fix(scan): accept string-encoded jwt exp claim (#311)

numericClaim only accepted exp as a json number, but some emitters
serialize it as a quoted string. a token with a past string exp was
reported as missing exp instead of expired, an expiry false negative
and a missing-claim false positive on the same token. also parse a
string value via strconv.ParseFloat, falling back to absent when it
does not parse.

internal/scan/jwt.go

@@ -24,6 +24,7 @@ import (
 	"io"
 	"net/http"
 	"regexp"
+	"strconv"
 	"strings"
 	"time"
 
@@ -375,14 +376,27 @@ func decodeJWTSegment(seg string) (map[string]any, error) {
 }
 
 // numericClaim pulls a numeric claim out of the payload. json numbers decode to
-// float64, so that's the only shape we accept.
+// float64, but some jwt libraries (older php/java stacks in particular) encode
+// NumericDate as a quoted string, so a string that parses as a number counts too.
+// otherwise a stringly-typed exp reads as absent and a genuinely expired token
+// slips through as "missing exp" instead of "expired".
 func numericClaim(payload map[string]any, key string) (float64, bool) {
 	v, ok := payload[key]
 	if !ok {
 		return 0, false
 	}
-	f, ok := v.(float64)
-	return f, ok
+	switch n := v.(type) {
+	case float64:
+		return n, true
+	case string:
+		f, err := strconv.ParseFloat(n, 64)
+		if err != nil {
+			return 0, false
+		}
+		return f, true
+	default:
+		return 0, false
+	}
 }
 
 // isHMACAlg reports whether alg is one of the HMAC family (HS256/HS384/HS512).

internal/scan/jwt_test.go

@@ -285,6 +285,61 @@ func TestJWT_SensitiveClaimFlagged(t *testing.T) {
 	}
 }
 
+func TestNumericClaim_AcceptsStringEncodedExp(t *testing.T) {
+	// some jwt stacks (older php/java libs) serialize NumericDate as a quoted
+	// string instead of a json number. a stringly-typed exp must still count
+	// as present, or a genuinely expired token reads as "missing exp" instead.
+	cases := []struct {
+		name    string
+		payload map[string]any
+		wantF   float64
+		wantOK  bool
+	}{
+		{"string past exp", map[string]any{"exp": "1000000000"}, 1000000000, true},
+		{"string future exp", map[string]any{"exp": "9999999999"}, 9999999999, true},
+		{"float exp still works", map[string]any{"exp": float64(1000000000)}, 1000000000, true},
+		{"garbage string exp", map[string]any{"exp": "not-a-number"}, 0, false},
+		{"missing exp", map[string]any{}, 0, false},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			f, ok := numericClaim(tc.payload, "exp")
+			if ok != tc.wantOK || f != tc.wantF {
+				t.Errorf("numericClaim(%+v) = (%v, %v), want (%v, %v)", tc.payload, f, ok, tc.wantF, tc.wantOK)
+			}
+		})
+	}
+}
+
+func TestJWT_StringExpStillDetectsExpiry(t *testing.T) {
+	// header {alg:none}, payload {sub:x, exp:"1000000000"} (string-typed, long past).
+	// exp is a valid number, just quoted, so this must flag "expired token" and
+	// must NOT also claim the expiry is missing.
+	jwtStringExpPast := "eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0." +
+		"eyJzdWIiOiJ4IiwiZXhwIjoiMTAwMDAwMDAwMCJ9."
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write([]byte(jwtStringExpPast))
+	}))
+	defer srv.Close()
+
+	result, err := JWT(srv.URL, 5*time.Second, "")
+	if err != nil {
+		t.Fatalf("JWT: %v", err)
+	}
+	if result == nil || len(result.Tokens) != 1 {
+		t.Fatalf("expected one token, got %+v", result)
+	}
+
+	token := &result.Tokens[0]
+	if !hasIssue(token, "expired token") {
+		t.Errorf("expected string-encoded past exp to be flagged expired, got %+v", token.Issues)
+	}
+	if hasIssue(token, "missing exp") {
+		t.Errorf("string-encoded exp must not be reported as missing, got %+v", token.Issues)
+	}
+}
+
 func TestJWT_NoTokens(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		_, _ = w.Write([]byte("nothing to see here"))