CVE-2026-42256 net-imap: Denial of service via high iteration count for `SCRAM-*` authentication
A malicious IMAP server can cause a denial of service in Ruby clients using SCRAM authentication by sending a high iteration count.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-42256?
CVE-2026-42256 is a medium-severity denial of service vulnerability in ruby net-imap, published on 9 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
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.
Which versions are affected by CVE-2026-42256?
6 open source packages ship an affected version, including net-imap. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
net-imap RubyGems | From 0.4.0 | 0.4.24 |
net-imap RubyGems | From 0.4.0 | 0.4.24 |
net-imap RubyGems | From 0.5.0 | 0.5.14 |
net-imap RubyGems | From 0.5.0 | 0.5.14 |
net-imap RubyGems | From 0.6.0 | 0.6.4 |
net-imap RubyGems | From 0.6.0 | 0.6.4 |
Linux distribution packages (3)
rmt-server SUSE:Linux Enterprise Module for Public Cloud 15 SP7 | All versions | 3.1.0-150700.3.28.1 |
rmt-server SUSE:Linux Enterprise Module for Server Applications 15 SP7 | All versions | 3.1.0-150700.3.28.1 |
rmt-server openSUSE:Leap 16.0 | All versions | 3.0.0-160000.1.1 |
Products include Ruby-Lang Net\.
How do I fix CVE-2026-42256?
Upgrade net-imap to a fixed version for your release line: 0.4.24, 0.5.14 or 0.6.4.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-42256 being exploited?
There is no known exploitation of CVE-2026-42256 in the wild and no public exploit code.
Other vulnerabilities in ruby net-imap
- CVE-2026-42246 net-imap vulnerable to STARTTLS stripping via invalid response timing
- CVE-2025-43857 net-imap rubygem vulnerable to possible DoS by memory exhaustion
- CVE-2026-42257 net-imap: Command Injection via "raw" arguments to multiple commands
- CVE-2026-42258 net-imap: Command Injection via unvalidated Symbol inputs
- CVE-2026-42245 net-imap: Quadratic complexity when reading response literals
Sources
Also known as ALPINE-CVE-2026-42256, BELL-CVE-2026-42256, CLEANSTART-2026-MC27877, CLEANSTART-2026-MH39201, DEBIAN-CVE-2026-42256, ECHO-0e5e-7c8e-8329 and 17 more.