CVE-2026-75837

The core Flex group blueprint system/blueprints/user/group.yaml (access field, lines 48-55) omits the security@: admin.super field guard that its sibling account blueprint carries (account.yaml:131/138/150, added by the CVE-2026-42613 fix). A delegated non-super operator holding admin.users.update can therefore save a group whose access map contains admin.super: true, which UserGroupObject::authorize then grants to every member of that group, a full privilege escalation to super-admin (scheduler/cron RCE, Twig eval). This is a distinct file, sink, and fix from all four related advisories.

Root Cause

The CVE-2026-42613 fix protected the account access/groups fields with a blueprint-level security@: admin.super gate, which Blueprint::dynamicSecurity() (system/src/Grav/Common/Data/Blueprint.php:644-662) uses to mark a field validate.ignore=true for non-super users so BlueprintSchema::filterArray() (BlueprintSchema.php:263-311) drops it. The functionally-identical group access field, a permission map granted to every member of the group, is declared in system/blueprints/user/group.yaml:48-55 with check_authorize: false and NO security@ guard. check_authorize has ZERO PHP enforcement (grep -rn check_authorize across the repo returns 0 PHP consumers; only two YAML blueprints reference it), so security@ is the sole real control. Because the group access field lacks security@, dynamicSecurity never flags it, filterArray retains it, and Validation::filterArray (type: array, value_type: bool) passes the nested admin.super:true leaf through. The core save path (FlexObject::update() then Framework/Flex/FlexObject.php:683 $blueprint->filter($data,true,true) then save()) persists it to user://config/groups.yaml.

Proof of Concept

As a non-super admin.users operator who is a member of group ops:

POST /admin/accounts/groups/ops   (or groups.json task:save)
data[access][admin][super]=1

user/config/groups.yaml gains ops: { access: { admin: { super: true } } }, so the operator is super-admin on the next request.

Attack Chain

1. Entry: authenticated delegated admin (admin.users.update, no admin.super) POSTs the group-edit form for a group they belong to (or a new group), body access[admin][super]=true. Guard: FlexAuthorizeTrait::isAuthorizedAction to admin.users.update. Bypass proof: user-groups.yaml exposes groups at admin.users:crudl; operator legitimately holds update.
2. Check (field guard): Blueprint::dynamicSecurity marks validate.ignore only for security@ fields. Guard: none on group access (no security@). Bypass proof: group.yaml:48-55 has no security@; git log -S 'security@' -- system/blueprints/user/group.yaml is empty.
3. Filter: BlueprintSchema::filterArray retains the non-ignored field; Validation::filterArray (value_type: bool) passes admin.super:true through. Bypass proof: field not flagged ignore/disabled; nested bool leaf preserved.
4. Sink (persist): FlexObject::update then filter then save writes to user://config/groups.yaml. Guard: none. Bypass proof: value already survived steps 2/3; storage performs no ACL filtering.
5. Impact: any member request triggers UserGroupObject::authorize('admin.super') (.../UserGroups/UserGroupObject.php:75) returning true, so the member is super-admin, enabling scheduler/Twig RCE.

Bypass Evidence

• system/blueprints/user/group.yaml:48-55: access block with check_authorize: false, no security@ (confirmed live on tag 2.0.12).
• git log -S 'security@' -- system/blueprints/user/group.yaml is empty (guard never existed; a permanent gap, not a regression).
• git log 2.0.12..HEAD -- system/blueprints/user/group.yaml is empty (no post-release fix).
• grep -rn check_authorize (whole repo) returns 0 PHP hits (guard unenforced in PHP).
• grep -rn "typePermissions|filterPermissions" system/src/Grav/Common/Data returns 0 (permissions map falls through to array-bool filtering that keeps nested keys).
• Guard asymmetry: account.yaml:131/138/150 carry security@: admin.super; group.yaml carries none. GHSA-h33v-82r9-v8pm's own text confirms the blueprint security@ gate is the sole Flex-backend strip for these keys.

Affected Versions

<= 2.0.12 (latest release; the guard never existed on group.yaml, so all 2.x are affected). The missing guard and the strip logic are both in core getgrav/grav; the group-edit UI is provided by the flex-objects/admin plugin, but the fix belongs in core.