VulnerabilityPublished 26 May 2026Updated 29 Jun 2026

CVE-2026-4646: Mattermost server · Mattermost doesn't validate user-supplied input in API request handlers

Affects github.com/mattermost/mattermost-serverSource GCVE Database

Compare

Not known to be exploited · CVSS 4.3 Medium : upgrade to 11.6.1 when convenient.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 3.1 base score

4.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 16% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 11.6.1, 11.5.4, 11.4.5 +2

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 26 May 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
Gogithub.com/mattermost/mattermost-server	Affected >=11.6.0 <11.6.1 =11.6.0	Fixed in 11.6.1
Gogithub.com/mattermost/mattermost-server	Affected >=11.5.0 <11.5.4	Fixed in 11.5.4
Gogithub.com/mattermost/mattermost-server	Affected >=11.4.0 <11.4.5	Fixed in 11.4.5
Gogithub.com/mattermost/mattermost-server	Affected >=10.11.0 <10.11.15	Fixed in 10.11.15
Gogithub.com/mattermost/mattermost-plugin-github	Affected <1.0.1-0.20260330164815-c2840e980b3c	Fixed in 1.0.1-0.20260330164815-c2840e980b3c

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

In the meantime, restricting authenticated user access to trusted parties may reduce risk.

Description

Mattermost versions 10.11.0 through 10.11.14, 11.4.0 through 11.4.4, 11.5.0 through 11.5.3, and 11.6.0 are affected by a vulnerability where user-supplied input in API request handlers is not properly validated. This flaw allows an authenticated attacker to crash the plugin process by sending a crafted HTTP request to the pull request details endpoint. The vulnerability has been assigned CVE-2026-4646 and is categorized under CWE-1287.

Impact

An authenticated attacker can exploit this vulnerability to crash the plugin process, resulting in denial of service conditions affecting the availability of the Mattermost server's plugin functionality. There is no impact on confidentiality or integrity according to the CVSS vector. No known exploits are reported in the wild at this time.

Summary

Mattermost server versions 10.11.x <= 10.11.14, 11.4.x <= 11.4.4, 11.5.x <= 11.5.3, and 11.6.0 fail to validate user input in API request handlers. This improper input validation enables an authenticated attacker to cause a denial of service by crashing the plugin process via a specially crafted HTTP request targeting the pull request details endpoint. The issue is tracked as CVE-2026-4646 and is considered a moderate severity vulnerability with a CVSS 3.1 vector indicating network attack vector, low attack complexity, low privileges required, no user interaction, unchanged scope, no confidentiality or integrity impact, and availability impact only.

CWE-1287Improper Validation of Specified Type of Input (opens in a new tab)
Base weakness
The product receives input that is expected to be of a certain type, but it does not validate or incorrectly validates that the input is actually of the expected type.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 29 Jun 2026, 22:13 UTC

WeaknessesCWE-1287

Technical details

References

Community reviews

Related threats