CVE-2026-41259: Mastodon is a free, open-source social network server based on ActivityPub

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-41259 is a high-severity flaw (CVSS 7.5) affecting mastodon. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Mastodon is a free, open-source social network server based on ActivityPub. Prior to v4.5.9, v4.4.16, and v4.3.22, Mastodon allows restricting new user sign-up based on e-mail domain names, and performs basic validation on e-mail addresses, but fails to restrict characters that are interpreted differently by some mailing servers. This vulnerability is fixed in v4.5.9, v4.4.16, and v4.3.22.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 88%)
CISA exploited list	No
Weakness	CWE-841
Published	2026-04-23
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: joinmastodon

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-23 19:17 UTCPublished CVE record published
2026-06-17 10:46 UTCUpdated Record last modified

joinmastodon mastodon UPGRADE

Affected: < 4.3.22 · >= 4.4.0 and < 4.4.16 · >= 4.5.0 and < 4.5.9

Fixed in: 4.3.22, 4.4.16, 4.5.9

Safe to downgrade to: a release before 4.4.0

1. Check installed version

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

2. UPGRADE to 4.3.22 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade mastodon

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 4.4.0 (Debian / Ubuntu)

apt-cache madison mastodon sudo apt install mastodon=<VERSION_BEFORE_4.4.0> sudo apt-mark hold mastodon

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

dnf --showduplicates list mastodon sudo dnf downgrade mastodon-<VERSION_BEFORE_4.4.0> sudo dnf versionlock add mastodon

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.

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-2026-41259?

mastodon: upgrade to 4.3.22, 4.4.16, 4.5.9. Exact commands for each OS are on this page.

Is CVE-2026-41259 being exploited?

CVE-2026-41259 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-2026-41259 affect?

CVE-2026-41259 affects mastodon. Severity is high (CVSS 7.5). Mastodon is a free, open-source social network server based on ActivityPub. Prior to v4.5.9, v4.4.16, and v4.3.22, Mastodon allows restricting new user sign-up based on e-mail domain names, and performs basic validation on e-mail addresses,

How urgent is CVE-2026-41259?

Priority 34/100. High severity; Remotely exploitable, no login, no user action

References

← Back to latest CVEs