Vulnerability-Lookup

CVE-2026-23990 (GCVE-0-2026-23990)

Vulnerability from cvelistv5 – Published: 2026-01-21 22:25 – Updated: 2026-01-22 16:49

Title

Flux Operator Web UI Impersonation Bypass via Empty OIDC Claims

Summary

The Flux Operator is a Kubernetes CRD controller that manages the lifecycle of CNCF Flux CD and the ControlPlane enterprise distribution. Starting in version 0.36.0 and prior to version 0.40.0, a privilege escalation vulnerability exists in the Flux Operator Web UI authentication code that allows an attacker to bypass Kubernetes RBAC impersonation and execute API requests with the operator's service account privileges. In order to be vulnerable, cluster admins must configure the Flux Operator with an OIDC provider that issues tokens lacking the expected claims (e.g., `email`, `groups`), or configure custom CEL expressions that can evaluate to empty values. After OIDC token claims are processed through CEL expressions, there is no validation that the resulting `username` and `groups` values are non-empty. When both values are empty, the Kubernetes client-go library does not add impersonation headers to API requests, causing them to be executed with the flux-operator service account's credentials instead of the authenticated user's limited permissions. This can result in privilege escalation, data exposure, and/or information disclosure. Version 0.40.0 patches the issue.

Severity ?

5.3 (Medium)


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

CWE

CWE-269 - Improper Privilege Management
CWE-862 - Missing Authorization

Assigner

GitHub_M

References

URL

Tags

	https://github.com/controlplaneio-fluxcd/flux-ope…	x_refsource_CONFIRM
	https://github.com/controlplaneio-fluxcd/flux-ope…	x_refsource_MISC
	https://github.com/controlplaneio-fluxcd/flux-ope…	x_refsource_MISC
	https://github.com/controlplaneio-fluxcd/flux-ope…	x_refsource_MISC

Impacted products

	Vendor	Product	Version
	controlplaneio-fluxcd	flux-operator	Affected: >= 0.36.0, < 0.40.0

Show details on NVD website

JSON

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GHSA-4XH5-JCJ2-CH8Q

Vulnerability from github – Published: 2026-01-21 22:23 – Updated: 2026-01-22 15:40

Summary

Flux Operator Web UI Impersonation Bypass via Empty OIDC Claims

Details

A privilege escalation vulnerability exists in the Flux Operator Web UI authentication code that allows an attacker to bypass Kubernetes RBAC impersonation and execute API requests with the operator's service account privileges.

After OIDC token claims are processed through CEL expressions, there is no validation that the resulting username and groups values are non-empty. When both values are empty, the Kubernetes client-go library does not add impersonation headers to API requests, causing them to be executed with the flux-operator service account's credentials instead of the authenticated user's limited permissions.

Impact

Privilege Escalation: Any authenticated user can escalate to operator-level read permissions and perform suspend/resume/reconcile actions
Data Exposure: Unauthorized read access to Flux resources across all namespaces, bypassing RBAC restrictions
Information Disclosure: View sensitive GitOps pipeline configurations, source URLs, and deployment status across the entire cluster

Attack Scenario

Prerequisite: Cluster admins must configure the Flux Operator with an OIDC provider that issues tokens lacking the expected claims (e.g., email, groups), or configure custom CEL expressions that can evaluate to empty values.

Cluster admin configures OIDC authentication with a provider that does not include email or groups claims in tokens
User authenticates with a valid token from that provider
The default CEL expressions evaluate to empty values:
Username: has(claims.email) ? claims.email : '' → ""
Groups: has(claims.groups) ? claims.groups : [] → []
Authentication succeeds (token signature is valid)
A userClient is created with empty impersonation config
All subsequent API requests bypass impersonation and execute as the flux-operator service account
User gains operator-level read access across all namespaces

Patches

This vulnerability was fixed in Flux Operator v0.40.0.

Workarounds

The workaround is to make the email and groups claims required in the web config impersonation section.

Example config:

apiVersion: web.fluxcd.controlplane.io/v1
kind: Config
spec:
  baseURL: https://flux.example.com
  authentication:
    type: OAuth2
    oauth2:
      provider: OIDC
      clientID: "<redacted>"
      clientSecret: "<redacted>"
      issuerURL: "https://login.microsoftonline.com/<redacted>/v2.0"
      scopes: [openid, profile, email, offline_access]
      impersonation:
        username: claims.email
        groups: claims.groups

References

See the Pull Request fixing this vulnerability https://github.com/controlplaneio-fluxcd/flux-operator/pull/610

Credits

This vulnerability was discovered by the Flux Operator maintainers during a debugging session with end-users.

Severity ?

