Current main branch and likely all versions with the channels feature.
Description
The GET /api/v1/channels/{id}/members endpoint only checks membership for group and dm channel types (lines 467-469). For standard channels — including private ones — there is no channel_has_access check before returning the member list. Any authenticated user who knows a private channel's UUID can enumerate all users with access to that channel.
# Line 467-469: only group/dm channels are checked
if channel.type in ['group', 'dm']:
if not Channels.is_user_channel_member(channel.id, user.id, db=db):
raise HTTPException(...)
# Standard channels fall through with NO access check
Compare with other channel endpoints (e.g., get_channel_messages at line 688) which correctly call channel_has_access(user.id, channel, permission='read') for standard channels.
CVSS 3.1 Breakdown
Metric
Value
Rationale
Attack Vector
Network (N)
Exploited remotely via API call
Attack Complexity
Low (L)
Single API call, no special conditions
Privileges Required
Low (L)
Requires a valid user account
User Interaction
None (N)
No victim interaction required
Scope
Unchanged (U)
Impact is within the channel authorization boundary
Confidentiality
Low (L)
Leaks user identities and details for a private channel
Integrity
None (N)
No data modification
Availability
None (N)
No denial of service
Attack Scenario
Attacker obtains a private standard channel's UUID (via logs, browser history, URL observation, or other API responses).
Attacker calls GET /api/v1/channels/{id}/members.
The server returns the full list of permitted users including their IDs, names, emails, roles, and profile images.
The attacker has no access to the channel's messages (those endpoints check access correctly), but now knows exactly who does.
Impact
Leaks the identity and personal details of every user with access to a private channel
Reveals organizational structure and project assignments
Enables targeted social engineering against channel members
Preconditions
Channels feature must be enabled (disabled by default)
Attacker must know the channel UUID (not guessable, but obtainable through indirect means)