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  <div class="kicker">Authz bypass</div>
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  <h1>Missing Channel-Level Authorization in Shared Channel Invite/Uninvite API Allows Private Channel Data Exfiltration</h1>
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<section><div class="wrap"><div class="content">
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<p><strong>Severity:</strong> High
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<strong>CVSS Score:</strong> 7.7
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<strong>CWE:</strong> CWE-862
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<strong>Type:</strong> Broken Access Control (Authorization Bypass)
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<strong>URL:</strong> <code>POST /api/v4/remotecluster/{remote_id}/channels/{channel_id}/invite</code></p>
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<h2>Summary</h2>
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<p>The <code>inviteRemoteClusterToChannel</code> and <code>uninviteRemoteClusterToChannel</code> API endpoints in <code>server/channels/api4/shared_channel.go</code> only enforce system-level <code>manage_shared_channels</code> permission via <code>RequirePermissionToManageSharedChannels()</code> (line 153/204). They do <strong>not</strong> verify that the calling user has channel-level access (<code>SessionHasPermissionToChannel</code>) to the target channel.</p>
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<p>This allows any user who holds the <code>manage_shared_channels</code> permission (granted via the <code>SharedChannelManager</code> role, which is a non-sysadmin role) to invite a remote cluster to <strong>any private channel on the instance</strong>, including channels they are not a member of and cannot read. Once invited, the remote cluster receives full message synchronization for that channel, effectively exfiltrating all private channel content to an attacker-controlled server.</p>
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<p>This is the same bug class as CVE-2025-11777 (authorization scope mismatch), where a system/team-level permission check was used instead of a channel-level check.</p>
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<p><strong>Inconsistency proof:</strong> In the same file, <code>getSharedChannelRemotes</code> (line 265) correctly calls <code>SessionHasPermissionToChannel(c.AppContext, *c.AppContext.Session(), c.Params.ChannelId, model.PermissionReadChannel)</code> before returning data. The invite/uninvite endpoints lack this check entirely.</p>
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<p><strong>The gap exists across the entire call chain:</strong>
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- API layer: <code>api4/shared_channel.go:153</code>, system-level only
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- App layer: <code>app/shared_channel.go:113</code>, zero authorization, passthrough
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- Plugin API: <code>plugin_api.go:1504</code>, zero authorization, any plugin can invoke
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- Service layer: <code>service_api.go:113</code>, zero authorization, will even auto-share a previously unshared channel via <code>shareIfNotShared=true</code></p>
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<h2>Reproduction Steps</h2>
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<ol>
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<li>
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<ol>
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<li>Deploy Mattermost with shared channels enabled (ExperimentalSettings.EnableSharedChannels=true, EnableRemoteClusterService=true) and at least one registered remote cluster.</li>
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</ol>
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</li>
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<li>
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<ol start="2">
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<li>As system admin, create a team and a PRIVATE channel. Post confidential data in the private channel.</li>
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</ol>
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</li>
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<li>
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<ol start="3">
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<li>Create a second user (attacker). Add attacker to the team but NOT to the private channel.</li>
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</ol>
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</li>
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<li>
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<ol start="4">
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<li>Verify attacker CANNOT read the private channel: GET /api/v4/channels/{private_channel_id}/posts returns HTTP 403.</li>
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</ol>
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</li>
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<li>
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<ol start="5">
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<li>Grant the attacker the manage_shared_channels permission, either by assigning the SharedChannelManager role or adding the permission to their existing role.</li>
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</ol>
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</li>
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<li>
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<ol start="6">
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<li>Re-authenticate as the attacker (new session to pick up permissions).</li>
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</ol>
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</li>
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<li>
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<ol start="7">
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<li>Verify attacker STILL cannot read the private channel: GET /api/v4/channels/{private_channel_id}/posts returns HTTP 403.</li>
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</ol>
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</li>
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<li>
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<ol start="8">
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<li>As attacker, invite a remote cluster to the private channel: POST /api/v4/remotecluster/{remote_id}/channels/{private_channel_id}/invite</li>
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</ol>
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</li>
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<li>
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<ol start="9">
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<li>Observe the request does NOT return 403. It either succeeds (200) or fails at a downstream step (400 invalid remote, 501 service not running), proving the channel-level permission check is missing.</li>
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</ol>
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</li>
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<li>
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<li>If a valid remote cluster ID is used: the private channel becomes shared and its messages sync to the remote cluster, exfiltrating confidential data.</li>
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</li>
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</ol>
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<pre><code class="language-bash">curl -X POST &quot;https://TARGET/api/v4/remotecluster/REMOTE_CLUSTER_ID/channels/PRIVATE_CHANNEL_ID/invite&quot; \
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  -H &quot;Authorization: Bearer SHARED_CHANNEL_MANAGER_TOKEN&quot; \
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  -H &quot;Content-Type: application/json&quot;
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</code></pre>
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<h2>Evidence</h2>
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<pre><code>**Docker verification on mattermost-preview:latest (v11.5.1):**
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Differential test - same endpoint, same private channel (created by user2, not accessible to either test user):
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</code></pre>
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<p>Admin (has manage_shared_channels via system_admin):
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  POST /api/v4/remotecluster/aaaabbbb&hellip;/channels/{private_channel_id}/invite
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  =&gt; HTTP 501: &ldquo;The remote cluster service is not enabled.&rdquo;
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  (PASSED auth check at line 153, reached GetRemoteClusterService at line 159)</p>
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<p>Regular user (no manage_shared_channels):
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  POST /api/v4/remotecluster/aaaabbbb&hellip;/channels/{private_channel_id}/invite
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  =&gt; HTTP 403: &ldquo;You do not have the appropriate permissions.&rdquo;
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  (BLOCKED at RequirePermissionToManageSharedChannels at line 153)</p>
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<pre><code>
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The 501 vs 403 differential proves the **only** authorization gate is the system-level `ManageSharedChannels` permission. No channel-level check (`SessionHasPermissionToChannel`) exists anywhere in the invite flow. On a production instance with the remote cluster service fully configured, the request would proceed to share the private channel.
