How severe is CVE-2026-40300?

CVE-2026-40300 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2026-40300 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (16th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-40300?

CVE-2026-40300 affects Zulip Zulip Server. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-40300?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-40300 have an EU (EUVD) identifier?

Yes. CVE-2026-40300 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-29537.

When was CVE-2026-40300 published?

CVE-2026-40300 was published on 2026-05-12 and last updated on 2026-06-17.

CVE-2026-40300

CVE-2026-40300 is a medium-severity vulnerability in Zulip Zulip Server with a CVSS 3.x base score of 6.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-284.

Key facts

Severity: Medium (CVSS 3.x base score 6.5)
CVSS v4: 6.0
EPSS exploit prediction: 0% (16th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-29537
Weakness: CWE-284
Affected product: Zulip Zulip Server
Published: 2026-05-12
Last modified: 2026-06-17

Description

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Prior to 12.0, With message_edit_history_visibility_policy set to "moves", /api/v1/messages/{id}/history still returns historical content values, allowing low-privilege users to recover text that was edited away from other users' messages.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.

Frequently asked questions

What is CVE-2026-40300?: 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Prior to 12.0, With message_edit_history_visibility_policy set to "moves", /api/v1/messages/{id}/history still returns historical content values, allowing low-privilege users to recover text that was edited away from other users' messages.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.
How severe is CVE-2026-40300?: CVE-2026-40300 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2026-40300 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (16th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-40300?: CVE-2026-40300 affects Zulip Zulip Server. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-40300?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-40300 have an EU (EUVD) identifier?: Yes. CVE-2026-40300 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-29537.
When was CVE-2026-40300 published?: CVE-2026-40300 was published on 2026-05-12 and last updated on 2026-06-17.

References

https://github.com/zulip/zulip/security/advisories/GHSA-jp8f-mvv6-89cr

Affected products (1)

cpe:2.3:a:zulip:zulip_server:10.0:*:*:*:*:*:*:*

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