diff --git a/lib/code_to_query/validator.rb b/lib/code_to_query/validator.rb
index f02f2d2..2c8341e 100644
--- a/lib/code_to_query/validator.rb
+++ b/lib/code_to_query/validator.rb
@@ -44,7 +44,7 @@ class Validator
     def validate(intent_hash, current_user: nil, allow_tables: nil)
       preprocessed = preprocess_exists_filters(intent_hash)
 
-      if !preprocessed.key?('limit') && CodeToQuery.config.default_limit
+      if fetch_value(preprocessed, :limit).nil? && CodeToQuery.config.default_limit
         preprocessed = preprocessed.merge('limit' => CodeToQuery.config.default_limit)
       end
 
@@ -56,17 +56,17 @@ def validate(intent_hash, current_user: nil, allow_tables: nil)
       original_metrics = intent_hash['_metrics'] || intent_hash[:_metrics]
       validated['_metrics'] = original_metrics if original_metrics.is_a?(Hash)
 
-      Array(validated['filters']).each_with_index do |f, idx|
-        op = f['op'].to_s
+      Array(fetch_value(validated, :filters)).each_with_index do |f, idx|
+        op = fetch_value(f, :op).to_s
         if %w[exists not_exists].include?(op)
-          unless f['related_table'].to_s.strip != '' && f['fk_column'].to_s.strip != ''
+          unless fetch_value(f, :related_table).to_s.strip != '' && fetch_value(f, :fk_column).to_s.strip != ''
             raise ArgumentError, "Invalid intent: filters[#{idx}] requires related_table and fk_column for #{op}"
           end
 
-          f['base_column'] ||= 'id'
-          f['column'] ||= 'id'
+          assign_default_value(f, :base_column, 'id')
+          assign_default_value(f, :column, 'id')
         else
-          unless f['column'].to_s.strip != ''
+          unless fetch_value(f, :column).to_s.strip != ''
             raise ArgumentError, "Invalid intent: filters[#{idx}].column must be filled"
           end
         end
@@ -77,12 +77,38 @@ def validate(intent_hash, current_user: nil, allow_tables: nil)
 
     private
 
+    def fetch_value(hash, key)
+      return unless hash.respond_to?(:key?) && hash.respond_to?(:[])
+
+      return hash[key] if hash.key?(key)
+
+      if key.is_a?(Symbol)
+        string_key = key.to_s
+        return hash[string_key] if hash.key?(string_key)
+      end
+
+      if key.is_a?(String)
+        symbol_key = key.to_sym
+        return hash[symbol_key] if hash.key?(symbol_key)
+      end
+
+      nil
+    end
+
+    def assign_default_value(hash, key, value)
+      return unless hash.respond_to?(:key?) && hash.respond_to?(:[]=)
+      return unless fetch_value(hash, key).nil?
+
+      hash[key.to_sym] = value
+    end
+
     def preprocess_exists_filters(intent_hash)
       intent = intent_hash.dup
+      filters_key = intent.key?('filters') ? 'filters' : :filters
 
-      if intent['filters'].is_a?(Array)
-        intent['filters'] = intent['filters'].map do |filter|
-          if filter.is_a?(Hash) && %w[exists not_exists].include?(filter['op'].to_s) && filter['column'].nil?
+      if intent[filters_key].is_a?(Array)
+        intent[filters_key] = intent[filters_key].map do |filter|
+          if filter.is_a?(Hash) && %w[exists not_exists].include?(fetch_value(filter, :op).to_s) && fetch_value(filter, :column).nil?
             filter.merge('column' => 'id')
           else
             filter
@@ -96,7 +122,7 @@ def preprocess_exists_filters(intent_hash)
     def enforce_allowlists!(intent, current_user:, allow_tables:)
       # Enforce table allowlist if provided (from user input)
       if Array(allow_tables).any?
-        table = intent['table']
+        table = fetch_value(intent, :table)
         if (table.to_s.strip != '') && !Array(allow_tables).map { |t| t.to_s.downcase }.include?(table.to_s.downcase)
           raise ArgumentError, "Invalid intent: table '#{table}' not allowed"
         end
@@ -106,7 +132,7 @@ def enforce_allowlists!(intent, current_user:, allow_tables:)
       adapter = CodeToQuery.config.policy_adapter
       return unless adapter.respond_to?(:call)
 
-      policy_info = safe_call_policy_adapter(adapter, current_user, table: intent['table'], intent: intent)
+      policy_info = safe_call_policy_adapter(adapter, current_user, table: fetch_value(intent, :table), intent: intent)
       if policy_info.nil?
         return handle_policy_failure('Policy adapter returned nil') if policy_adapter_fail_open?
 
