diff --git a/lib/permittable.rb b/lib/permittable.rb
index 745f905..f116add 100644
--- a/lib/permittable.rb
+++ b/lib/permittable.rb
@@ -744,6 +744,17 @@ def cast_string(value)
       end
     end
 
+    # Kernel#Integer/Float and BigDecimal() all accept underscore digit
+    # separators and surrounding whitespace — a convenience for a NUMBER
+    # LITERAL IN RUBY SOURCE, not for a request body. "1_8" is not how a
+    # client spells eighteen, and " 99 " is not how one spells ninety-nine;
+    # silently accepting either is the same kind of leniency as the
+    # NaN/Infinity/`0e10` cases below, just arriving from a different door.
+    # Checked against the raw String before any of those delegate, so a
+    # non-canonical spelling never reaches them at all.
+    INTEGER_FORMAT = /\A[+-]?\d+\z/
+    NUMERIC_FORMAT = /\A[+-]?(?:\d+(?:\.\d+)?|\.\d+)(?:[eE][+-]?\d+)?\z/
+
     def cast_integer(value)
       case value
       when Integer then [:ok, value]
@@ -751,7 +762,7 @@ def cast_integer(value)
       # RangeError, which the ArgumentError rescue below does not catch), and
       # no integer is what either one sent. Same rule as finite_float.
       when Float then value.finite? && value == value.truncate ? [:ok, value.to_i] : [:error, "invalid_type"]
-      when String then [:ok, Integer(value, 10)]
+      when String then value.match?(INTEGER_FORMAT) ? [:ok, Integer(value, 10)] : [:error, "invalid_type"]
       else [:error, "invalid_type"]
       end
     rescue ArgumentError
@@ -761,7 +772,7 @@ def cast_integer(value)
     def cast_float(value)
       case value
       when Numeric then finite_float(value.to_f)
-      when String then finite_float(Float(value), source: value)
+      when String then value.match?(NUMERIC_FORMAT) ? finite_float(Float(value), source: value) : [:error, "invalid_type"]
       else [:error, "invalid_type"]
       end
     rescue ArgumentError
@@ -792,7 +803,8 @@ def nonzero_significand?(source)
 
     def cast_decimal(value)
       case value
-      when Numeric, String then finite_decimal(BigDecimal(value.to_s))
+      when Numeric then finite_decimal(BigDecimal(value.to_s))
+      when String then value.match?(NUMERIC_FORMAT) ? finite_decimal(BigDecimal(value)) : [:error, "invalid_type"]
       else [:error, "invalid_type"]
       end
     rescue ArgumentError
diff --git a/spec/permittable_spec.rb b/spec/permittable_spec.rb
index 5cc9e93..08b2da4 100644
--- a/spec/permittable_spec.rb
+++ b/spec/permittable_spec.rb
@@ -511,6 +511,15 @@ def include?(value) = any? { |candidate| candidate.to_s.include?(value.to_s) }
       expect(violations_for({ n: "4.5" }) { permit_params(:create) { required :n, :integer } }.details.first[:code]).to eq("invalid_type")
     end
 
+    it "rejects underscore digit separators and surrounding whitespace for :integer" do
+      # Integer() treats both as harmless formatting for a SOURCE LITERAL —
+      # neither is how a client spells a number in a request body.
+      ["1_8", " 99 "].each do |value|
+        e = violations_for({ n: value }) { permit_params(:create) { required :n, :integer } }
+        expect(e.details).to eq([{ param: "n", code: "invalid_type" }]), "for #{value.inspect}"
+      end
+    end
+
     it "accepts a whole Float for :integer but rejects a fractional one (JSON numbers)" do
       klass = permittable_class { permit_params(:create) { required :n, :integer } }
       expect(controller(klass, params: { n: 42.0 }).permitted_params[:n]).to eq(42)
@@ -696,6 +705,15 @@ def rejected(key, value, &decl)
       end
     end
 
+    it "rejects underscore digit separators and surrounding whitespace for :float and :decimal" do
+      # Float()/BigDecimal() treat both as harmless formatting for a SOURCE
+      # LITERAL, same leniency as the :integer version of this rule above.
+      ["1_8.5", " 1.5 "].each do |value|
+        expect(rejected(:f, value, &decl)).to eq([{ param: "f", code: "invalid_type" }]), "for :float #{value.inspect}"
+        expect(rejected(:d, value, &decl)).to eq([{ param: "d", code: "invalid_type" }]), "for :decimal #{value.inspect}"
+      end
+    end
+
     it "keeps accepting the large exponents BigDecimal genuinely represents" do
       expect(permit({ d: "1e400" }, &decl)[:d]).to eq(BigDecimal("1e400"))
       expect(permit({ d: "0.0000000000000000001" }, &decl)[:d]).to eq(BigDecimal("1e-19"))