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**Source code proof (GitHub HEAD):**
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```go
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// api4/shared_channel.go:142 - VULNERABLE
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func inviteRemoteClusterToChannel(c *Context, w http.ResponseWriter, r *http.Request) {
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    c.RequireRemoteId()
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    c.RequireChannelId()
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    c.RequirePermissionToManageSharedChannels()  // Line 153: system-level ONLY
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    // MISSING: c.App.SessionHasPermissionToChannel(... c.Params.ChannelId, model.PermissionReadChannel)
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    ...
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    c.App.InviteRemoteToChannel(c.Params.ChannelId, c.Params.RemoteId, ...)  // Line 180
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}
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// web/context.go:837 - confirms system scope
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func (c *Context) RequirePermissionToManageSharedChannels() *Context {
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    if !c.App.SessionHasPermissionTo(*c.AppContext.Session(), model.PermissionManageSharedChannels) {
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        c.SetPermissionError(model.PermissionManageSharedChannels)
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    }
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    return c
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}
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// api4/shared_channel.go:265 - SECURE (same file, inconsistent)
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func getSharedChannelRemotes(c *Context, w http.ResponseWriter, r *http.Request) {
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    if ok, _ := c.App.SessionHasPermissionToChannel(c.AppContext, *c.AppContext.Session(),
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        c.Params.ChannelId, model.PermissionReadChannel); !ok {
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        c.SetPermissionError(model.PermissionReadChannel)  // HAS channel check
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        return
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    }
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}
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</code></pre>
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<pre><code>
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## Impact
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**Confidentiality: HIGH** - An attacker with `SharedChannelManager` role (non-sysadmin) can exfiltrate all messages from any private channel on the instance by inviting their controlled remote cluster to it. This includes:
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- Private channel message history (full sync)
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- File attachments shared in the channel
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- User metadata of channel members
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- Ongoing real-time message sync
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**Integrity: MEDIUM** - The attacker could also uninvite legitimate remotes from shared channels (via the uninvite endpoint with the same missing check), disrupting authorized cross-cluster collaboration.
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**Additional attack surface:** The Plugin API (`PluginAPI.InviteRemoteToChannel` at plugin_api.go:1504) performs zero authorization checks. Any installed plugin can share any channel with any remote without any permission validation, extending this vulnerability to plugin-based attacks.
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**Scope escalation:** The `shareIfNotShared=true` parameter (passed by the API handler) causes the service layer to auto-share a previously unshared private channel before inviting the remote - silently converting a channel that was never intended to be shared.
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## Root Cause
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Authorization scope mismatch. The `RequirePermissionToManageSharedChannels()` function (web/context.go:842) calls `SessionHasPermissionTo()` which is a **system-level** permission check. It verifies the user holds the `manage_shared_channels` permission globally, but does not verify the user has any access to the specific channel being shared.
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The correct fix requires adding a channel-level check - `SessionHasPermissionToChannel(channelId, PermissionReadChannel)` - before proceeding with the invite, consistent with how `getSharedChannelRemotes` (line 265 in the same file) already checks channel access.
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The `ManageSharedChannels` permission is defined with `PermissionScopeSystem` (model/permission.go:836), which is architecturally correct for controlling who can manage shared channels in general. But the invite/uninvite endpoints need an **additional** channel-scoped check to enforce that the user can only share channels they have access to.
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## Suggested Fix
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Add `SessionHasPermissionToChannel` check in both `inviteRemoteClusterToChannel` and `uninviteRemoteClusterToChannel` handlers, after the existing `RequirePermissionToManageSharedChannels` check:
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```go
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// api4/shared_channel.go - inviteRemoteClusterToChannel (after line 156)
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if ok, _ := c.App.SessionHasPermissionToChannel(c.AppContext, *c.AppContext.Session(),
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    c.Params.ChannelId, model.PermissionReadChannel); !ok {
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    c.SetPermissionError(model.PermissionReadChannel)
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    return
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}
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</code></pre>
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<p>Apply the same fix to:
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1. <code>uninviteRemoteClusterToChannel</code> (after line 207)
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2. <code>PluginAPI.InviteRemoteToChannel</code> (plugin_api.go:1504), add channel membership validation
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3. <code>PluginAPI.UninviteRemoteFromChannel</code> (plugin_api.go:1508), same
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4. The <code>/share invite</code> slash command handler (command_share.go), already implicitly scoped to current channel but should be explicitly validated</p>
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<hr><p style="color:var(--faint);font-size:12.5px;font-family:ui-monospace,Menlo,monospace">Source &middot; github.com/zionboggan/security-research-notebook &middot; writeups/grafana/mattermost-shared-channel-authz-bypass.md</p>
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</div></div></section>
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<footer><div class="wrap row">
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  <div class="links"><a href="/">Portfolio</a><a href="https://www.linkedin.com/in/zion-boggan">LinkedIn</a><a href="/security-research-notebook/">Notebook</a><a href="mailto:zionboggan0@gmail.com">Email</a></div>
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  <div class="note">Coordinated-disclosure research. Findings appear here only after the program's disclosure window closed, the patch shipped, or a CVE was published. No customer data was accessed.</div>
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</div></footer>
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