GHSA-34g8-9fpp-46ch CVE-2026-2456 GO-2026-4726
Mattermost fails to limit the size of responses from integration action endpoints
- Severity
- 5.3 / 10
- Medium · CVSS 3.1
- Exploit chance (EPSS)
- 0.17%
- Chance of exploitation in the next 30 days. Higher than 6.10% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-789 Memory Allocation with Excessive Size Value
- Fix
- Available
- 2 packages have a fixed version
Description
Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 Mattermost fails to limit the size of responses from integration action endpoints, which allows an authenticated attacker to cause server memory exhaustion and denial of service via a malicious integration server that returns an arbitrarily large response when a user clicks an interactive message button. Mattermost Advisory ID: MMSA-2026-00571
Affected versions
Go
| Package | Vulnerable | Fixed in |
|---|---|---|
| github.com/mattermost/mattermost-server | < 5.3.2-0.20260127165411-fe3052073dc6 |
5.3.2-0.20260127165411-fe3052073dc6 |
| github.com/mattermost/mattermost-server | >= 10.11.0-rc1, < 10.11.11 |
10.11.11 |
| github.com/mattermost/mattermost-server | >= 11.2.0-rc1, < 11.2.3 |
11.2.3 |
| github.com/mattermost/mattermost-server | >= 11.3.0-rc1, < 11.3.1 |
11.3.1 |
| github.com/mattermost/mattermost-server | >= 10.11.0-rc1+incompatible, < 10.11.11+incompatible>= 11.2.0-rc1+incompatible, < 11.2.3+incompatible>= 11.3.0-rc1+incompatible, < 11.3.1+incompatible |
10.11.11+incompatible11.2.3+incompatible11.3.1+incompatible |
| github.com/mattermost/mattermost-server/v5 | all versions |
No fix yet |
| github.com/mattermost/mattermost-server/v6 | all versions |
No fix yet |
| github.com/mattermost/mattermost/server/v8 | < 8.0.0-20260127165411-fe3052073dc6 |
8.0.0-20260127165411-fe3052073dc6 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
github.com/mattermost/mattermost-server Go → 5.3.2-0.20260127165411-fe3052073dc6
- go
go get github.com/mattermost/mattermost-server@v5.3.2-0.20260127165411-fe3052073dc6 && go mod tidy
github.com/mattermost/mattermost/server/v8 Go → 8.0.0-20260127165411-fe3052073dc6
- go
go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260127165411-fe3052073dc6 && go mod tidy
Severity breakdown
CVSS 3.1 5.3
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
Weakness
CWE-789: Memory Allocation with Excessive Size Value
The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated.
How to prevent it
- Implementation, Architecture and Design: Perform adequate input validation against any value that influences the amount of memory that is allocated. Define an appropriate strategy for handling requests that exceed the limit, and consider supporting a configuration option so that the administrator can extend the amount of memory to be used if necessary.
- Operation: Run your program using system-provided resource limits for memory. This might still cause the program to crash or exit, but the impact to the rest of the system will be minimized.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-xmfh-3ccg-c9fx Mattermost has an Incorrect Authorization issue | Low | Also affects github.com/mattermost/mattermost-server |
| GHSA-h998-hxxj-8q83 Mattermost doesn't invalidate cached authentication state for active WebSocket connections during global session revocation | Medium | Also affects github.com/mattermost/mattermost-server |
| GHSA-g5vr-6pgg-74qv Mattermost doesn't enforce bot-specific permission checks on the user active status endpoint | Low | Also affects github.com/mattermost/mattermost-server |
| GHSA-jqgv-39mg-7c2r Mattermost doesn't authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret | Medium | Also affects github.com/mattermost/mattermost-server |
| GHSA-mxq2-5jpg-7474 Mattermost doesn't validate channel ownership of an existing subscription before applying edits | Medium | Also affects github.com/mattermost/mattermost-server |
| GHSA-2g8v-grq3-hq2g Mattermost doesn't enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler | Medium | Also affects github.com/mattermost/mattermost-server |
| GHSA-6hxm-w4hv-vgvw Mattermost doesn't require role-management authorization when setting the scheme_admin flag on group syncable link and patch endpoints | High | Also affects github.com/mattermost/mattermost-server |
| GHSA-9p44-r552-4wp9 Mattermost doesn't sanitize the Remote Cluster API response on PATCH operations | Medium | Also affects github.com/mattermost/mattermost-server |
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