NVD / CVE Program· CNA Mattermost

CVE-2023-40703: Denial of Service via specially crafted block fields in Mattermost Boards

Noroxi assessment

Priority: Monitor (action score 30/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.72% (FIRST EPSS); higher than 54% of all records.
Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
Fixed version: github.com/mattermost/mattermost-server/v6 (Go) 7.8.13; github.com/mattermost/mattermost/server/v8 (Go) 9.0.2, 9.1.1, 8.1.4.
Severity is disputed: the assigning authority (CNA) scores it 4.3, NVD 7.5.

Low priority for now. Upgrade to github.com/mattermost/mattermost-server/v6 (Go) 7.8.13; github.com/mattermost/mattermost/server/v8 (Go) 9.0.2, 9.1.1, 8.1.4 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

Mattermost fails to properly limit the characters allowed in different fields of a block in Mattermost Boards allowing a attacker to consume excessive resources, possibly leading to Denial of Service, by patching the field of a block using a specially crafted string.

HighCVSS 7.5 · v3.1—No exploit Fix available

GHSA-c37r-v8jx-7cv2

Published: Nov 27, 2023
Updated: Jun 17, 2026
EPSS: 0.7% · 54th percentile
CWE: CWE-400

Report tools

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Action score

Monitor

Low priority for now.

CVSS: 30 / 40 · 7.5 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.7%

CISA SSVC decision

Exploitation: none
Automatable: no
Technical impact: partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: 7.5
CNA · Mattermost: 4.3; 3.2 point gap

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

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
mattermost	mattermost	cpe:2.3:a:mattermost:mattermost

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

mattermost mattermostup to 7.8.12 inclusive
mattermost mattermost8.0.0 and later · up to 8.1.3 inclusive
mattermost mattermost9.0.0 and later · up to 9.0.1 inclusive
mattermost mattermost9.1.0

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Mattermost). Independent of NVD's CPE analysis and usually ahead of it.

Mattermost Mattermost
- 9.1.1, 9.0.2, 7.8.13, 8.1.4not affected
- up to and including 8.1.3affected · semver
- up to and including 9.0.1affected · semver
- up to and including 7.8.12affected · semver
- up to and including 9.1.0affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
Go	github.com/mattermost/mattermost-server/v6	before 7.8.13	7.8.13
Go	github.com/mattermost/mattermost/server/v8	from 9.0.0 · before 9.0.2	9.0.2
Go	github.com/mattermost/mattermost/server/v8	from 9.1.0 · before 9.1.1	9.1.1
Go	github.com/mattermost/mattermost/server/v8	before 8.1.4	8.1.4

Same product

mattermost: all records

Other highest-scoring records for the same primary product.

CVE-2024-39777Malicious remote can invite itself to an arbitrary local channel
38Monitor
CVE-2023-2193Oauth authorization codes do not expire when deauthorizing an oauth2 app
36Monitor
CVE-2019-20851An issue was discovered in Mattermost Mobile Apps before 1.26.0.
36Monitor

Published before and after this one in the same product

← EarlierCVE-2023-47865Nov 27, 2023 · Username and Icon override can be used by members when Hardened Mode is enabled
17Monitor
← EarlierCVE-2023-35075Nov 27, 2023 · HTML injection via channel autocomplete
21Monitor
Later →CVE-2023-43754Nov 27, 2023 · Permalink previews displayed for posts in archived channels even if users are disallowed to view archived channels
17Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
Go:github.com/mattermost/mattermost-server/v6	7.8.13	Package registry (OSV)
Go:github.com/mattermost/mattermost/server/v8	8.1.4	Package registry (OSV)
Go:github.com/mattermost/mattermost/server/v8	9.0.2	Package registry (OSV)
Go:github.com/mattermost/mattermost/server/v8	9.1.1	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

vultza (vultza)finder

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

CVE-2023-482680 days apartDenial of Service via Board Import Zip Bomb
30Monitor
CVE-2023-519659 days apartDoS via Channel Notification Properties
26Monitor
CVE-2023-2793164 days apartStack exhaustion in PreparePostForClientWithEmbedsAndImages
26Monitor
CVE-2023-2831164 days apartDenial of Service while unescaping a Markdown string
26Monitor
CVE-2022-2406501 days apartMalicious imports can lead to Denial of Service
26Monitor
CVE-2023-483690 days apartLog Flooding due to specially crafted requests in different endpoints
21Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

Mattermost Public Bug Bounty Engagement Bugcrowd · paid · up to $2,000

The product’s vendor appears in a public program. Matched by name; verify the scope text in the program.

Technical details

Attack conditions

Anyone who can reach it over the internet can trigger it.
No account or password is required.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: none
Integrity: none
Availability: high · the service can be disrupted

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality impact: None
Integrity impact: None
Availability impact: High

Weakness class (CWE)

CWE-400 · Uncontrolled Resource Consumption

CVSS vector

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

nvd-primary

Root cause

Code that parses nested structures sets no depth or size limit. A small input can turn into a disproportionate processing load.

The bug in code

The whole CWE-400 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

1function expand(node) {
2  return node.children.map(expand);
3}

Fixed

1const MAX_DEPTH = 32;
2function expand(node, depth = 0) {
3  if (depth > MAX_DEPTH) throw new Error("too deep");
4  return node.children.map((c) => expand(c, depth + 1));
5}

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Attack patterns (CAPEC)

ATT&CK techniques

Endpoint Denial of ServiceT1499

Change log

Fix✗ → ✓Sep 28, 2026, 10:47 PM UTC

For records you follow, these changes also arrive as notifications. →

References

mattermost.com/security-updates

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records