A new vulnerability has been identified in net-imap, the Ruby library that provides IMAP client support. Labeled as CVE-2025-25186, this flaw allows a denial of service (DoS) attack due to unchecked memory consumption triggered by maliciously crafted server responses. This exclusive post explores how the issue arises, who it impacts, how to exploit it, and what you can do to stay secure.

What is net-imap and the IMAP Protocol?

IMAP (Internet Message Access Protocol) is the standard way for email clients to retrieve messages from a mail server. Ruby’s net-imap lets programs read, search, and manage mailboxes over IMAP using a straightforward API.

Here's a sample of how someone might connect to an IMAP server using Ruby

require 'net/imap'

imap = Net::IMAP.new('imap.mailserver.com', 993, true)
imap.login('user', 'password')
imap.select('INBOX')
uids = imap.search(["ALL"])
puts "Found #{uids.length} messages."
imap.disconnect

A. Root Cause

Inside net-imap (versions .3.2 up to but not including .3.8, .4.19, and .5.6), there's a flaw in how incoming server responses are handled—specifically, in parsing the IMAP uid-set data. IMAP allows servers to respond with a sequence like 100:99999999, asking the client to interpret this as all message IDs from 100 to 99,999,999.

To turn these into usable message IDs, net-imap uses Ruby's Range#to_a, expanding these into massive arrays in memory without any restriction on the range size.

Here’s a simplified code snippet similar to the vulnerable code

def parse_uid_set(uid_set)
  # Example: uid_set = "100:99999999"
  ranges = uid_set.split(",").map do |part|
    if part.include?(":")
      start_uid, end_uid = part.split(":").map(&:to_i)
      (start_uid..end_uid).to_a  # Danger: can produce HUGE arrays
    else
      part.to_i
    end
  end.flatten
end

With a malicious server sending 1:100000000, this call would try to create an array of 1 billion items, crashing or freezing the client with memory exhaustion.

B. How Could It Be Exploited?

At any time during the IMAP connection, a malicious or compromised server could send a UID range so large it overwhelms the client:

* 1 FETCH (UID 1:100000000)

The Ruby process handling this will try to make an array of a billion integers, eating up system memory and likely killing the process via an out-of-memory (OOM) error. This is a classic Denial of Service (DoS) mechanism.

Exploit Demo (for test environments)

require 'net/imap'

# WARNING: Only connect to a safe, test server!
imap = Net::IMAP.new('malicious.test.server', 993, true)
imap.select('INBOX')
# The following would be triggered by the server sending the malicious response.

>= .5. and < .5.6

Anyone connecting to an IMAP server they don’t trust, or to servers potentially open to compromise or malice, is at risk. This includes standard email clients, automated inbox checkers, and any Ruby script that uses net-imap to talk to IMAP servers.

Example Gemfile update

gem 'net-imap', '>= .3.8' # Or newer

Then run

bundle update net-imap

If you can’t upgrade Ruby or gems right away, avoid connecting to untrusted IMAP servers. If you must continue, you could patch the code to limit the maximum range size, but this is only a quick stopgap. Official fixes also protect you from other, similar issues.

References and Resources

- Official CVE-2025-25186 at NVD
- GitHub Security Advisory for net-imap
- net-imap RubyGems page
- Net::IMAP Documentation

Conclusion

CVE-2025-25186 is a good reminder that even well-built libraries can become vulnerable if server responses aren’t properly validated or capped. Always keep dependencies up to date and take care when integrating third-party services.

If you’re maintaining Ruby applications that rely on net-imap, update now, and ensure your production environments use only the patched versions. For more details or migration tips, see the GitHub advisory for backward compatibility help.

Timeline

Published on: 02/10/2025 16:15:39 UTC