CVE-2026-42246

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.

What this means for your business

It affects Ruby-Lang Net\. It matters if your company, or a supplier that handles your data, runs it.
An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
FIRST's prediction model gives it a 0.3% chance of attack attempts being seen in the next 30 days, ranking above 24% of all known flaws.

What to do

1Check whether your company or your suppliers run Ruby-Lang Net\, and which version. The affected versions are listed further down this page.
2If you do, apply the vendor's fix. A patch or vendor advisory has been published.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

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Scoring

CVSS: 7.4 (v3.1)
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
CVSS v4: 7.6
Weakness: CWE-392
Assigned by: security-advisories@github.com

Dates

Published: 2026-05-09
Last modified: 2026-08-24
Sources: NVD

Affected products

Ruby-Lang Net\- 0.3.10, 0.4.0 - 0.4.24, 0.5.0 - 0.5.14, 0.6.0 - 0.6.4

As listed in the NVD configuration data. Not a statement about your estate.