Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6 fail to properly validate synced reactions, when shared channels are enabled, which allows a malicious remote to create arbitrary reactions on arbitrary posts
{
"versions": [
{
"introduced": "9.5.0"
},
{
"fixed": "9.5.7"
},
{
"introduced": "0"
},
{
"last_affected": "9.9.0"
}
]
}