithms only Brainpool P-512 curves are affected and one presumably can attack ECDH key negotiation. Impact was not analyzed in detail, because pre-requisites for attack are considered unlikely. Namely multiple clients have to choose the curve in question and the server has to share the private key among them, neither of which is default behaviour. Even then only clients that chose the curve will be affected.