CVE-2026-33868: Mastodon is a free, open-source social network server based on ActivityPub

Routine

MEDIUM 4.3UPGRADE

CVE-2026-33868 is a medium-severity flaw (CVSS 4.3) 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 versions 4.5.8, 4.4.15, and 4.3.21, an unauthenticated Open Redirect vulnerability (CWE-601) exists in the `/web/*` route due to improper handling of URL-encoded path segments. An attacker can craft a specially encoded URL that causes the application to redirect users to an arbitrary external domain, enabling phishing attacks and potential OAuth credential theft.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.5% (top 59%)
CISA exploited list	No
Weakness	CWE-601
Published	2026-03-27
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

2026-03-27 20:16 UTCPublished CVE record published
2026-06-17 10:38 UTCUpdated Record last modified

joinmastodon mastodon UPGRADE

Affected: < 4.3.21 · >= 4.4.0 and < 4.4.15 · >= 4.5.0 and < 4.5.8

Fixed in: 4.3.21, 4.4.15, 4.5.8

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 4.3.21 (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.

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Nuclei scan template: Mastodon - Open Redirect

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-2026-33868?

mastodon: upgrade to 4.3.21, 4.4.15, 4.5.8. Exact commands for each OS are on this page.

Is CVE-2026-33868 being exploited?

CVE-2026-33868 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-2026-33868 affect?

CVE-2026-33868 affects mastodon. Severity is medium (CVSS 4.3). Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.5.8, 4.4.15, and 4.3.21, an unauthenticated Open Redirect vulnerability (CWE-601) exists in the `/web/*` route due to improper handling of URL-e

How urgent is CVE-2026-33868?

Priority 25/100. No elevated risk signals yet.

Related CVEs

CVE-2026-41259 HIGH 7.5 UPGRADE Mastodon is a free, open-source social network server based on ActivityPub
CVE-2026-33869 MEDIUM 4.8 UPGRADE Mastodon is a free, open-source social network server based on ActivityPub

References

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