Race Condition in Guest Magic Link Authentication Allows Token Reuse

published 2026-04-15 6 mo ago modified 2026-06-25 4 mo ago also GHSA-mh4x-rmrx-3hp4 , GO-2026-5507 , EUVD-2026-22915

cvss: 6.5; 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 4%
public exploits: 0; none known

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests.. Mattermost Advisory ID: MMSA-2026-00624

Patch / fix

fixed in 3 distros 1

go github.com/mattermost/mattermost/server/v8 8.0.0-20250723052842-4cb8d8940332
go github.com/mattermost/mattermost-server 10.11.13, 10.11.13+incompatible, 11.3.3, 11.3.3+incompatible, 11.4.3, 11.4.3+incompatible, 11.5.0, 11.5.0+incompatible
Mattermost Mattermost 10.11.13, 11.3.3, 11.4.3, 11.5.1, 11.6.0

govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-3590 updated 2026-06-28 cvss 6.5

mattermost: race condition in guest magic link authentication allows token reuse

Red Hat All currently supported Red Hat products
not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 6 mo ago (2026-04-15)

Exploit prediction

0.15% chance of exploitation in the next 30 days, higher than 4% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	6.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
GitHub	3.1	6.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Affected packages

ecosystem	package	affected	fixed in
go	github.com/mattermost/mattermost/server/v8	>=8.0.0-20250721062209-4952acea88ce, <8.0.0-20250723052842-4cb8d8940332	8.0.0-20250723052842-4cb8d8940332
go	github.com/mattermost/mattermost-server	>=10.11.0-rc1, <10.11.13 >=11.5.0-rc1, <11.5.0 >=11.4.0-rc1, <11.4.3 >=11.3.0-rc1, <11.3.3 >=10.11.0-rc1+incompatible, <10.11.13+incompatible >=11.3.0-rc1+incompatible, <11.3.3+incompatible >=11.4.0-rc1+incompatible, <11.4.3+incompatible >=11.5.0-rc1+incompatible, <11.5.0+incompatible	10.11.13 11.5.0 11.4.3 11.3.3 10.11.13+incompatible 11.3.3+incompatible 11.4.3+incompatible 11.5.0+incompatible
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-367 Time-of-check Time-of-use (TOCTOU) Race Condition

References

mattermost.com /security-updates
vendor-advisory, web
nvd.nist.gov /vuln/detail/CVE-2026-3590
advisory
github.com /mattermost/mattermost
package
github.com /advisories/GHSA-mh4x-rmrx-3hp4
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