CVE-2026-6339Published

Missing request origin validation on burn-on-read reveal endpoint

Collected exploitation and prediction evidence

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

cve-program · CVE-2026-6339

Published

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

Source published: 18 May 2026 08:05:30 UTC
Source modified: 18 May 2026 12:42:01 UTC
Observed here: 23 Oct 2026 15:48:27 UTC
Source terms: CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 4.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

euvd · EUVD-2026-30749

Published

EUVD source date text has no stated timezone and is retained without inventing a UTC instant. Related bulletin membership is not an alias, common cause or applicability claim. A missing vector with a zero score is treated as unrated. Complete vendor/product and underlying source evidence remains in the original JSON.

Original publisher date fields

datePublished: May 18, 2026, 8:05:30 AM

dateUpdated: May 18, 2026, 12:42:01 PM

Source published: May 18, 2026, 8:05:30 AM (timezone not stated)
Source modified: May 18, 2026, 12:42:01 PM (timezone not stated)
Observed here: 9 Nov 2026 14:12:39 UTC
Source terms: ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 4.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

nvd · CVE-2026-6339

Published

NVD analysis and CPE criteria are source context, not proof that a package or your environment is affected. Complete affected data, logical configurations and SSVC are retained in the original JSON.

Source analysis status: Analyzed

Weakness classifications: CWE-346

Original publisher date fields

published: 2026-05-18T09:16:23.573

lastModified: 2026-10-14T20:10:00.190

1 source CPE criteria · context only

cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*

The original JSON retains the logical configuration operators, exclusions and version bounds.

Source published: 18 May 2026 09:16:23 UTC
Source modified: 14 Oct 2026 20:10:00 UTC
Observed here: 25 Oct 2026 14:19:28 UTC
Source terms: NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 4.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

CVSS 3.1 · [email protected] · Primary · 4.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

osv · GHSA-xvcx-mgpc-5xh3

Published

Source published: 18 May 2026 09:31:48 UTC
Source modified: 25 Jun 2026 23:11:08 UTC
Observed here: 24 Oct 2026 13:19:41 UTC
Source terms: GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

Publisher rating · MODERATE

osv · GO-2026-5769

Published

Mattermost doesn't validate the X-Requested-With header on the burn-on-read reveal endpoint in github.com/mattermost/mattermost-server. NOTE: The source advisory for this report contains additional versions that could not be automatically mapped to standard Go module versions. (If this is causing false-positive reports from vulnerability scanners, please suggest an edit to the report.) The additional affected modules and versions are: github.com/mattermost/mattermost-server/v5 before v5.3.2-0.20260327001745-7a339a6438f5.

Source published: 25 Jun 2026 22:34:41 UTC
Source modified: 25 Jun 2026 23:01:41 UTC
Observed here: 24 Oct 2026 13:19:41 UTC
Source terms: OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.