From f77fa561933de0e372b71e8de2aae2f6741e2bbd Mon Sep 17 00:00:00 2001
From: Anders Eknert <anders.eknert@bisnode.com>
Date: Thu, 15 Aug 2019 19:35:16 +0200
Subject: [PATCH] Honour nbf claim if present in ID token

---
 oidc.go        |  1 +
 verify.go      | 13 ++++++++++++-
 verify_test.go | 28 ++++++++++++++++++++++++++++
 3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/oidc.go b/oidc.go
index 15632bb7..508b39d3 100644
--- a/oidc.go
+++ b/oidc.go
@@ -321,6 +321,7 @@ type idToken struct {
 	Audience     audience               `json:"aud"`
 	Expiry       jsonTime               `json:"exp"`
 	IssuedAt     jsonTime               `json:"iat"`
+	NotBefore    *jsonTime              `json:"nbf"`
 	Nonce        string                 `json:"nonce"`
 	AtHash       string                 `json:"at_hash"`
 	ClaimNames   map[string]string      `json:"_claim_names"`
diff --git a/verify.go b/verify.go
index bb8a3b6f..ff7555db 100644
--- a/verify.go
+++ b/verify.go
@@ -270,10 +270,21 @@ func (v *IDTokenVerifier) Verify(ctx context.Context, rawIDToken string) (*IDTok
 		if v.config.Now != nil {
 			now = v.config.Now
 		}
+		nowTime := now()
 
-		if t.Expiry.Before(now()) {
+		if t.Expiry.Before(nowTime) {
 			return nil, fmt.Errorf("oidc: token is expired (Token Expiry: %v)", t.Expiry)
 		}
+
+		// If nbf claim is provided in token, ensure that it is indeed in the past.
+		if token.NotBefore != nil {
+			nbfTime := time.Time(*token.NotBefore)
+			leeway := 1 * time.Minute
+
+			if nowTime.Add(leeway).Before(nbfTime) {
+				return nil, fmt.Errorf("oidc: current time %v before the nbf (not before) time: %v", nowTime, nbfTime)
+			}
+		}
 	}
 
 	switch len(jws.Signatures) {
diff --git a/verify_test.go b/verify_test.go
index 5eaacac2..d2fffa9c 100644
--- a/verify_test.go
+++ b/verify_test.go
@@ -107,6 +107,34 @@ func TestVerify(t *testing.T) {
 			},
 			signKey: newRSAKey(t),
 		},
+		{
+			name: "nbf in future",
+			idToken: `{"iss":"https://foo","nbf":` + strconv.FormatInt(time.Now().Add(time.Hour).Unix(), 10) +
+				`,"exp":` + strconv.FormatInt(time.Now().Add(time.Hour).Unix(), 10) + `}`,
+			config: Config{
+				SkipClientIDCheck: true,
+			},
+			signKey: newRSAKey(t),
+			wantErr: true,
+		},
+		{
+			name: "nbf in past",
+			idToken: `{"iss":"https://foo","nbf":` + strconv.FormatInt(time.Now().Add(-time.Hour).Unix(), 10) +
+				`,"exp":` + strconv.FormatInt(time.Now().Add(time.Hour).Unix(), 10) + `}`,
+			config: Config{
+				SkipClientIDCheck: true,
+			},
+			signKey: newRSAKey(t),
+		},
+		{
+			name: "nbf in future within clock skew tolerance",
+			idToken: `{"iss":"https://foo","nbf":` + strconv.FormatInt(time.Now().Add(30*time.Second).Unix(), 10) +
+				`,"exp":` + strconv.FormatInt(time.Now().Add(time.Hour).Unix(), 10) + `}`,
+			config: Config{
+				SkipClientIDCheck: true,
+			},
+			signKey: newRSAKey(t),
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.name, test.run)