@@ -119,15 +145,15 @@ def enforce_allowlists!(intent, current_user:, allow_tables:)
         raise CodeToQuery::PolicyAdapterError, message
       end
 
-      allowed_tables = Array(policy_info[:allowed_tables] || policy_info['allowed_tables']).map { |t| t.to_s.downcase }
+      allowed_tables = Array(fetch_value(policy_info, :allowed_tables)).map { |t| t.to_s.downcase }
       if allowed_tables.any?
-        table = intent['table']
+        table = fetch_value(intent, :table)
         if (table.to_s.strip != '') && !allowed_tables.include?(table.to_s.downcase)
           raise ArgumentError, "Invalid intent: table '#{table}' not permitted by policy"
         end
       end
 
-      allowed_columns = policy_info[:allowed_columns] || policy_info['allowed_columns'] || {}
+      allowed_columns = fetch_value(policy_info, :allowed_columns) || {}
       return if allowed_columns.nil? || allowed_columns.empty?
 
       # Normalize map keys to strings with lowercase table and column names
@@ -136,10 +162,10 @@ def enforce_allowlists!(intent, current_user:, allow_tables:)
         normalized[tbl.to_s.downcase] = Array(cols).map { |c| c.to_s.downcase }
       end
 
-      main_table = intent['table'].to_s.downcase
+      main_table = fetch_value(intent, :table).to_s.downcase
 
       # Columns in SELECT
-      Array(intent['columns']).each do |col|
+      Array(fetch_value(intent, :columns)).each do |col|
         next if col == '*'
         next unless normalized[main_table]&.any?
         unless normalized[main_table].include?(col.to_s.downcase)
@@ -148,8 +174,8 @@ def enforce_allowlists!(intent, current_user:, allow_tables:)
       end
 
       # ORDER BY columns
-      Array(intent['order']).each do |o|
-        col = o['column']
+      Array(fetch_value(intent, :order)).each do |o|
+        col = fetch_value(o, :column)
         next if col.nil?
         next unless normalized[main_table]&.any?
         unless normalized[main_table].include?(col.to_s.downcase)
@@ -158,7 +184,7 @@ def enforce_allowlists!(intent, current_user:, allow_tables:)
       end
 
       # DISTINCT ON columns
-      Array(intent['distinct_on']).each do |col|
+      Array(fetch_value(intent, :distinct_on)).each do |col|
         next unless normalized[main_table]&.any?
         unless normalized[main_table].include?(col.to_s.downcase)
           raise ArgumentError, "Invalid intent: distinct_on column '#{col}' not permitted on '#{main_table}'"
@@ -166,7 +192,7 @@ def enforce_allowlists!(intent, current_user:, allow_tables:)
       end
 
       # GROUP BY
-      Array(intent['group_by']).each do |col|
+      Array(fetch_value(intent, :group_by)).each do |col|
         next unless normalized[main_table]&.any?
         unless normalized[main_table].include?(col.to_s.downcase)
           raise ArgumentError, "Invalid intent: group_by column '#{col}' not permitted on '#{main_table}'"
@@ -174,22 +200,22 @@ def enforce_allowlists!(intent, current_user:, allow_tables:)
       end
 
       # WHERE filters
-      Array(intent['filters']).each do |f|
-        op = f['op'].to_s
+      Array(fetch_value(intent, :filters)).each do |f|
+        op = fetch_value(f, :op).to_s
         if %w[exists not_exists].include?(op)
-          related_table = f['related_table']
+          related_table = fetch_value(f, :related_table)
           rel_cols = normalized[related_table.to_s.downcase]
           next if rel_cols.nil? || rel_cols.empty?
 
