Vulnerability Instance

net-imap rubygem vulnerable to possible DoS by memory exhaustion
### Summary

There is a possibility for denial of service by memory exhaustion when `net-imap` reads server responses.  At any time while the client is connected, a malicious server can send can send a "literal" byte count, which is automatically read by the client's receiver thread.  The response reader immediately allocates memory for the number of bytes indicated by the server response.

This should not be an issue when securely connecting to trusted IMAP servers that are well-behaved.  It can affect insecure connections and buggy, untrusted, or compromised servers (for example, connecting to a user supplied hostname).

### Details

The IMAP protocol allows "literal" strings to be sent in responses, prefixed with their size in curly braces (e.g. `{1234567890}\r\n`).  When `Net::IMAP` receives a response containing a literal string, it calls `IO#read` with that size.  When called with a size, `IO#read` immediately allocates memory to buffer the entire string before processing continues.  The server does not need to send any more data.  There is no limit on the size of literals that will be accepted.

### Fix
#### Upgrade
Users should upgrade to `net-imap` 0.5.7 or later.  A configurable `max_response_size` limit has been added to `Net::IMAP`'s response reader.  The `max_response_size` limit has also been backported to `net-imap` 0.2.5, 0.3.9, and 0.4.20.

To set a global value for `max_response_size`, users must upgrade to `net-imap` ~> 0.4.20, or > 0.5.7.

#### Configuration

To avoid backward compatibility issues for secure connections to trusted well-behaved servers, the default `max_response_size` for `net-imap` 0.5.7 is _very high_ (512MiB), and the default `max_response_size` for `net-imap` ~> 0.4.20, ~> 0.3.9, and 0.2.5 is `nil` (unlimited).

When connecting to untrusted servers or using insecure connections, a much lower `max_response_size` should be used.
```ruby
# Set the global max_response_size (only ~> v0.4.20, > 0.5.7)
Net::IMAP.config.max_response_size = 256 << 10 # 256 KiB

# Set when creating the connection
imap = Net::IMAP.new(hostname, ssl: true,
                     max_response_size: 16 << 10) # 16 KiB

# Set after creating the connection
imap.max_response_size = 256 << 20 # 256 KiB
# flush currently waiting read, to ensure the new setting is loaded
imap.noop
```

_**Please Note:**_ `max_response_size` only limits the size _per response_.  It does not prevent a flood of individual responses and it does not limit how many unhandled responses may be stored on the responses hash.  Users are responsible for adding response handlers to prune excessive unhandled responses.

#### Compatibility with lower `max_response_size`

A lower `max_response_size` may cause a few commands which legitimately return very large responses to raise an exception and close the connection.  The `max_response_size` could be temporarily set to a higher value, but paginated or limited versions of commands should be used whenever possible.  For example, to fetch message bodies:

```ruby
imap.max_response_size = 256 << 20 # 256 KiB
imap.noop # flush currently waiting read

# fetch a message in 252KiB chunks
size = imap.uid_fetch(uid, "RFC822.SIZE").first.rfc822_size
limit = 252 << 10
message = ((0..size) % limit).each_with_object("") {|offset, str|
  str << imap.uid_fetch(uid, "BODY.PEEK[]<#{offset}.#{limit}>").first.message(offset:)
}

imap.max_response_size = 16 << 20 # 16 KiB
imap.noop # flush currently waiting read
```

### References

* PR to introduce max_response_size: https://github.com/ruby/net-imap/pull/444
  * Specific commit: [0ae8576c1 - lib/net/imap/response_reader.rb](https://github.com/ruby/net-imap/pull/444/commits/0ae8576c1a90bcd9573f81bdad4b4b824642d105#diff-53721cb4d9c3fb86b95cc8476ca2df90968ad8c481645220c607034399151462)
* Backport to 0.4: https://github.com/ruby/net-imap/pull/445
* Backport to 0.3: https://github.com/ruby/net-imap/pull/446
* Backport to 0.2: https://github.com/ruby/net-imap/pull/447