5.3 (Medium)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/controlplaneio-fluxcd/flux-operator"
      },
      "ranges": [
        {
          "events": [
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              "introduced": "0.36.0"
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    "github_reviewed_at": "2026-01-21T22:23:33Z",
    "nvd_published_at": "2026-01-21T23:15:52Z",
    "severity": "MODERATE"
  },
  "details": "A privilege escalation vulnerability exists in the Flux Operator Web UI authentication code that allows an attacker to bypass Kubernetes RBAC impersonation and execute API requests with the operator\u0027s service account privileges.\n\nAfter OIDC token claims are processed through CEL expressions, there is no validation that the resulting `username` and `groups` values are non-empty. When both values are empty, the Kubernetes client-go library does not add impersonation headers to API requests, causing them to be executed with the flux-operator service account\u0027s credentials instead of the authenticated user\u0027s limited permissions.\n\n### Impact\n\n- **Privilege Escalation**: Any authenticated user can escalate to operator-level read permissions and perform suspend/resume/reconcile actions\n- **Data Exposure**: Unauthorized read access to Flux resources across all namespaces, bypassing RBAC restrictions\n- **Information Disclosure**: View sensitive GitOps pipeline configurations, source URLs, and deployment status across the entire cluster\n\n### Attack Scenario\n\n**Prerequisite**: Cluster admins must configure the Flux Operator with an OIDC provider that issues tokens lacking the expected claims (e.g., `email`, `groups`), or configure custom CEL expressions that can evaluate to empty values.\n\n1. Cluster admin configures OIDC authentication with a provider that does not include `email` or `groups` claims in tokens\n2. User authenticates with a valid token from that provider\n3. The default CEL expressions evaluate to empty values:\n   - Username: `has(claims.email) ? claims.email : \u0027\u0027` \u2192 `\"\"`\n   - Groups: `has(claims.groups) ? claims.groups : []` \u2192 `[]`\n4. Authentication succeeds (token signature is valid)\n5. A userClient is created with empty impersonation config\n6. All subsequent API requests bypass impersonation and execute as the flux-operator service account\n7. User gains operator-level read access across all namespaces\n\n### Patches\n\nThis vulnerability was fixed in Flux Operator v0.40.0.\n\n### Workarounds\n\nThe workaround is to make the `email` and `groups` claims required in the web config `impersonation` section.\n\nExample config:\n\n```yaml\napiVersion: web.fluxcd.controlplane.io/v1\nkind: Config\nspec:\n  baseURL: https://flux.example.com\n  authentication:\n    type: OAuth2\n    oauth2:\n      provider: OIDC\n      clientID: \"\u003credacted\u003e\"\n      clientSecret: \"\u003credacted\u003e\"\n      issuerURL: \"https://login.microsoftonline.com/\u003credacted\u003e/v2.0\"\n      scopes: [openid, profile, email, offline_access]\n      impersonation:\n        username: claims.email\n        groups: claims.groups\n```\n\n### References\n\nSee the Pull Request fixing this vulnerability https://github.com/controlplaneio-fluxcd/flux-operator/pull/610 \n\n### Credits\n\nThis vulnerability was discovered by the Flux Operator maintainers during a debugging session with end-users.",
  "id": "GHSA-4xh5-jcj2-ch8q",
  "modified": "2026-01-22T15:40:22Z",
  "published": "2026-01-21T22:23:33Z",
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      "url": "https://github.com/controlplaneio-fluxcd/flux-operator/security/advisories/GHSA-4xh5-jcj2-ch8q"
    },
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      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-23990"
    },
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      "type": "CVSS_V3"
    }
  ],
  "summary": "Flux Operator Web UI Impersonation Bypass via Empty OIDC Claims"
}

FKIE_CVE-2026-23990

Vulnerability from fkie_nvd - Published: 2026-01-21 23:15 - Updated: 2026-01-21 23:15

Severity ?

5.3 (Medium) - CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/controlplaneio-fluxcd/flux-operator/commit/084540424f6de8ba5d88fb1fd1e8472ba29afd7e
	security-advisories@github.com	https://github.com/controlplaneio-fluxcd/flux-operator/pull/610
	security-advisories@github.com	https://github.com/controlplaneio-fluxcd/flux-operator/releases/tag/v0.40.0
	security-advisories@github.com	https://github.com/controlplaneio-fluxcd/flux-operator/security/advisories/GHSA-4xh5-jcj2-ch8q

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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    }
  ],
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  "lastModified": "2026-01-21T23:15:52.930",
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        "exploitabilityScore": 1.6,
        "impactScore": 3.6,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-01-21T23:15:52.930",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/controlplaneio-fluxcd/flux-operator/commit/084540424f6de8ba5d88fb1fd1e8472ba29afd7e"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/controlplaneio-fluxcd/flux-operator/pull/610"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/controlplaneio-fluxcd/flux-operator/releases/tag/v0.40.0"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/controlplaneio-fluxcd/flux-operator/security/advisories/GHSA-4xh5-jcj2-ch8q"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-269"
        },
        {
          "lang": "en",
          "value": "CWE-862"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}

Sightings

Author	Source	Type	Date

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-23990 (GCVE-0-2026-23990)

GHSA-4XH5-JCJ2-CH8Q

Impact

Attack Scenario

Patches

Workarounds

References

Credits

FKIE_CVE-2026-23990

Tags

Sightings

Nomenclature