CVE-2026-42246 net-imap vulnerable to STARTTLS stripping via invalid response timing
A man-in-the-middle attacker can bypass TLS encryption in the Ruby net-imap library, exposing email data to interception.
- 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-2026-42246?
CVE-2026-42246 is a high-severity 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. Prior to versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4, a man-in-the-middle attacker can cause Net::IMAP#starttls to return "successfully", without starting TLS. This issue has been patched in versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4.
Weakness: CWE-325, CWE-841, CWE-754, CWE-636, CWE-393, CWE-392
Which versions are affected by CVE-2026-42246?
8 open source packages ship an affected version, including net-imap. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
net-imap RubyGems | All versions | 0.3.10 |
net-imap RubyGems | All versions | 0.3.10 |
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 |
2 more packages
net-imap RubyGems | From 0.6.0 | 0.6.4 |
net-imap RubyGems | From 0.6.0 | 0.6.4 |
Products include Ruby-Lang Net\.
How do I fix CVE-2026-42246?
Upgrade net-imap to a fixed version for your release line: 0.3.10, 0.4.24, 0.5.14 or 0.6.4.
Is CVE-2026-42246 being exploited?
There is no known exploitation of CVE-2026-42246 in the wild and no public exploit code.
Other vulnerabilities in ruby net-imap
- CVE-2026-42256 net-imap: Denial of service via high iteration count for `SCRAM-*` authentication
- 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-2026-42246, BELL-CVE-2026-42246, CLEANSTART-2026-FA36805, CLEANSTART-2026-MW08049, CLSA-2026-1783525446, DEBIAN-CVE-2026-42246 and 20 more.