Security readout for executives and security teams
Plain-English summary
Mattermost disclosed a high-severity reflected cross-site scripting issue in OAuth-enabled Mattermost instances. Successful abuse would require a user interaction but could expose sensitive data or let an attacker act in the user’s browser context. The source bundle does not show active exploitation.
Executive priority
Treat this as a high-priority patch and validation item for Mattermost environments using OAuth. It is not confirmed as actively exploited in the supplied sources, but potential confidentiality and integrity impact is high.
Technical view
CVE-2021-37859 is a CWE-79 reflected XSS bypass in Mattermost’s OAuth flow. CVSS 3.1 is 7.1 with network attack vector, high attack complexity, no privileges required, user interaction required, and high confidentiality and integrity impact.
Likely exposure
Exposure is most relevant to Mattermost deployments using OAuth, especially versions identified in the source bundle: 5.34.5, 5.35.4, and 5.32.0. Non-OAuth deployments may not match the described vulnerable condition, but should still confirm against vendor guidance.
Exploitation context
The bundle states KEV is false and provides no evidence of in-the-wild exploitation. The CVSS vector indicates exploitation requires user interaction and higher attack complexity, lowering immediate likelihood but not reducing business impact if a privileged user is targeted.
Researcher notes
Evidence is limited to the CVE record and Mattermost security update reference. Do not assume affected branches or fixed versions beyond vendor guidance. Focus validation on OAuth-enabled Mattermost instances and reflected XSS behavior in the OAuth flow without creating weaponized proof of concept material.
Mitigation direction
- Check Mattermost security updates for the vendor-listed fixed release.
- Upgrade affected OAuth-enabled Mattermost instances according to vendor guidance.
- Prioritize internet-facing or externally accessible Mattermost deployments.
- Review OAuth configuration and disable unused providers where operationally feasible.
- Monitor for suspicious OAuth-flow errors or user reports of unexpected redirects.
Validation and detection
- Inventory Mattermost versions and identify OAuth-enabled instances.
- Confirm whether any listed affected versions are deployed.
- Review vendor security notes for the applicable release branch.
- Run safe XSS validation in staging using approved internal tooling.
- Check relevant web and application logs for anomalous OAuth-flow activity.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-79: User-session and phishing behavior lookup
Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-37859 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.1 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:L
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:L1.65.5Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.1HighVector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:L
Source materials
- CVE List V5 sourceCVE List V5
- https://mattermost.com/security-updates/CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.