Account Takeover via Substring Matching in OpenID Connect Authentication

published 2026-03-25 7 mo ago modified 2026-06-25 4 mo ago also GHSA-fg35-5rf6-qg3g , GO-2026-5360 , EUVD-2026-15805

cvss: 5.7; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.2%; EPSS 0.2% chance of exploitation in 30 days, percentile 7%
public exploits: 0; none known

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590

Patch / fix

fixed in 3 distros 1

go github.com/mattermost/mattermost-server 10.11.12, 10.11.12+incompatible, 11.2.4, 11.2.4+incompatible, 11.3.2, 11.3.2+incompatible, 11.4.1, 11.4.1+incompatible
go github.com/mattermost/mattermost/server/v8 8.0.0-20260217110922-b7d4a1f1f59b
Mattermost Mattermost 10.11.12, 11.2.4, 11.3.2, 11.4.1, 11.5.0

govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: total
Assessed: 7 mo ago (2026-03-26)

Exploit prediction

0.18% chance of exploitation in the next 30 days, higher than 7% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	5.7	CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N
NVD	3.1	6.1	CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:U/C:H/I:H/A:N
GitHub	3.1	5.7	CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N

Affected packages

ecosystem	package	affected	fixed in
go	github.com/mattermost/mattermost-server	>=11.4.0-rc1, <11.4.1 >=11.3.0-rc1, <11.3.2 >=11.2.0-rc1, <11.2.4 >=10.11.0-rc1, <10.11.12 >=10.11.0-rc1+incompatible, <10.11.12+incompatible >=11.2.0-rc1+incompatible, <11.2.4+incompatible >=11.3.0-rc1+incompatible, <11.3.2+incompatible >=11.4.0-rc1+incompatible, <11.4.1+incompatible	11.4.1 11.3.2 11.2.4 10.11.12 10.11.12+incompatible 11.2.4+incompatible 11.3.2+incompatible 11.4.1+incompatible
go	github.com/mattermost/mattermost/server/v8	>=8.0.0-20260105080200-d27a2195068d, <8.0.0-20260217110922-b7d4a1f1f59b	8.0.0-20260217110922-b7d4a1f1f59b
go	github.com/mattermost/mattermost-server/v5	all versions	no fix listed
go	github.com/mattermost/mattermost-server/v6	all versions	no fix listed

Affected products

Mattermost Mattermost

Classification

Weakness

CWE-303 Incorrect Implementation of Authentication Algorithm

References

mattermost.com /security-updates
vendor-advisory, web
nvd.nist.gov /vuln/detail/CVE-2026-27656
advisory
github.com /mattermost/mattermost
package
github.com /advisories/GHSA-fg35-5rf6-qg3g
advisory

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary