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CVE-2026-5132

Unbounded zlib decompression in Calls SDP WebSocket messages

MEDIUM
6.5
CVSS Score
Published: September 14, 2026 at 10:41 AM UTC
Updated: October 7, 2026 at 03:43 PM UTC
2 products affected across 1 vendorMattermost ServerMattermost

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Severity
MEDIUM6.5
Exploit prediction
EPSS 0.4%
SSVC
No known exploit

Description

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

Affected

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

Mattermost Server· Mattermost
95%
10.11.0 ≤ v < 10.11.2311.7.0 ≤ v < 11.7.811.8.0 ≤ v < 11.8.511.9.0 ≤ v < 11.9.1
Affected range stops below 10.11.23, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.7.8, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.8.5, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.9.1, so it looks safe — our inference, not a vendor statement
Mattermost· Mattermost
60%
10.11.0 ≤ v ≤ 10.11.2211.7.0 ≤ v ≤ 11.7.711.8.0 ≤ v ≤ 11.8.410.11.011.7.011.8.011.9.0
Vendor states this is fixed in 10.11.23 (unaffected)
Vendor states this is fixed in 11.10.0 (unaffected)
Vendor states this is fixed in 11.7.8 (unaffected)
Vendor states this is fixed in 11.8.5 (unaffected)
Vendor states this is fixed in 11.9.1 (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

6.5MEDIUMMattermost
Attack Vector
NETWORK
Complexity
LOW
Privileges
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
NONE
Integrity
NONE
Availability
HIGH
Vector?

Quick Info

StatusANALYZED
CWE
CWE-409

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