CVE-2026-81318 in ash_sql
Summary
by MITRE • 08/30/2026
Incorrect Authorization vulnerability in ash-project ash_sql allows a caller in a schema-based multitenant application to receive aggregate values computed from another tenant's rows.
When an aggregate is computed over a distinct query, AshSql.AggregateQuery.add_single_aggs/5 rebuilds the outer query from query.from.source alone, which is only the {table, schema} tuple and does not carry query.prefix or query.from.prefix. For strategy(:context) multitenancy those hold the tenant schema, so the rebuilt outer query reads the repo's default schema while the inner correlated subquery still reads the tenant schema, and the two are joined only on primary key. The aggregate, and any relationship join added off the prefix-less binding, is then computed against the wrong tenant's rows. The neighbouring limit and exists branches instead wrap the query with subquery/1, which preserves the prefix.
This issue affects ash_sql: from 0.1.0 before 0.7.1.
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Analysis
by VulDB Data Team • 08/30/2026
The vulnerability identified in AshSql represents a critical failure in authorization logic within schema-based multitenant applications, specifically affecting versions of the library ranging from 0.1.0 up to but not including 0.7.1. This flaw allows an authenticated caller operating under one tenant context to retrieve aggregate values that are computed using data rows belonging to a different tenant. In secure multitenant architectures, strict isolation between tenants is paramount to prevent cross-tenant data leakage and ensure compliance with privacy regulations such as GDPR or HIPAA. The core of this vulnerability lies in the incorrect handling of database schema prefixes during query reconstruction for specific aggregate operations.
The technical root cause resides within the AshSql.AggregateQuery.add_single_aggs/5 function, which is responsible for rebuilding outer queries when computing aggregates over distinct results. During this process, the function reconstructs the outer query using only the source information from the original query's FROM clause, specifically extracting a tuple containing the table name and schema identifier. Crucially, it fails to preserve or propagate the query.prefix and query.from.prefix attributes. In environments utilizing strategy(:context) multitenancy, these prefix fields are essential as they define the specific tenant schema where data resides. By omitting these prefixes during reconstruction, the rebuilt outer query defaults to reading from the repository's default public schema rather than the intended tenant-specific schema.
This discrepancy creates a dangerous misalignment between the inner and outer components of the generated SQL statement. While the inner correlated subquery correctly retains the tenant schema prefix and thus filters data based on the current user's tenant context, the outer query operates in the wrong schema namespace. These two parts are joined solely on primary keys without any additional filtering that would enforce cross-schema isolation or validate that the rows accessed by the outer query belong to the same tenant as those processed by the inner subquery. Consequently, when aggregate functions such as SUM, COUNT, or AVG are executed over this misaligned result set, they compute values against a mixture of data from multiple tenants or entirely incorrect tenant data, leading to significant data integrity issues and unauthorized information disclosure.
The operational impact of this vulnerability is severe for any application relying on AshSql for multitenant data isolation. Attackers who can trigger these specific aggregate queries can extract sensitive aggregated metrics belonging to other organizations or users within the same system. This could reveal competitive intelligence, financial totals, user counts, or other business-critical aggregates that should remain strictly confined to their respective tenant boundaries. The existence of neighboring code branches for limit and exists operations highlights the inconsistency in the library's security posture; these alternative paths correctly wrap queries with subquery/1 calls which preserve necessary prefixes and maintain isolation, whereas the aggregate path fails to do so, indicating a lack of comprehensive regression testing or consistent application of multitenancy principles across all query types.
To mitigate this vulnerability, organizations must immediately upgrade AshSql to version 0.7.1 or later, where the issue has been addressed by ensuring that schema prefixes are correctly propagated during aggregate query reconstruction. For applications unable to update immediately due to dependency constraints, a temporary workaround involves avoiding the use of distinct aggregates over complex queries in multitenant contexts until an upgrade is feasible. Additionally, developers should implement rigorous input validation and ensure that all database access layers explicitly enforce tenant context through middleware or application-level checks rather than relying solely on ORM-generated SQL isolation.
From a classification perspective, this vulnerability aligns with CWE-269 Improper Privilege Management, as it involves an actor obtaining privileges to view data outside their authorized scope due to flawed authorization logic. It also maps directly to ATT&CK technique T1530 Data from Cloud Storage Object Misconfiguration or more broadly to T1537 Transient Infrastructure if the multitenancy is cloud-based, but primarily fits within the context of unauthorized access through broken object level permissions (BOLA) as defined in OWASP API Security Top 10 under API4:2019 Broken Function Level Authorization. The failure lies not in authentication but in function-level authorization where the system fails to verify that the data being accessed belongs to the authenticated tenant, resulting in a breach of confidentiality and integrity for multitenant SaaS applications.