CVE-2026-115635 in ash_postgres
Summary
by MITRE • 10/25/2026
Incorrect Authorization vulnerability in ash-project ash_postgres allows a user to read another tenant's record through a skipped upsert.
When Ash.bulk_create/4 or a single create runs as an upsert with return_skipped_upsert?: true and the upsert is skipped (for example because its upsert_condition does not hold), AshPostgres.DataLayer reads the existing row back by the upsert identity in do_bulk_create/3 and upsert/4. That lookup did not apply the resource's attribute multitenancy, so on a resource using strategy :attribute it could match and return the row of another tenant with the same identity values, exposing its fields to the caller.
This issue affects ash_postgres: from 2.6.22 before 2.15.2.
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Analysis
by VulDB Data Team • 10/25/2026
The vulnerability identified in AshPostgres represents a critical failure in multi-tenant data isolation mechanisms within the Elixir-based web framework ecosystem. This flaw specifically impacts versions of the library ranging from 2.6.22 up to, but not including, version 2.15.2. The core issue stems from an incorrect authorization logic during bulk creation operations that utilize upsert functionality with a specific return configuration. In multi-tenant architectures where data is segregated by tenant identifiers such as organization IDs or user groups, maintaining strict boundaries between these tenants is paramount for both security and regulatory compliance. When the application attempts to insert new records while handling potential duplicates via an upsert mechanism, it relies on identity columns to determine if a record already exists. If the condition for updating an existing row fails, typically because specific attribute values do not match the update criteria, the system falls back to returning the existing record rather than creating a new one or raising an error.
The technical root cause lies in how AshPostgres retrieves this existing record when the upsert is skipped due to the return_skipped_upsert option being set to true. During this retrieval phase within the do_bulk_create and upsert functions, the database layer queries for the row based on its unique identity values without applying the resource's configured multitenancy strategy. In resources that utilize an attribute-based multi-tenancy strategy, data is filtered by a specific tenant identifier column during normal read operations to ensure users only see their own data. However, this internal lookup bypassed that filtering logic. Consequently, if two different tenants possess records with identical identity values in the upsert key columns, the query will match and return the row belonging to any of those tenants, not necessarily the one associated with the current authenticated user or context. This effectively breaks the logical isolation between tenants, allowing a malicious actor who controls one tenant's account to access sensitive data from another tenant by triggering this specific code path.
The operational impact of this vulnerability is severe, primarily categorized as an unauthorized information disclosure that leads to cross-tenant data leakage. An attacker can exploit this flaw by crafting requests that trigger bulk create operations with upsert conditions designed to fail for existing records but match the identity keys of target tenant data. By observing the returned data or side effects, the attacker gains access to private fields such as customer details, financial information, or proprietary business logic belonging to other organizations sharing the same database instance. This violates fundamental principles of SaaS security and can lead to significant regulatory penalties under frameworks like GDPR or HIPAA due to the exposure of personally identifiable information or protected health data across organizational boundaries. The severity is heightened by the fact that this occurs during standard write operations, making it difficult to detect through typical monitoring tools focused on read queries.
From a classification perspective, this vulnerability aligns with CWE-269, which denotes Improper Privilege Management, specifically regarding authorization failures in multi-tenant environments. It also maps closely to CWE-862, Missing Authorization, as the system fails to enforce access controls during data retrieval operations triggered by write actions. In terms of offensive security tactics, this behavior can be leveraged within MITRE ATT&CK technique T1005, Data from Local System Retrieval, or more specifically in cloud contexts under T1530, Data from Cloud Storage Object Discovery, as it involves discovering and exfiltrating data stored in a shared database structure. The attack vector is typically remote via the application interface, requiring authentication to one tenant account but allowing access to another, which classifies it under privilege escalation or lateral movement within the SaaS platform context.
Mitigation strategies must focus on immediate patching and architectural review. The primary remediation is to upgrade the ash_postgres dependency to version 2.15.2 or later, where this logic has been corrected to ensure that multitenancy filters are applied during all data retrieval operations, including those triggered by skipped upserts. For organizations unable to immediately patch due to compatibility constraints with other dependencies, a temporary workaround involves disabling the return_skipped_upsert option in bulk create and single create calls if it is not strictly required for business logic. Additionally, developers should audit their Ash resources to ensure that all read operations explicitly enforce tenant scoping through middleware or query filters. Implementing comprehensive logging around upsert operations can also aid in detecting potential exploitation attempts by monitoring for unusual patterns of data retrieval following failed insertions. Regular security assessments and penetration testing focused on multi-tenant isolation are recommended to verify the effectiveness of these controls and identify any similar flaws in custom business logic that may bypass standard framework safeguards.