-          Array(f['related_filters']).each do |rf|
-            col = rf['column']
+          Array(fetch_value(f, :related_filters)).each do |rf|
+            col = fetch_value(rf, :column)
             next if col.nil?
             unless rel_cols.include?(col.to_s.downcase)
               raise ArgumentError, "Invalid intent: filter column '#{col}' not permitted on '#{related_table}'"
             end
           end
         else
-          col = f['column']
+          col = fetch_value(f, :column)
           next if col.nil?
 
           cols = normalized[main_table]
diff --git a/spec/code_to_query/guardrails/adversarial_fixture_spec.rb b/spec/code_to_query/guardrails/adversarial_fixture_spec.rb
new file mode 100644
index 0000000..11ee80a
--- /dev/null
+++ b/spec/code_to_query/guardrails/adversarial_fixture_spec.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+# rubocop:disable RSpec/LeakyLocalVariable
+
+require 'spec_helper'
+require 'yaml'
+
+adversarial_sql_fixture_path = File.expand_path('../../fixtures/adversarial/sql_linter.yml', __dir__)
+adversarial_sql_cases = YAML.safe_load_file(adversarial_sql_fixture_path, permitted_classes: [], aliases: false).freeze
+adversarial_sql_adapters = %w[postgres mysql sqlite].freeze
+adversarial_sql_allowed_adapters = (adversarial_sql_adapters + %w[any]).freeze
+adversarial_sql_expected_categories = %w[
+  comment_obfuscation
+  dangerous_function
+  encoded_payload
+  literal_smuggling
+  prompt_injection
+  stacked_statement
+  system_table
+  union
+].freeze
+
+RSpec.describe 'Adversarial SQL fixture corpus' do
+  adversarial_sql_cases.each do |test_case|
+    adapter = test_case.fetch('adapter')
+    adapters = adapter == 'any' ? adversarial_sql_adapters : [adapter]
+
+    adapters.each do |adapter_name|
+      context "with #{test_case.fetch('category')} fixture #{test_case.fetch('id')} on #{adapter_name}" do
+        it 'rejects the SQL with the expected reason' do
+          linter = CodeToQuery::Guardrails::SqlLinter.new(
+            stub_config(adapter: adapter_name.to_sym, max_limit: 1000, max_joins: 3, policy_adapter: nil),
+            allow_tables: %w[users orders]
+          )
+
+          expect { linter.check!(test_case.fetch('sql')) }
+            .to raise_error(SecurityError, /#{Regexp.escape(test_case.fetch('reason'))}/)
+        end
+      end
+    end
+  end
+
+  it 'keeps every fixture categorized with an expected rejection reason' do
+    categories = adversarial_sql_cases.map { |test_case| test_case.fetch('category') }.uniq
+    adapters = adversarial_sql_cases.map { |test_case| test_case.fetch('adapter') }.uniq
+
+    expect(categories).to match_array(adversarial_sql_expected_categories)
+    expect(adapters - adversarial_sql_allowed_adapters).to be_empty
+    expect(adversarial_sql_cases).to all(include('id', 'category', 'adapter', 'sql', 'reason'))
+  end
+end
+# rubocop:enable RSpec/LeakyLocalVariable
diff --git a/spec/code_to_query/validator_adversarial_fixture_spec.rb b/spec/code_to_query/validator_adversarial_fixture_spec.rb
new file mode 100644
index 0000000..685fe3b
--- /dev/null
+++ b/spec/code_to_query/validator_adversarial_fixture_spec.rb
@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+# rubocop:disable RSpec/LeakyLocalVariable
+
+require 'spec_helper'
+require 'yaml'
+
+adversarial_validator_fixture_path = File.expand_path('../fixtures/adversarial/validator.yml', __dir__)
+adversarial_validator_cases = YAML.safe_load_file(adversarial_validator_fixture_path, permitted_classes: [], aliases: false).freeze
+adversarial_validator_expected_categories = %w[
+  literal_smuggling
+  policy_bypass
+  prompt_injection
+  system_table
+].freeze
+
+RSpec.describe CodeToQuery::Validator do
+  after do
+    CodeToQuery.config.policy_adapter = nil
+    CodeToQuery.config.policy_adapter_fail_open = false
+  end
+
+  adversarial_validator_cases.each do |test_case|
+    context "with #{test_case.fetch('category')} fixture #{test_case.fetch('id')}" do
+      it 'rejects the intent with the expected reason' do
+        policy = test_case['policy']
+        policy_adapter = policy && lambda do |_user, **_context|
+          {
+            allowed_tables: policy.fetch('allowed_tables', []),
+            allowed_columns: policy.fetch('allowed_columns', {})
+          }
+        end
+
+        stub_config(policy_adapter: policy_adapter, policy_adapter_fail_open: false)
+
+        expect do
+          described_class.new.validate(
+            test_case.fetch('intent'),
+            allow_tables: test_case.fetch('allow_tables', nil)
+          )
+        end.to raise_error(ArgumentError, /#{Regexp.escape(test_case.fetch('reason'))}/)
+      end
+    end
+  end
+
+  it 'keeps every fixture categorized with an expected rejection reason' do
+    categories = adversarial_validator_cases.map { |test_case| test_case.fetch('category') }.uniq
+
+    expect(categories).to match_array(adversarial_validator_expected_categories)
+    expect(adversarial_validator_cases).to all(include('id', 'category', 'intent', 'reason'))
+  end
+end
+# rubocop:enable RSpec/LeakyLocalVariable
diff --git a/spec/code_to_query/validator_spec.rb b/spec/code_to_query/validator_spec.rb
index 094fa2c..ab59372 100644
--- a/spec/code_to_query/validator_spec.rb
+++ b/spec/code_to_query/validator_spec.rb
@@ -6,6 +6,11 @@
   let(:config) { stub_config(adapter: :postgres, default_limit: 100) }
   let(:validator) { described_class.new }
 
