CVE-2026-40300: Zulip is an open-source team collaboration tool

Routine

MEDIUM 6.5DOWNGRADE

CVE-2026-40300 is a medium-severity flaw (CVSS 6.5) affecting zulip server. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Zulip is an open-source team collaboration tool. Prior to 12.0, With message_edit_history_visibility_policy set to "moves", /api/v1/messages/{id}/history still returns historical content values, allowing low-privilege users to recover text that was edited away from other users' messages. This vulnerability is fixed in 12.0.

Why this priority

Public exploit reference

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.2% (top 84%)
CISA exploited list	No
Weakness	CWE-284
Published	2026-05-12
Last modified	2026-06-17

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

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

Am I affected?

Timeline

2026-05-12 17:16 UTCPublished CVE record published
2026-06-17 10:44 UTCUpdated Record last modified

zulip zulip server DOWNGRADE

Affected: 10.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 10.0

1. Check installed version

apt-cache policy zulip-server || rpm -q zulip-server || zulip-server --version

2. No fixed release: DOWNGRADE to a release before 10.0 (Debian / Ubuntu)

apt-cache madison zulip-server sudo apt install zulip-server=<VERSION_BEFORE_10.0> sudo apt-mark hold zulip-server

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list zulip-server sudo dnf downgrade zulip-server-<VERSION_BEFORE_10.0> sudo dnf versionlock add zulip-server

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-2026-40300?

zulip server: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-40300 being exploited?

CVE-2026-40300 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-2026-40300 affect?

CVE-2026-40300 affects zulip server. Severity is medium (CVSS 6.5). Zulip is an open-source team collaboration tool. Prior to 12.0, With message_edit_history_visibility_policy set to "moves", /api/v1/messages/{id}/history still returns historical content values, allowing low-privilege users to recover text

How urgent is CVE-2026-40300?

Priority 32/100. Public exploit reference

References

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