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Mattermost EUVDEUVD-2026-77370

| CVE-2026-5132 MEDIUM
Improper Handling of Highly Compressed Data (Data Amplification) (CWE-409)
2026-09-14 Mattermost GHSA-9c74-gj4h-5458
Denial Of Service Mattermost
6.5
CVSS 3.1 · Vendor: Mattermost
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Severity by source

Vendor (Mattermost) PRIMARY
6.5 MEDIUM
AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
vuln.today AI
6.5 MEDIUM

Network-reachable, trivial to execute once authenticated (PR:L); pure availability impact only, no confidentiality or integrity loss possible.

3.1 AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Primary rating from Vendor (Mattermost).

CVSS VectorVendor: Mattermost

Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

1
Analysis Generated
Sep 14, 2026 - 11:28 vuln.today

DescriptionCVE.org

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to limit size of unpacked SDP messages compressed with zlib, which allows attacker to deny service or crash server via sending many SDP messages that unpack to large size.. Mattermost Advisory ID: MMSA-2026-00643

AnalysisAI

Mattermost server crashes or becomes unavailable when an authenticated user sends multiple SDP messages compressed with zlib that decompress to arbitrarily large sizes, exploiting the absence of any decompressed-size cap. Affected are all Mattermost instances running versions 11.9.0, 11.8.4 and earlier in the 11.8.x line, 11.7.7 and earlier in the 11.7.x line, and 10.11.22 and earlier in the 10.11.x line. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

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Exploit
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Execution
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Vulnerability AssessmentAI

Exploitation Exploitation requires: (1) a valid Mattermost user account at any privilege level (PR:L per CVSS - guest or member accounts suffice); (2) network access to the Mattermost server's API or WebSocket endpoint; (3) the Mattermost Calls or SDP-handling subsystem to be reachable (it is enabled by default in standard deployments). … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The official CVSS 3.1 vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H scores 6.5 (Medium), which is well-calibrated here: the attack is network-reachable, requires no elevated privileges beyond a standard user account (PR:L), and reliably produces a server availability impact. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario Full exploit scenario with step-by-step reproduction available after sign-in.
Remediation The primary fix is to upgrade to a patched release as listed in the Mattermost security advisory MMSA-2026-00643 at https://mattermost.com/security-updates - the vendor has released corrections across all affected stable branches. … Detailed patch versions, workarounds, and compensating controls in full report.
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EUVD-2026-77370 vulnerability details – vuln.today
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