CVE-2026-42257: Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby

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CRITICAL 9.8REMOTEUPGRADE

CVE-2026-42257 is a critical-severity flaw (CVSS 9.8) affecting net\. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, several Net::IMAP commands accept a raw string argument that is sent to the server without validation or escaping. If this string is derived from user-controlled input, it may contain contain CRLF sequences, which an attacker can use to inject arbitrary IMAP commands. This issue has been patched in versions 0.4.2

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 63%)
CISA exploited list	No
Weakness	CWE-77, CWE-93
Published	2026-05-09
Last modified	2026-07-24

Publisher (CNA): GitHub, Inc. · Vendor: ruby-lang

Last updated 2026-07-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-09 20:16 UTCPublished CVE record published
2026-07-24 21:10 UTCUpdated Record last modified

ruby-lang net\ UPGRADE

Affected: < 0.4.24 · >= 0.5.0 and < 0.5.14 · >= 0.6.0 and < 0.6.4

Fixed in: 0.4.24, 0.5.14, 0.6.4

Safe to downgrade to: a release before 0.5.0

1. Check installed version

apt-cache policy net || rpm -q net || net --version

2. UPGRADE to 0.4.24 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade net

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade net

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 0.5.0 (Debian / Ubuntu)

apt-cache madison net sudo apt install net=<VERSION_BEFORE_0.5.0> sudo apt-mark hold net

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list net sudo dnf downgrade net-<VERSION_BEFORE_0.5.0> sudo dnf versionlock add net

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Advisories and coverage

Official advisories

Ruby News: Ruby 3.3.12 Released 2026-07-16

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-42257?

net\: upgrade to 0.4.24, 0.5.14, 0.6.4. Exact commands for each OS are on this page.

Is CVE-2026-42257 being exploited?

CVE-2026-42257 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-42257 affect?

CVE-2026-42257 affects net\. Severity is critical (CVSS 9.8). Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, several Net::IMAP commands accept a raw string argument that is sent to the server without validation or

How urgent is CVE-2026-42257?

Priority 49/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

Ruby vulnerabilities

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References

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