Injection Flaws in Sinatra with Mongodb

Injection flaws occur when untrusted data is interpreted as part of a command or query. In Sinatra applications that use Mongodb, the risk centers on how user-controlled input is incorporated into database operations. Sinatra does not enforce any schema or query structure, which means developers construct queries directly in Ruby code and may concatenate or interpolate parameters into BSON conditions.

For example, building a query with string interpolation or merging raw params into a find call allows an attacker to manipulate the query logic. Consider a route that retrieves a user profile by username:

get '/profile' do
  collection = Mongo::Client.new(['127.0.0.1:27017'], database: 'app_db')[:users]
  username = params['username']
  collection.find({ 'username' => username }).first
end

If the developer mistakenly uses eval or builds the filter as a string and parses it, the query can become fully controllable by the attacker. Another common pattern is using JSON.parse on user input and merging it into a query document, which can unintentionally change the query shape or introduce operators like $where or $ne that lead to unintended data access.

Mongodb’s query language supports operators that can be abused if input is not strictly validated. Injection is not limited to SQL; in Mongodb, injection can mean the difference between retrieving a single document and extracting an entire collection or performing a destructive operation. Attack patterns include bypassing authentication by injecting {'$ne': null} into a login filter, or causing information disclosure via error messages when invalid operators are used.

In a Sinatra context, the lack of an ORM or automatic sanitization increases the surface area. If developers rely on client-side validation or assume that the Mongodb driver will safely escape inputs, they may overlook the need for strict allow-lists and type checks. The attack surface is further expanded when APIs accept JSON payloads that are directly merged into update operations without validation.

Compliance mappings such as OWASP API Top 10 (API1:2023 Broken Object Level Authorization often coexists with injection risks) and standards like PCI-DSS and SOC2 require controls that prevent injection. middleBrick’s LLM/AI Security checks and API scanning can detect patterns where user input reaches the database without adequate sanitization, providing findings with severity and remediation guidance.

Remediation focuses on ensuring that user input never alters the structure of the query or update document. Use explicit field extraction, type validation, and allow-lists instead of merging raw input.

Safe query construction

Always construct query documents programmatically and avoid string interpolation. Validate and cast each parameter to the expected type.

helpers do
  def safe_username_param
    return nil unless params['username']
    # Allow only alphanumeric usernames, 3–32 chars
    username = params['username'].to_s.strip
    username if username.match?(\A[a-zA-Z0-9_]{3,32}\z)
  end
end

get '/profile' do
  username = safe_username_param
  halt 400, { error: 'invalid username' }.to_json unless username
  collection = Mongo::Client.new(['127.0.0.1:27017'], database: 'app_db')[:users]
  collection.find({ 'username' => username }).first || { error: 'not found' }
end

Safe updates with whitelisted fields

For updates, explicitly permit which fields can be modified and cast values to their expected types.

post '/profile/:id' do
  id = params['id']
  halt 400, { error: 'invalid id' }.to_json unless id.match?(\A[a-f0-9]{24}\z)
  body = JSON.parse(request.body.read, symbolize_names: true)
  # Whitelist editable fields
  update_fields = {}
  update_fields['email'] = body[:email].to_s.strip.downcase if body[:email]
  update_fields['preferences'] = body[:preferences].to_hash if body[:preferences]
  halt 400, { error: 'no fields to update' }.to_json if update_fields.empty?
  collection = Mongo::Client.new(['127.0.0.1:27017'], database: 'app_db')[:users]
  result = collection.find({ '_id' => id }).update_one({ '$set' => update_fields })
  { updated: result.modified_count > 0 }.to_json
end

Avoiding operator injection

Never parse user input as part of a query operator. If dynamic querying is required, map user choices to known safe keys and operators.

get '/search' do
  field = params['field']
  value = params['value']
  allowed_fields = { 'name' => 'username', 'email' => 'email' }
  op = params['operator'] || 'eq'
  safe_op = { 'eq' => '$eq', 'ne' => '$ne' }[op]
  halt 400, { error: 'invalid field or operator' }.to_json unless allowed_fields.key?(field) && safe_op
  collection = Mongo::Client.new(['127.0.0.1:27017'], database: 'app_db')[:users]
  collection.find(allowed_fields[field] => { safe_op => value }).to_a
end

Using the official Mongodb Ruby driver, ensure that the connection uses strong TLS encryption and that the driver is kept up to date to avoid known CVEs. middleBrick’s free scan can be used to verify that no endpoints expose unsafe query patterns.

Products like the middleBrick CLI (middlebrick scan <url>) and GitHub Action help integrate checks into development workflows. The Pro plan adds continuous monitoring so that new endpoints are automatically evaluated for injection risks. The MCP Server enables scanning APIs directly from AI coding assistants, helping developers catch unsafe patterns before code is committed.