Injection Flaws in Restify with Dynamodb
Injection Flaws in Restify with Dynamodb — how this specific combination creates or exposes the vulnerability
Injection flaws occur when untrusted data is concatenated into commands or queries without proper validation or parameterization. In a Restify service that uses DynamoDB, the typical pattern is to build a DocumentClient request from HTTP query parameters, path parameters, or JSON payloads. If these values are interpolated into expression attribute values or key condition strings, the behavior can resemble injection even though DynamoDB itself does not support SQL. The misconception that NoSQL databases are immune to injection leads to insecure coding patterns that expose the raw request to DynamoDB’s query API.
Consider a search endpoint in Restify that builds a Select expression from a query parameter:
app.get('/items/:category', (req, res, next) => {
const category = req.params.category;
const expression = `category = :cat`;
const expressionAttributeValues = { ':cat': category };
const params = {
TableName: 'Items',
Select: 'ALL_ATTRIBUTES',
FilterExpression: expression,
ExpressionAttributeValues: expressionAttributeValues
};
docClient.scan(params, (err, data) => {
if (err) return next(new HttpError(500, err.message));
res.send(data.Items);
return next();
});
});If the caller controls category and supplies a value like electronics' OR '1'='1, the resulting expression becomes category = :cat OR '1'='1'. Because :cat is bound safely, the literal string comparison fails to exploit this in many cases, but more complex key condition patterns can expose broader issues. For example, using Query with a partition key built from concatenation can bypass expected partition boundaries and access unintended items when the service trusts client input for the key schema.
In DynamoDB, injection-like behavior often maps to the broader OWASP API Top 10 categories such as API1:2023-Broken Object Level Authorization and API9:2023-Security Misconfiguration, because flawed expression construction can allow horizontal privilege escalation. An attacker may provide a crafted payload that changes the logical structure of the request, such as injecting a condition that always evaluates to true or manipulating the partition key to access other users’ data. Because Restify does not inherently sanitize incoming parameters, the developer must ensure that all input is validated and that DynamoDB expression names and values are used strictly as intended.
Another realistic scenario involves a POST endpoint that accepts a filter object and directly forwards keys to docClient.query:
app.post('/search', (req, res, next) => {
const { filterKey, filterValue } = req.body;
const params = {
TableName: 'UserData',
KeyConditionExpression: `${filterKey} = :val`,
ExpressionAttributeValues: { ':val': filterValue }
};
docClient.query(params, (err, data) => {
if (err) return next(new HttpError(500, err.message));
res.send(data.Items);
return next();
});
});If filterKeyuserId = :val AND attribute_exists(gsiKey), effectively turning the query into a more complex expression that may bypass intended access controls. This pattern demonstrates how injection-like behavior arises not from a SQL parser but from unsafe string assembly in the application layer.
To summarize, injection flaws in Restify with DynamoDB emerge when developers build expressions by concatenating untrusted input, misuse expression attribute names, or trust client data for key construction. These patterns can lead to unauthorized data access, privilege escalation, and unexpected query behavior even though the database does not execute traditional SQL.
Dynamodb-Specific Remediation in Restify — concrete code fixes
Remediation focuses on strict input validation, using DynamoDB expression attribute names for schema identifiers, and keeping expression attribute values separate from the structure of the request. Never concatenate user input into key condition strings or filter expressions. Instead, map incoming values to predefined safe parameters and validate them against an allowlist where possible.
For the earlier search example, rewrite the handler to validate category against an allowlist and use expression attribute names if the attribute name itself is dynamic:
const ALLOWED_CATEGORIES = ['electronics', 'books', 'clothing'];
app.get('/items/:category', (req, res, next) => {
const category = req.params.category;
if (!ALLOWED_CATEGORIES.includes(category)) {
return next(new HttpError(400, 'Invalid category'));
}
const params = {
TableName: 'Items',
Select: 'ALL_ATTRIBUTES',
FilterExpression: '#cat = :val',
ExpressionAttributeNames: { '#cat': 'category' },
ExpressionAttributeValues: { ':val': category }
};
const docClient = new AWS.DynamoDB.DocumentClient();
docClient.scan(params, (err, data) => {
if (err) return next(new HttpError(500, err.message));
res.send(data.Items);
return next();
});
});For the POST search endpoint, validate filterKey against a known set of safe keys and use expression attribute names to avoid any risk of injecting expression syntax:
const SAFE_FILTER_KEYS = ['userId', 'orgId', 'status'];
app.post('/search', (req, res, next) =>
const { filterKey, filterValue } = req.body;
if (!SAFE_FILTER_KEYS.includes(filterKey)) {
return next(new HttpError(400, 'Invalid filter key'));
}
const params = {
TableName: 'UserData',
KeyConditionExpression: '#key = :val',
ExpressionAttributeNames: { '#key': filterKey },
ExpressionAttributeValues: { ':val': filterValue }
};
const docClient = new AWS.DynamoDB.DocumentClient();
docClient.query(params, (err, data) => {
if (err) return next(new HttpError(500, err.message));
res.send(data.Items);
return next();
});
);Additional remediation practices include using middleware in Restify to normalize and sanitize inputs, enabling AWS SDK logging for debugging expression construction, and integrating middleBrick to scan your endpoints. The middleBrick CLI can be used locally with middlebrick scan <url> to detect injection-like patterns in unauthenticated scans, while the GitHub Action can enforce a maximum risk score in CI/CD pipelines. For comprehensive visibility across your API surface, the middleBrick Dashboard can track scores over time and highlight findings tied to OWASP API Top 10 and compliance frameworks.
By adopting parameterized patterns and strict allowlists, you reduce the attack surface significantly. Remember that DynamoDB’s safety depends on how expressions are assembled in Restify; keeping structure and values separate is the cornerstone of secure integration.