Mattermost fails to check if a custom emoji reaction exists when sending it to a post and to limit the amount of custom emojis allowed to be added in a post, allowing an attacker sending a huge amount of non-existent custom emojis in a post to crash the mobile app of a user seeing the post and to crash the server due to overloading when clients attempt to retrive the aforementioned post.
| Package (Ecosystem) | Introduced | Fixed | Limit |
|---|---|---|---|
| github.com/mattermost/mattermost/server/v8(Go) | 0 | 8.1.8 | N/A |
| github.com/mattermost/mattermost/server/v8(Go) | 9.2.0 | 9.2.4 | N/A |
| github.com/mattermost/mattermost/server/v8(Go) | 9.1.0 | 9.1.5 | N/A |
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