Affected: ruby net-imap

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. From versions 0.4.0 to before 0.4.24, 0.5.0 to before 0.5.14, and 0.6.0 to before 0.6.4, when authenticating a connection with SCRAM-SHA1 or SCRAM-SHA256, a hostile server can perform a computational denial-of-service attack on the client process by sending a big iteration count value. This issue has been patched in versions 0.4.24, 0.5.14, and 0.6.4.

Affected: RubyGems: net-imap

### Summary

When authenticating a connection with `SCRAM-SHA1` or `SCRAM-SHA256`, a hostile server can perform a computational denial-of-service attack on the client process by sending a big iteration count value.

### Details

A hostile IMAP server can send an arbitrarily large PBKDF2 iteration count in the SCRAM server-first-message, causing the client to perform an expensive `OpenSSL::KDF.pbkdf2_hmac` call. Because the PBKDF2 function is a blocking C extension and holds onto Ruby’s Global VM Lock, it can freeze the entire Ruby VM for the duration of the computation.

OpenSSL enforces an effective maximum by using a 32-bit signed integer for the iteration count, Depending on hardware capabilities and OpenSSL version, this iteration count may be sufficient for to block all Ruby threads in the process for over seven minutes.

This is listed as one of the "Security Considerations", in [RFC 7804](https://www.rfc-editor.org/rfc/rfc7804.html#page-15): > A hostile server can perform a computational denial-of-service attack on clients by sending a big iteration count value. In order to defend against that, a client implementation can pick a maximum iteration count that it is willing to use and reject any values that exceed that threshold (in such cases, the client, of course, has to fail the authentication).

### Impact

During SCRAM authentication to a hostile server, the entire Ruby VM will be locked for the duration of the computation. Depending on hardware capabilities…