diff --git a/java/src/org/openqa/selenium/json/JsonInput.java b/java/src/org/openqa/selenium/json/JsonInput.java
index d40fb51f2b7b2..10e891e87eb17 100644
--- a/java/src/org/openqa/selenium/json/JsonInput.java
+++ b/java/src/org/openqa/selenium/json/JsonInput.java
@@ -133,7 +133,6 @@ public JsonType peek() {
         return JsonType.NULL;
 
       case '-':
-      case '+':
       case '0':
       case '1':
       case '2':
@@ -223,52 +222,88 @@ public String nextName() {
    */
   public Number nextNumber() {
     expect(JsonType.NUMBER);
-    boolean mightBeDecimal = false;
     StringBuilder builder = new StringBuilder();
-    // We know it's safe to use a do/while loop since the first character was a number
-    boolean read = true;
-    do {
-      switch (input.peek()) {
-        case '-':
-        case '+':
-        case '0':
-        case '1':
-        case '2':
-        case '3':
-        case '4':
-        case '5':
-        case '6':
-        case '7':
-        case '8':
-        case '9':
-          builder.append((char) input.read());
-          break;
-        case '.':
-        case 'e':
-        case 'E':
-          mightBeDecimal = true;
-          builder.append((char) input.read());
-          break;
-        default:
-          read = false;
+    boolean isDecimal = false;
+
+    // Optional leading minus. (Per RFC 8259 §6, a leading '+' is not allowed.)
+    if (input.peek() == '-') {
+      builder.append((char) input.read());
+    }
+
+    // Integer part: either "0" or [1-9] [0-9]*.
+    int first = input.peek();
+    if (first == '0') {
+      builder.append((char) input.read());
+      // Leading zeros ("00", "01", ...) are not allowed.
+      if (isDigit(input.peek())) {
+        throw new JsonException("Leading zeros are not permitted in JSON numbers. " + input);
+      }
+    } else if (first >= '1' && first <= '9') {
+      while (isDigit(input.peek())) {
+        builder.append((char) input.read());
       }
-    } while (read);
+    } else {
+      throw new JsonException("Expected digit but saw " + describeChar(first) + ". " + input);
+    }
+
+    // Optional fractional part: '.' 1*DIGIT
+    if (input.peek() == '.') {
+      isDecimal = true;
+      builder.append((char) input.read());
+      if (!isDigit(input.peek())) {
+        throw new JsonException(
+            "Expected at least one digit after '.' but saw "
+                + describeChar(input.peek())
+                + ". "
+                + input);
+      }
+      while (isDigit(input.peek())) {
+        builder.append((char) input.read());
+      }
+    }
+
+    // Optional exponent part: ('e' | 'E') ('+' | '-')? 1*DIGIT
+    if (input.peek() == 'e' || input.peek() == 'E') {
+      isDecimal = true;
+      builder.append((char) input.read());
+      if (input.peek() == '+' || input.peek() == '-') {
+        builder.append((char) input.read());
+      }
+      if (!isDigit(input.peek())) {
+        throw new JsonException(
+            "Expected at least one digit in exponent but saw "
+                + describeChar(input.peek())
+                + ". "
+                + input);
+      }
+      while (isDigit(input.peek())) {
+        builder.append((char) input.read());
+      }
+    }
 
     try {
-      // The JSON Schema does state the decimal point should not be used distinguish between
-      // integers and floating point values.
-      // Therefore, using a Long is only a fast path here, but we should not rely on the `double`
-      // value below is a real floating point.
-      if (!mightBeDecimal) {
+      // Fast path for integers: Long-valued when no fraction/exponent was present.
+      if (!isDecimal) {
         return Long.valueOf(builder.toString());
       }
-
-      return new BigDecimal(builder.toString()).doubleValue();
+      double value = new BigDecimal(builder.toString()).doubleValue();
+      if (Double.isInfinite(value) || Double.isNaN(value)) {
+        throw new JsonException("Number is out of range for a double: " + builder + ". " + input);
+      }
+      return value;
     } catch (NumberFormatException e) {
       throw new JsonException("Unable to parse to a number: " + builder + ". " + input, e);
     }
   }
 
+  private static boolean isDigit(int c) {
+    return c >= '0' && c <= '9';
+  }
+
+  private static String describeChar(int c) {
+    return c == Input.EOF ? "<EOF>" : "'" + (char) c + "'";
+  }
+
   /**
    * Read the next element of the JSON input stream as a string.
    *
diff --git a/java/test/org/openqa/selenium/json/JsonInputTest.java b/java/test/org/openqa/selenium/json/JsonInputTest.java
index 99737c49b8618..037c5714c5ca7 100644
--- a/java/test/org/openqa/selenium/json/JsonInputTest.java
+++ b/java/test/org/openqa/selenium/json/JsonInputTest.java
@@ -325,6 +325,65 @@ void shouldRejectUnescapedControlCharactersInStrings() {
     }
   }
 
+  @Test
+  void shouldRejectSpecInvalidNumbers() {
+    // See RFC 8259 §6. Each of these was previously accepted by nextNumber().
+    for (String bad :
+        new String[] {
+          "+5", // leading plus not allowed
+          "01", // leading zero not allowed
+          "007", // leading zero not allowed
+          ".5", // no digit before decimal
+          "5.", // no digit after decimal
+          "1e", // exponent without digits
+          "-", // sign without digits
+        }) {
+      try (JsonInput input = newInput(bad)) {
+        assertThatExceptionOfType(JsonException.class)
+            .describedAs("Input %s should be rejected", bad)
+            .isThrownBy(input::nextNumber);
+      }
+    }
+  }
+
+  @Test
+  void shouldStillAcceptSpecValidNumbers() {
+    assertThat(parseNumber("0")).isEqualTo(0L);
+    assertThat(parseNumber("-0")).isEqualTo(0L);
+    assertThat(parseNumber("42")).isEqualTo(42L);
+    assertThat(parseNumber("-17")).isEqualTo(-17L);
+    assertThat(parseNumber("3.14")).isEqualTo(3.14d);
+    assertThat(parseNumber("-2.5e10")).isEqualTo(-2.5e10d);
+    assertThat(parseNumber("1E+2")).isEqualTo(100.0d);
+    assertThat(parseNumber("0.0")).isEqualTo(0.0d);
+  }
+
+  @Test
+  void shouldRejectDoubleOverflow() {
+    try (JsonInput input = newInput("1e9999")) {
+      assertThatExceptionOfType(JsonException.class)
+          .isThrownBy(input::nextNumber)
+          .withMessageContaining("out of range");
+    }
+  }
+
+  private Number parseNumber(String raw) {
+    try (JsonInput input = newInput(raw)) {
+      return input.nextNumber();
+    }
+  }
+
+  @Test
+  void shouldReportEndOfInputAsEofNotAsAUnicodeReplacement() {
+    // If the number ends prematurely, the diagnostic should say "<EOF>" rather than "'￿'"
+    // (which is a valid literal character elsewhere).
+    try (JsonInput input = newInput("5.")) {
+      assertThatExceptionOfType(JsonException.class)
+          .isThrownBy(input::nextNumber)
+          .withMessageContaining("<EOF>");
+    }
+  }
+
   @Test
   void nullInputsShouldCoerceAsNullValues() throws IOException {
     try (InputStream is = new ByteArrayInputStream(new byte[0]);