Security readout for executives and security teams
Plain-English summary
Kanboard deployments using reverse proxy authentication can let an unauthenticated attacker sign in as any user, including administrators, if running version 1.2.48 or earlier. This is a high-urgency issue because it can expose and alter project data without needing credentials. The vendor fixed it in version 1.2.49.
Executive priority
Treat as urgent for any Kanboard system using reverse proxy authentication. Prioritize internet-facing or broadly accessible instances first. The business risk is unauthorized access to project data and administrative control, with no credentials required under the vulnerable configuration.
Technical view
CVE-2026-21881 is a CWE-287 authentication bypass in Kanboard before 1.2.49 when REVERSE_PROXY_AUTH is enabled. The application trusted authentication-related HTTP headers without confirming the request came from a trusted reverse proxy. CVSS is 9.1, network exploitable, low complexity, no privileges, no user interaction, with high confidentiality and integrity impact.
Likely exposure
Exposure is limited to Kanboard instances below 1.2.49 that have REVERSE_PROXY_AUTH enabled, especially if reachable over a network where clients can directly influence authentication headers. Instances not using this feature are less likely to be affected based on the advisory description.
Exploitation context
No active exploitation is stated in the provided sources, and the CVE is not marked KEV. However, the advisory describes a simple unauthenticated authentication bypass condition with administrator impersonation potential, making externally reachable affected deployments urgent to review.
Researcher notes
The public sources identify the vulnerable condition, affected versions, CVSS vector, fixing release, and associated commit. They do not provide evidence of exploitation in the wild. Analysis should focus on configuration reachability, trusted proxy boundaries, and confirming the 1.2.49 fix in deployed environments.
Mitigation direction
- Upgrade Kanboard to version 1.2.49 or later.
- Review vendor advisory and release notes for deployment-specific guidance.
- If immediate upgrade is not possible, reassess use of REVERSE_PROXY_AUTH.
- Restrict direct access so Kanboard only receives traffic through the intended trusted proxy.
Validation and detection
- Inventory Kanboard instances and identify versions below 1.2.49.
- Check whether REVERSE_PROXY_AUTH is enabled in each deployment.
- Confirm clients cannot reach Kanboard directly around the reverse proxy.
- Review access logs for unexpected administrator sessions or unusual authentication patterns.
- Verify the upgrade to 1.2.49 is deployed successfully.
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-287: Credential and account abuse lookup
Authentication and credential weaknesses can make valid-account abuse and credential telemetry useful review starting points. 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 lookupCredential and access behavior lookup
The CVE wording references authentication or credential exposure, so valid-account and credential-access review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2026-21881 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
- Critical
- CVSS
- 9.1 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
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:L/PR:N/UI:N/S:U/C:H/I:H/A:N3.95.2Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
9.1CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/kanboard/kanboard/security/advisories/GHSA-wwpf-3j4p-739wCVE reference · x_refsource_CONFIRM
- https://github.com/kanboard/kanboard/commit/7af6143e2ad25b5c15549cca8af4341c7ac4e2fcCVE reference · x_refsource_MISC
- https://github.com/kanboard/kanboard/releases/tag/v1.2.49CVE 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 Authentication
Improper Authentication represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.