+  after do
+    CodeToQuery.config.policy_adapter = nil
+    CodeToQuery.config.policy_adapter_fail_open = false
+  end
+
   describe '#validate' do
     context 'with valid basic intent' do
       let(:intent) do
@@ -98,6 +103,28 @@
         expect(result[:filters].first[:op]).to eq('exists')
         expect(result[:filters].first[:related_table]).to eq('orders')
       end
+
+      it 'adds default exists columns using the validated intent key style' do
+        intent = {
+          'type' => 'select',
+          'table' => 'users',
+          'columns' => ['*'],
+          'filters' => [
+            {
+              'op' => 'exists',
+              'related_table' => 'orders',
+              'fk_column' => 'user_id'
+            }
+          ]
+        }
+
+        result = validator.validate(intent)
+        filter = result[:filters].first
+
+        expect(filter[:column]).to eq('id')
+        expect(filter[:base_column]).to eq('id')
+        expect(filter).not_to include('column', 'base_column')
+      end
     end
 
     context 'with not_exists filter' do
diff --git a/spec/fixtures/adversarial/sql_linter.yml b/spec/fixtures/adversarial/sql_linter.yml
new file mode 100644
index 0000000..7cb5205
--- /dev/null
+++ b/spec/fixtures/adversarial/sql_linter.yml
@@ -0,0 +1,86 @@
+# Fixture-driven adversarial SQL corpus for CodeToQuery guardrails.
+# Each case must be rejected by SqlLinter with the expected reason category.
+
+- id: union_select_literal_smuggling
+  category: union
+  adapter: any
+  sql: "SELECT id FROM users WHERE id = $1 UNION SELECT id FROM users LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: stacked_drop_statement
+  category: stacked_statement
+  adapter: any
+  sql: "SELECT * FROM users LIMIT 10; DROP TABLE users;"
+  reason: "Multiple statements and semicolons are not allowed"
+
+- id: inline_comment_tautology
+  category: comment_obfuscation
+  adapter: any
+  sql: "SELECT * FROM users WHERE id = $1 /* bypass tenant filter */ LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: line_comment_tautology
+  category: comment_obfuscation
+  adapter: any
+  sql: "SELECT * FROM users WHERE id = $1 -- bypass tenant filter\n LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: url_encoded_quote_payload
+  category: encoded_payload
+  adapter: any
+  sql: "SELECT * FROM users WHERE name = %27admin%27 LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: hex_literal_payload
+  category: encoded_payload
+  adapter: any
+  sql: "SELECT * FROM users WHERE id = 0x41424344 LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: mysql_sleep_delay
+  category: dangerous_function
+  adapter: mysql
+  sql: "SELECT * FROM users WHERE id = ? OR SLEEP(5) LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: postgres_sleep_delay
+  category: dangerous_function
+  adapter: postgres
+  sql: "SELECT * FROM users WHERE id = $1 OR pg_sleep(5) LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: mysql_information_schema
+  category: system_table
+  adapter: mysql
