Medium
Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost
Type: advisorySources: OSV
Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2026-6850
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 3 Nov 2026, 11:31 UTC · Revision 1
Source evidence
Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost
core-osv · Original sourcePublished 13 Jul 2026, 08:13 UTC
Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658
osv:CVE-2026-6850
Complete source record
{
"id": "34f8af682e9f858da0fa800ce6723f708e0546d8b77fbf5be5316c3c1edc8573",
"url": "https://osv.dev/vulnerability/CVE-2026-6850",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost",
"summary": "Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658",
"evidence": "osv:CVE-2026-6850",
"products": [],
"severity": "medium",
"structured": {
"id": "CVE-2026-6850",
"aliases": [
"CVE-2026-6850"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/mattermost/mattermost",
"type": "GIT",
"events": [
{
"introduced": "5b955468ea1ea8b56c0fb10feb258a36a767d7bb"
},
{
"fixed": "31b97d9ee1e020ca63f2169e4daab4aa0519a727"
},
{
"introduced": "d4f4e78835e86e2e76f76bfe797623076f5c9835"
},
{
"fixed": "b94397a68c6129bb904dcb0ab7643b188f2717b0"
},
{
"introduced": "a9e574a82633915f22071f0d7ca2b006f249ec2a"
},
{
"fixed": "28b63b15f73e0ed02d0da0018e06d88c47a77c48"
}
],
"database_specific": {
"extracted_events": [
{
"cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "10.11.0"
},
{
"fixed": "10.11.20"
}
],
"source": "CPE_RANGE"
},
{
"cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "11.6.0"
},
{
"fixed": "11.6.5"
}
],
"source": "CPE_RANGE"
},
{
"cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "11.7.0"
},
{
"fixed": "11.7.3"
}
],
"source": "CPE_RANGE"
}
]
}
}
],
"versions": [
"v10.11.19",
"v11.6.4",
"v10.11.19-rc1",
"v11.7.2",
"v11.6.3",
"v10.11.18",
"v11.7.1",
"v11.6.2",
"v10.11.17",
"v10.11.16",
"v11.7.0",
"mattermost-redux@11.7.0",
"@mattermost/types@11.7.0",
"@mattermost/shared@11.7.0",
"@mattermost/client@11.7.0",
"v10.11.15-rc1",
"v10.11.15",
"v11.6.1-rc1",
"v11.6.1",
"v10.11.14",
"v11.6.0",
"v10.11.14-rc1",
"v10.11.13",
"v10.11.13-rc1",
"v10.11.12",
"v10.11.11-rc2",
"v10.11.11",
"v10.11.11-rc1",
"v10.11.10",
"v10.11.9",
"v10.11.9-rc1",
"v10.11.8",
"v10.11.7",
"v10.11.6",
"v10.11.5",
"v10.11.4-rc3",
"v10.11.4",
"v10.11.4-rc2",
"v10.11.4-rc1",
"v10.11.3",
"v10.11.2-rc2",
"v10.11.2",
"v10.11.2-rc1",
"v10.11.1",
"v10.11.1-rc1",
"v10.11.0-rc3",
"v10.11.0",
"mattermost-redux@10.11.0",
"@mattermost/types@10.11.0",
"@mattermost/client@10.11.0"
],
"database_specific": {
"source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-6850.json"
}
}
],
"modified": "2026-10-28T02:30:28.229898670Z",
"provider": "osv",
"references": [
"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/6xxx/CVE-2026-6850.json",
"https://mattermost.com/security-updates",
"https://nvd.nist.gov/vuln/detail/CVE-2026-6850"
],
"provider_metadata": {
"cwe_ids": [
"CWE-1333"
],
"cna_assigner": "Mattermost",
"unresolved_ranges": [
{
"extracted_events": [
{
"range": [
{
"introduced": "11.7.0"
},
{
"last_affected": "11.7.2"
},
{
"introduced": "11.6.0"
},
{
"last_affected": "11.6.4"
},
{
"introduced": "10.11.0"
},
{
"last_affected": "10.11.19"
}
],
"source": "AFFECTED_FIELD",
"strategy": "StandardRange"
}
]
}
],
"osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/6xxx/CVE-2026-6850.json"
}
},
"related_cves": [
{
"value": "CVE-2026-6850",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 494980,
"published_at": "2026-07-13T08:13:02.883+00:00"
}