How an Untamed Filter Parameter Unlocked SQL Injection
Background — When Query Builders Trust User Input
During a penetration test of a multi-tenant ERP platform, our team uncovered a critical SQL-injection vulnerability in the OData $filter parameter used by several REST endpoints. The application dynamically merged user-supplied filters into SQL WHERE clauses without parameterisation, leaving the data layer wide-open while the API presented a façade of strict access control. This is the classic “string-building trap”—security checks live on the client, but the final query assembly happens unchecked on the server.
Discovery — Two Quotes, Full Control
During a penetration test of a multi-tenant ERP platform, our team uncovered a critical SQL-injection vulnerability in the OData $filter parameter used by several REST endpoints. The application dynamically merged user-supplied filters into SQL WHERE clauses without parameterisation, leaving the data layer wide-open while the API presented a façade of strict access control. This is the classic “string-building trap”—security checks live on the client, but the final query assembly happens unchecked on the server.
GET /api/v1/<service>/GenXDatas
?$filter=TypeCode eq 'HotKeys'
and ProductID eq 'Ep'
and Key1 eq 'manager'''
Authorization: Bearer <JWT>
Result – The server replied 200 OK with a valid JSON payload, demonstrating that the payload reached the database unfiltered. A follow-up payload such as
Key1 eq 'manager''' or 1=1 --
returned full record sets, confirming Boolean-based SQL injection.
Technical Root Cause — Dynamic SQL in $filter Handling
// Simplified pseudo-code
var where = $"TypeCode = '{type}' AND ProductID = '{product}' AND Key1 = '{key1}'";
var sql = $"SELECT * FROM GenXData WHERE {where}";
No prepared statements — variables are spliced directly into SQL.
Missing escaping/validation — a single quote ends the literal and appends attacker code.
Shared helper — the same builder assembles queries for search, reporting and admin views, multiplying impact.
Real-World Risks — What Attackers Could Do
Bulk data exfiltration – Read sensitive configuration and business records across tenants.
Підвищення привілеїв – Manipulate auth tables to grant admin roles.
Data tampering – Alter pricing, insert fraudulent defaults, disrupt production schedules.
Denial of service – Run heavy queries (WAITFOR DELAY, expensive joins) to exhaust resources.
Broader Lessons — SQL Injection Isn’t a “Legacy” Issue
Modern APIs (OData, GraphQL, serverless functions) still fall to SQLi when dev teams assume “the framework sanitises it.” Typical patterns include:
• Dynamic string templates for “flexible” reporting.
• Conditional clause assembly instead of parameter arrays.
• Debug toggles that disable ORM parameterisation and later reach production.
If you concatenate anything into SQL, you need a threat-model review—framework or not.
Recommendations — Fix Fast, Prevent Future Slips
✅ Parameterise every query
• In .NET: SqlParameter, Entity Framework lambdas—never raw string interpolation.
✅ Harden query builders
• Whitelist permitted fields/operators in $filter.
• Strip comments (--, /* */) and block stacked queries (;).
✅ Centralised validation layer
• Reject payloads with unmatched quotes or suspicious keywords.
✅ Least-privilege DB accounts
• Application credentials need only minimal SELECT/EXEC rights—no ALTER, DROP.
✅ Continuous monitoring
• Log anomalous $filter lengths and “always true” patterns (or 1=1).
• Alert on spikes in query time or cardinality.
Conclusion — Query Safety Is a Continuous Discipline
Parameterized queries add a few lines of code but save entire businesses. Here, a single unescaped quote transformed a harmless filter into a master key for the ERP’s data layer. Every time you build a SQL string, an attacker smiles—unless you lock it down first.
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