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Fencer Vulns
high 7.5

CVE-2026-5740 json

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

published 2026-05-22 5 mo ago modified 2026-07-23 3 mo ago also GHSA-w9m8-p4cc-4qj9 , GO-2026-5838 , EUVD-2026-31426

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 27%
public exploits
0
none known

Summary

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647

Patch / fix

  • go github.com/mattermost/mattermost-server 10.11.15, 10.11.15+incompatible, 11.4.5, 11.4.5+incompatible, 11.5.4, 11.5.4+incompatible, 11.6.1, 11.6.1+incompatible
  • go github.com/mattermost/mattermost/server/v8 8.0.0-20260410202636-17939826efa2
  • Mattermost Mattermost 10.11.15, 11.4.5, 11.5.4, 11.6.1, 11.7.0
  • govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
5 mo ago

Exploit prediction

0.34% chance of exploitation in the next 30 days, higher than 27% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
go github.com/mattermost/mattermost-server
>=11.6.0, <11.6.1
>=11.5.0, <11.5.4
>=11.4.0, <11.4.5
>=10.11.0, <10.11.15
>=10.11.0+incompatible, <10.11.15+incompatible
>=11.4.0+incompatible, <11.4.5+incompatible
>=11.5.0+incompatible, <11.5.4+incompatible
>=11.6.0+incompatible, <11.6.1+incompatible
11.6.1
11.5.4
11.4.5
10.11.15
10.11.15+incompatible
11.4.5+incompatible
11.5.4+incompatible
11.6.1+incompatible
go github.com/mattermost/mattermost/server/v8
<8.0.0-20260410202636-17939826efa2
8.0.0-20260410202636-17939826efa2
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-789 Memory Allocation with Excessive Size Value

References

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.