CVE-2026-58092: Unauthorized credential switching
In FreeBSD 15.0, the kernel structure used to represent user credentials changed: previously the primary group ID was stored in the first element of the array containing the list of supplementary group IDs, whereas now the primary group ID is stored in a dedicated field. This change was largely internal to the kernel and not user-visible.
One function, groupisprimary(), was not properly updated as a part of this transition. This function is used by macdo to determine the primary group ID of the credential after applying a transition rule, used when the rule target does not explicitly specify a group.
As a result, with certain macdo rules, it is possible for a credential switch to incorrectly set the primary group ID to the ID stored in the first element of the original credential's supplementary group array.
If the list of supplementary groups is empty, this value will be 0, corresponding to the "wheel" group. For example, a rule such as "uid=1001>uid=1002" can be abused to set the primary group ID to 0 even if the process did not originally belong to group 0.
Certain macdo rules can be abused to set a process' group ID to 0. Note however, that the rule must apply to the caller in order for the bug to be triggered, e.g., given the ruleset "uid=1001>uid=1002", the user must have user ID 1001 in order to trigger the bug.
Further, logged-in users will in general have a non-empty supplementary group list, in which case the bug can at worst be used to set the credential's first supplementary group ID as its primary group ID. Processes must explicitly remove themselves from all supplementary groups, using the privileged setgroups(2) system call, in order to exploit the bug to set 0 as the primary group ID.
Since membership in group 0 is often used to enable controlled privilege escalation, the bug might be further exploitable to obtain root privileges, depending on the system configuration. For instance, a ruleset such as the following could be exploited by a process running as user 1001 and with an empty supplementary group list: "uid=1001>uid=1002;gid=0>uid=0".
Affected Software
Event History
Frequently Asked Questions
Which mac_do rules are relevant to this issue?
Rules that apply a credential transition but do not explicitly specify a target group are affected. A rule such as "uid=1001>uid=1002" is an example, because the target group is inferred during the transition.
Under what condition can the resulting primary group become wheel (group 0)?
The original credential must have an empty supplementary-group list when an affected mac_do transition is applied. In that case, the incorrectly read array element has the value 0, which corresponds to the wheel group.
Does the process need to have belonged to wheel before the transition?
No. The example describes a process gaining primary group ID 0 even when it did not originally belong to group 0.