kerberos: check ticket and authenticator times in KerberosSSP acceptor

scapy/layers/kerberos.py

@@ -4949,6 +4949,10 @@ class KerberosSSP(SSP):
 
     auth_type = 0x10
 
+    # [RFC4120] sect 1.6: "a reasonable value for allowable clock skew is
+    # 5 minutes"
+    KRB_MaxClockSkew = 5 * 60
+
     class STATE(SSP.STATE):
         INIT = 1
         CLI_SENT_TGTREQ = 2
@@ -5930,6 +5934,36 @@ def GSS_Accept_sec_context(
             Context.STSessionKey = tkt.key.toKey()
             authenticator = ap_req.authenticator.decrypt(Context.STSessionKey)
 
+            # Check the ticket validity period and the authenticator timestamp
+            # [RFC4120] sect 3.2.3
+            now = now_time.timestamp()
+            starttime = (tkt.starttime or tkt.authtime).datetime
+            endtime = tkt.endtime.datetime
+            ctime = authenticator.ctime.datetime
+            errorCode = None
+            if not starttime or starttime.timestamp() - now > self.KRB_MaxClockSkew:
+                errorCode = "KRB_AP_ERR_TKT_NYV"
+            elif not endtime or now - endtime.timestamp() > self.KRB_MaxClockSkew:
+                errorCode = "KRB_AP_ERR_TKT_EXPIRED"
+            elif not ctime or abs(ctime.timestamp() - now) > self.KRB_MaxClockSkew:
+                errorCode = "KRB_AP_ERR_SKEW"
+            # Don't check times in passive mode: the capture may be old.
+            if errorCode and not Context.passive:
+                warning("KerberosSSP: %s" % errorCode)
+                err = KRB_GSSAPI_Token(
+                    innerToken=KRB_InnerToken(
+                        TOK_ID=b"\x03\x00",
+                        root=KRB_ERROR(
+                            errorCode=errorCode,
+                            stime=ASN1_GENERALIZED_TIME(now_time),
+                            realm=ap_req.ticket.realm,
+                            sname=ap_req.ticket.sname,
+                            eData=None,
+                        ),
+                    )
+                )
+                return Context, err, GSS_S_DEFECTIVE_CREDENTIAL
+
             # Compute an application session key ([MS-KILE] sect 3.1.1.2)
             subkey = None
             if ap_req.apOptions.val[2] == "1":  # mutual-required

test/scapy/layers/dcerpc.uts

@@ -884,6 +884,12 @@ t.import_krb(
     KRB_Ticket(bytes.fromhex("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")),
     key=SRVKEY,
 )
+# The captured ticket is expired: refresh its validity period
+from datetime import datetime, timedelta, timezone
+tkt = t.dec_ticket(0, key=SRVKEY)
+tkt.starttime = ASN1_GENERALIZED_TIME(datetime.now(timezone.utc))
+tkt.endtime = ASN1_GENERALIZED_TIME(datetime.now(timezone.utc) + timedelta(hours=10))
+t.update_ticket(0, tkt, hash=SRVKEY.key)
 clissp = t.ssp(0)
 
 rpcserver = MyRPCServer.spawn(

test/scapy/layers/kerberos.uts

@@ -1894,6 +1894,45 @@ try:
 except ValueError:
     pass
 
+= GSS_Accept_sec_context rejects expired tickets and stale authenticators
+
+from datetime import datetime, timedelta, timezone
+from scapy.layers.gssapi import GSS_S_COMPLETE, GSS_S_DEFECTIVE_CREDENTIAL
+from scapy.layers.kerberos import EncryptedData, EncryptionKey, EncTicketPart, KRB_ERROR, PrincipalName, TransitedEncoding
+
+srvkey = Key(EncryptionType.AES256_CTS_HMAC_SHA1_96, key=b"S" * 32)
+tktkey = Key(EncryptionType.AES256_CTS_HMAC_SHA1_96, key=b"K" * 32)
+now = datetime(2024, 3, 5, 16, 52, 55, tzinfo=timezone.utc)
+
+def make_ap_req(endtime, ctime):
+    st = KRB_Ticket(realm="DOMAIN.LOCAL", sname=PrincipalName.fromSPN("cifs/dc1"), encPart=EncryptedData())
+    st.encPart.encrypt(srvkey, EncTicketPart(
+        transited=TransitedEncoding(trType=ASN1_INTEGER(0), contents=ASN1_STRING(b"")),
+        key=EncryptionKey.fromKey(tktkey),
+        crealm="DOMAIN.LOCAL",
+        cname=PrincipalName.fromUPN("User1@DOMAIN.LOCAL"),
+        authtime=ASN1_GENERALIZED_TIME(endtime - timedelta(hours=10)),
+        starttime=ASN1_GENERALIZED_TIME(endtime - timedelta(hours=10)),
+        endtime=ASN1_GENERALIZED_TIME(endtime),
+    ))
+    krbcli = KerberosSSP(UPN="User1@DOMAIN.LOCAL", SPN="cifs/dc1", ST=KRB_Ticket(bytes(st)), KEY=tktkey)
+    with mock.patch('scapy.layers.kerberos.datetime', mock.MagicMock(now=lambda tz=None: ctime)):
+        _, tok, _ = krbcli.GSS_Init_sec_context(None, req_flags=0)
+    return tok
+
+krbsrv = KerberosSSP(SPN="cifs/dc1", KEY=srvkey)
+with mock.patch('scapy.layers.kerberos.datetime', date_mock):
+    _, tok, status = krbsrv.GSS_Accept_sec_context(None, make_ap_req(now + timedelta(hours=1), now))
+    assert status == GSS_S_COMPLETE
+    # Expired ticket
+    _, tok, status = krbsrv.GSS_Accept_sec_context(None, make_ap_req(now - timedelta(days=1), now))
+    assert status == GSS_S_DEFECTIVE_CREDENTIAL
+    assert tok.innerToken.root.errorCode == "KRB_AP_ERR_TKT_EXPIRED"
+    # Stale (replayed) authenticator
+    _, tok, status = krbsrv.GSS_Accept_sec_context(None, make_ap_req(now + timedelta(hours=1), now - timedelta(hours=1)))
+    assert status == GSS_S_DEFECTIVE_CREDENTIAL
+    assert tok.innerToken.root.errorCode == "KRB_AP_ERR_SKEW"
+
 = Create client and server KerberosSSP (raw)
 
 client = KerberosSSP(