diff --git a/src/ckdb.c b/src/ckdb.c
index 3554a11e..c43ae064 100644
--- a/src/ckdb.c
+++ b/src/ckdb.c
@@ -174,8 +174,12 @@ const char *mailpatt = "^[A-Za-z0-9_-][A-Za-z0-9_\\.-]*@[A-Za-z0-9][A-Za-z0-9\\.
 const char *idpatt = "^[_A-Za-z][_A-Za-z0-9]*$";
 const char *intpatt = "^[0-9][0-9]*$";
 const char *hashpatt = "^[A-Fa-f0-9]*$";
-// TODO: bitcoind will check it properly
-const char *addrpatt = "^[A-Za-z0-9]*$";
+/* BTC addresses start with '1' (one) or '3' (three),
+ *  exclude: capital 'I' (eye), capital 'O' (oh),
+ *  lowercase 'l' (elle) and '0' (zero)
+ *  and with a simple test must be ADDR_MIN_LEN to ADDR_MAX_LEN (ckdb.h)
+ * bitcoind is used to fully validate them when required */
+const char *addrpatt = "^[13][A-HJ-NP-Za-km-z1-9]*$";
 
 // So the records below have the same 'name' as the klist
 const char Transfer[] = "Transfer";
diff --git a/src/ckdb.h b/src/ckdb.h
index dd245ef3..f9a216c0 100644
--- a/src/ckdb.h
+++ b/src/ckdb.h
@@ -52,7 +52,7 @@
 
 #define DB_VLOCK "1"
 #define DB_VERSION "0.9.4"
-#define CKDB_VERSION DB_VERSION"-0.631"
+#define CKDB_VERSION DB_VERSION"-0.640"
 
 #define WHERE_FFL " - from %s %s() line %d"
 #define WHERE_FFL_HERE __FILE__, __func__, __LINE__
@@ -80,6 +80,14 @@ extern const char *intpatt;
 extern const char *hashpatt;
 extern const char *addrpatt;
 
+/* If a trimmed username is like an address but this many or more characters,
+ * disallow it */
+#define ADDR_USER_CHECK 16
+
+// BTC address size
+#define ADDR_MIN_LEN 26
+#define ADDR_MAX_LEN 34
+
 typedef struct loadstatus {
 	tv_t oldest_sharesummary_firstshare_n;
 	tv_t newest_sharesummary_firstshare_a;
@@ -1334,6 +1342,7 @@ extern PGconn *dbconnect();
 
 extern char *safe_text(char *txt);
 extern void username_trim(USERS *users);
+extern bool like_address(char *username);
 
 extern void _txt_to_data(enum data_type typ, char *nam, char *fld, void *data, size_t siz, WHERE_FFL_ARGS);
 
diff --git a/src/ckdb_cmd.c b/src/ckdb_cmd.c
index c7c580c8..e3443858 100644
--- a/src/ckdb_cmd.c
+++ b/src/ckdb_cmd.c
@@ -15,7 +15,7 @@ static char *cmd_adduser(PGconn *conn, char *cmd, char *id, tv_t *now, char *by,
 {
 	char reply[1024] = "";
 	size_t siz = sizeof(reply);
-	K_ITEM *i_username, *i_emailaddress, *i_passwordhash, *u_item;
+	K_ITEM *i_username, *i_emailaddress, *i_passwordhash, *u_item = NULL;
 
 	LOGDEBUG("%s(): cmd '%s'", __func__, cmd);
 
@@ -23,18 +23,30 @@ static char *cmd_adduser(PGconn *conn, char *cmd, char *id, tv_t *now, char *by,
 	if (!i_username)
 		return strdup(reply);
 
-	i_emailaddress = require_name(trf_root, "emailaddress", 7, (char *)mailpatt, reply, siz);
-	if (!i_emailaddress)
-		return strdup(reply);
+	/* If a username added from the web site looks like an address
+	 *  then disallow it - a false positive is not an issue
+	 * Allowing it will create a security issue - someone could create
+	 *  an account with someone else's, as yet unused, payout address
+	 *  and redirect the payout to another payout address.
+	 *  ... and the person who owns the payout address can't check that
+	 *  in advance, they'll just find out with their first payout not
+	 *  arriving at their payout address */
+	if (!like_address(transfer_data(i_username))) {
+		i_emailaddress = require_name(trf_root, "emailaddress", 7,
+					      (char *)mailpatt, reply, siz);
+		if (!i_emailaddress)
+			return strdup(reply);
 
-	i_passwordhash = require_name(trf_root, "passwordhash", 64, (char *)hashpatt, reply, siz);
-	if (!i_passwordhash)
-		return strdup(reply);
+		i_passwordhash = require_name(trf_root, "passwordhash", 64,
+					      (char *)hashpatt, reply, siz);
+		if (!i_passwordhash)
+			return strdup(reply);
 
-	u_item = users_add(conn, transfer_data(i_username),
-				 transfer_data(i_emailaddress),
-				 transfer_data(i_passwordhash),
-				 by, code, inet, now, trf_root);
+		u_item = users_add(conn, transfer_data(i_username),
+					 transfer_data(i_emailaddress),
+					 transfer_data(i_passwordhash),
+					 by, code, inet, now, trf_root);
+	}
 
 	if (!u_item) {
 		LOGERR("%s() %s.failed.DBE", __func__, id);
@@ -223,7 +235,8 @@ static char *cmd_userset(PGconn *conn, char *cmd, char *id,
 				email = NULL;
 			}
 			i_address = optional_name(trf_root, "address",
-						  27, (char *)addrpatt,
+						  ADDR_MIN_LEN,
+						  (char *)addrpatt,
 						  reply, siz);
 			if (i_address)
 				address = transfer_data(i_address);
diff --git a/src/ckdb_data.c b/src/ckdb_data.c
index e712f520..b799648d 100644
--- a/src/ckdb_data.c
+++ b/src/ckdb_data.c
@@ -67,6 +67,55 @@ void username_trim(USERS *users)
 	}
 }
 
+/* Is the trimmed username like an address?
+ * False positive is OK (i.e. 'like')
+ * Before checking, it is trimmed to avoid web display confusion
+ * Length check is done before trimming - this may give a false
+ *  positive on any username with lots of trim characters ... which is OK */
+bool like_address(char *username)
+{
+	char *tmp, *front, *trail;
+	size_t len;
+	regex_t re;
+	int ret;
+
+	len = strlen(username);
+	if (len < ADDR_USER_CHECK)
+		return false;
+
+	tmp = strdup(username);
+	front = tmp;
+	while (*front && TRIM_IGNORE(*front))
+		front++;
+
+	trail = front + strlen(front) - 1;
+	while (trail >= front) {
+		if (TRIM_IGNORE(*trail))
+			*(trail--) = '\0';
+		else
+			break;
+	}
+
+	if (regcomp(&re, addrpatt, REG_NOSUB) != 0) {
+		LOGEMERG("%s(): failed to compile addrpatt '%s'",
+			 __func__, addrpatt);
+		free(tmp);
+		// This will disable adding any new usernames ...
+		return true;
+	}
+
+	ret = regexec(&re, front, (size_t)0, NULL, 0);
+	regfree(&re);
+
+	if (ret == 0) {
+		free(tmp);
+		return true;
+	}
+
+	free(tmp);
+	return false;
+}
+
 void _txt_to_data(enum data_type typ, char *nam, char *fld, void *data, size_t siz, WHERE_FFL_ARGS)
 {
 	char *tmp;