diff --git a/pkg/ffapi/restfilter_json.go b/pkg/ffapi/restfilter_json.go
index 63a8348..3132906 100644
--- a/pkg/ffapi/restfilter_json.go
+++ b/pkg/ffapi/restfilter_json.go
@@ -17,10 +17,12 @@
 package ffapi
 
 import (
+	"bytes"
 	"context"
 	"database/sql/driver"
 	"encoding/json"
 	"fmt"
+	"math/big"
 	"strconv"
 	"strings"
 
@@ -146,13 +148,24 @@ func SkipFieldValidation() *JSONBuildFilterOpt {
 
 func (js *SimpleFilterValue) UnmarshalJSON(b []byte) error {
 	var v interface{}
-	err := json.Unmarshal(b, &v)
+	d := json.NewDecoder(bytes.NewReader(b))
+	d.UseNumber()
+	err := d.Decode(&v)
 	if err != nil {
 		return err
 	}
 	switch vi := v.(type) {
-	case float64:
-		*js = (SimpleFilterValue)(strconv.FormatFloat(vi, 'f', -1, 64))
+	case json.Number:
+		// Parsed exactly (rather than as a float64) so large values such as a uint256 are not
+		// rounded, and bounded to 256 bits so an exponent cannot expand into an enormous string
+		if !boundedLiteral(vi.String()) {
+			return i18n.NewError(context.Background(), i18n.MsgJSONQueryValueUnsupported, string(b))
+		}
+		r, ok := new(big.Rat).SetString(vi.String()) // #nosec G113 - length and exponent bounded by boundedLiteral above
+		if !ok || r.Num().BitLen() > maxFilterNumberBits || r.Denom().BitLen() > maxFilterNumberBits {
+			return i18n.NewError(context.Background(), i18n.MsgJSONQueryValueUnsupported, string(b))
+		}
+		*js = (SimpleFilterValue)(formatDecimal(r))
 		return nil
 	case string:
 		*js = (SimpleFilterValue)(vi)
@@ -165,6 +178,40 @@ func (js *SimpleFilterValue) UnmarshalJSON(b []byte) error {
 	}
 }
 
+const (
+	maxFilterNumberBits   = 256
+	maxFilterNumberDigits = 78                                              // decimal digits in 2^256
+	maxFilterNumberLength = maxFilterNumberDigits + maxFilterNumberBits + 8 // integer digits, decimal places (2^-255 needs 255), sign, point and exponent
+)
+
+// boundedLiteral reports whether s, a valid JSON number, is short enough and has a small enough
+// exponent that the value might fit in maxFilterNumberBits.
+// Check before parsing to big.Rat (per gosec G113)
+func boundedLiteral(s string) bool {
+	if len(s) > maxFilterNumberLength {
+		return false
+	}
+	i := strings.IndexAny(s, "eE")
+	if i < 0 {
+		return true
+	}
+	exp, err := strconv.Atoi(s[i+1:])
+	limit := maxFilterNumberLength + maxFilterNumberDigits
+	return err == nil && exp >= -limit && exp <= limit
+}
+
+// formatDecimal renders r, which was parsed from a decimal literal, as a plain decimal with
+// no exponent and no trailing zeros (so "5.0" and "1e3" arrive as "5" and "1000")
+func formatDecimal(r *big.Rat) string {
+	if r.IsInt() {
+		return r.Num().String()
+	}
+	// The denominator is 2^a*5^b, which terminates within max(a,b) places - and BitLen
+	// is always at least that
+	s := r.FloatString(r.Denom().BitLen())
+	return strings.TrimRight(s, "0")
+}
+
 func (js SimpleFilterValue) String() string {
 	return (string)(js)
 }
diff --git a/pkg/ffapi/restfilter_json_test.go b/pkg/ffapi/restfilter_json_test.go
index c13097f..5520989 100644
--- a/pkg/ffapi/restfilter_json_test.go
+++ b/pkg/ffapi/restfilter_json_test.go
@@ -21,6 +21,7 @@ import (
 	"database/sql/driver"
 	"encoding/json"
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -777,3 +778,45 @@ func TestBuildQueryAndFail(t *testing.T) {
 	_, err = qf.BuildFilter(context.Background(), TestQueryFactory)
 	assert.Regexp(t, "FF00142.*color", err)
 }
+
+func TestSimpleFilterValueNumbers(t *testing.T) {
+	for in, expected := range map[string]string{
+		`9007199254740993`: "9007199254740993", // 2^53+1, not representable as a float64
+		`115792089237316195423570985008687907853269984665640564039457584007913129639935`: "115792089237316195423570985008687907853269984665640564039457584007913129639935", // 2^256-1
+		`-57896044618658097711785492504343953926634992332820282019728792003956564819968`: "-57896044618658097711785492504343953926634992332820282019728792003956564819968", // -2^255
+		`-12345678901234567890123`: "-12345678901234567890123",
+		`42`:                       "42",
+		`5.0`:                      "5",
+		`1e3`:                      "1000",
+		`1.5`:                      "1.5",
+		`-0.000123`:                "-0.000123",
+		`1.25e-2`:                  "0.0125",
+		`0.0009765625`:             "0.0009765625", // 1/1024 - more places than denominator digits
+		`9007199254740993e0`:       "9007199254740993",
+		`123456789012345678901.5`:  "123456789012345678901.5",
+		`1e30`:                     "1000000000000000000000000000000",
+		// 2^-255, the most decimal places a value within 256 bits can need
+		`0.000000000000000000000000000000000000000000000000000000000000000000000000000017272337110188889250772703725600799142232000728872562770047406940337183606324854115943015006944576453121094587892299327193990197893663893387306007554116149549372494220733642578125`: "0.000000000000000000000000000000000000000000000000000000000000000000000000000017272337110188889250772703725600799142232000728872562770047406940337183606324854115943015006944576453121094587892299327193990197893663893387306007554116149549372494220733642578125",
+		`true`:                     "true",
+		`"str"`:                    "str",
+	} {
+		var js SimpleFilterValue
+		err := json.Unmarshal([]byte(in), &js)
+		assert.NoError(t, err, in)
+		assert.Equal(t, expected, js.String(), in)
+	}
+
+	var js SimpleFilterValue
+	err := js.UnmarshalJSON([]byte(`115792089237316195423570985008687907853269984665640564039457584007913129639936`)) // 2^256
+	assert.Regexp(t, "FF00241", err)
+	err = js.UnmarshalJSON([]byte(`1e999999`))
+	assert.Regexp(t, "FF00241", err)
+	err = js.UnmarshalJSON([]byte(`1e99999999999999999999`)) // exponent overflows an int
+	assert.Regexp(t, "FF00241", err)
+	err = js.UnmarshalJSON([]byte(`1e80`)) // within the exponent bound, but over 256 bits
+	assert.Regexp(t, "FF00241", err)
+	err = js.UnmarshalJSON([]byte(strings.Repeat("9", 1_000_000))) // too long to parse
+	assert.Regexp(t, "FF00241", err)
+	err = js.UnmarshalJSON([]byte(`1e-400`))
+	assert.Regexp(t, "FF00241", err)
+}