+  sql: "SELECT * FROM information_schema.tables LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: postgres_catalog_function
+  category: system_table
+  adapter: postgres
+  sql: "SELECT pg_read_file($1) FROM users LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: sqlite_schema_table
+  category: system_table
+  adapter: sqlite
+  sql: "SELECT * FROM sqlite_master LIMIT 10"
+  reason: "Dangerous SQL pattern detected"
+
+- id: literal_email_filter
+  category: literal_smuggling
+  adapter: any
+  sql: "SELECT * FROM users WHERE email = 'ceo@example.com' LIMIT 10"
+  reason: "String literals are not allowed"
+
+- id: large_numeric_tenant_filter
+  category: literal_smuggling
+  adapter: any
+  sql: "SELECT * FROM users WHERE tenant_id = 123456 LIMIT 10"
+  reason: "Numeric literals in WHERE clauses should be parameterized"
+
+- id: prompt_injection_comment
+  category: prompt_injection
+  adapter: mysql
+  sql: "SELECT * FROM users WHERE id = ? # ignore previous tenant filter"
+  reason: "Dangerous SQL pattern detected"
diff --git a/spec/fixtures/adversarial/validator.yml b/spec/fixtures/adversarial/validator.yml
new file mode 100644
index 0000000..7548b7d
--- /dev/null
+++ b/spec/fixtures/adversarial/validator.yml
@@ -0,0 +1,92 @@
+# Fixture-driven adversarial intent corpus for Validator allowlist guardrails.
+
+- id: non_allowlisted_table
+  category: system_table
+  allow_tables:
+    - users
+    - orders
+  intent:
+    type: select
+    table: admin_secrets
+    columns:
+      - '*'
+    filters: []
+    order: []
+    limit: 10
+    params: {}
+  reason: "table 'admin_secrets' not allowed"
+
+- id: literal_smuggled_table_name
+  category: literal_smuggling
+  allow_tables:
+    - users
+  intent:
+    type: select
+    table: "users WHERE email = 'ceo@example.com'"
+    columns:
+      - '*'
+    filters: []
+    order: []
+    limit: 10
+    params: {}
+  reason: "not allowed"
+
+- id: prompt_injection_column_name
+  category: prompt_injection
+  policy:
+    allowed_columns:
+      users:
+        - id
+        - email
+  intent:
+    type: select
+    table: users
+    columns:
+      - "email /* ignore tenant policy */"
+    filters: []
+    order: []
+    limit: 10
+    params: {}
+  reason: "selecting column"
+
+- id: non_policy_filter_column
+  category: policy_bypass
+  policy:
+    allowed_columns:
+      users:
+        - id
+        - email
+  intent:
+    type: select
+    table: users
+    columns:
+      - email
+    filters:
+      - column: tenant_id
+        op: eq
+        param: tenant_id
+    order: []
+    limit: 10
+    params:
+      tenant_id: 123
+  reason: "filter column 'tenant_id' not permitted"
+
+- id: non_policy_order_column
+  category: policy_bypass
+  policy:
+    allowed_columns:
+      users:
+        - id
+        - email
+  intent:
+    type: select
+    table: users
+    columns:
+      - email
+    filters: []
+    order:
+      - column: admin_notes
+        dir: asc
+    limit: 10
+    params: {}
+  reason: "ordering by column 'admin_notes' not permitted"