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CVE-2026-6673

Mattermost Jira plugin had unauthenticated {{/ac/installed}} lifecycle callback during pending Jira Cloud install

MEDIUM
6.4
CVSS Score
Published: June 22, 2026 at 01:38 PM UTC
Updated: June 23, 2026 at 09:04 PM UTC
2 products affected across 1 vendorMattermost ServerMattermost

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Severity
MEDIUM6.4
Exploit prediction
EPSS 0.3%
SSVC
No known exploit

Description

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret and disrupt the Jira integration via POST to /ac/installed during the pending-install window.. Mattermost Advisory ID: MMSA-2026-00654

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Mattermost Server· Mattermost
95%
10.11.0 ≤ v < 10.11.1811.5.0 ≤ v < 11.5.611.6.0 ≤ v < 11.6.311.7.0 ≤ v < 11.7.1
Affected range stops below 10.11.18, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.5.6, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.6.3, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.7.1, so it looks safe — our inference, not a vendor statement
Mattermost· Mattermost
60%
10.11.0 ≤ v ≤ 10.11.1711.5.0 ≤ v ≤ 11.5.511.6.0 ≤ v ≤ 11.6.210.11.011.5.011.6.011.7.0
Vendor states this is fixed in 10.11.18 (unaffected)
Vendor states this is fixed in 11.5.6 (unaffected)
Vendor states this is fixed in 11.6.3 (unaffected)
Vendor states this is fixed in 11.7.1 (unaffected)
Vendor states this is fixed in 11.8.0 (unaffected)

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (1)

Vendor Advisory
Source: cverecord
https://mattermost.com/security-updates

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

6.4MEDIUMMattermost
Attack Vector
NETWORK
Complexity
HIGH
Privileges
LOW
User Interaction
REQUIRED
Scope
UNCHANGED
Confidentiality
NONE
Integrity
HIGH
Availability
HIGH
Vector?

Quick Info

StatusANALYZED
CWE
CWE-306

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