diff --git a/compat/getgrouplist.c b/compat/getgrouplist.c
index 8573c3498..6e035be84 100644
--- a/compat/getgrouplist.c
+++ b/compat/getgrouplist.c
@@ -69,9 +69,10 @@ getgrouplist(const char *name, gid_t basegid, gid_t *groups, int *ngroupsp)
     aix_setauthdb((char *) name);
 #endif
     if ((grset = getgrset(name)) != NULL) {
+	const char *errstr;
 	for (cp = strtok(grset, ","); cp != NULL; cp = strtok(NULL, ",")) {
-	    gid = atoi(cp);
-	    if (gid != basegid) {
+	    gid = atoid(cp, NULL, NULL, &errstr);
+	    if (errstr == NULL && gid != basegid) {
 		if (ngroups == grpsize)
 		    goto done;
 		groups[ngroups++] = gid;
diff --git a/plugins/group_file/getgrent.c b/plugins/group_file/getgrent.c
index 68e004e49..6fb750f3a 100644
--- a/plugins/group_file/getgrent.c
+++ b/plugins/group_file/getgrent.c
@@ -64,6 +64,8 @@ struct group *mygetgrent(void);
 struct group *mygetgrnam(const char *);
 struct group *mygetgrgid(gid_t);
 
+extern id_t atoid(const char *str, const char *sep, char **endp, const char **errstr);
+
 void
 mysetgrfile(const char *file)
 {
@@ -101,25 +103,31 @@ mygetgrent(void)
     static struct group gr;
     static char grbuf[LINE_MAX], *gr_mem[GRMEM_MAX+1];
     size_t len;
+    id_t id;
     char *cp, *colon;
+    const char *errstr;
     int n;
 
+next_entry:
     if ((colon = fgets(grbuf, sizeof(grbuf), grf)) == NULL)
 	return NULL;
 
     memset(&gr, 0, sizeof(gr));
     if ((colon = strchr(cp = colon, ':')) == NULL)
-	return NULL;
+	goto next_entry;
     *colon++ = '\0';
     gr.gr_name = cp;
     if ((colon = strchr(cp = colon, ':')) == NULL)
-	return NULL;
+	goto next_entry;
     *colon++ = '\0';
     gr.gr_passwd = cp;
     if ((colon = strchr(cp = colon, ':')) == NULL)
-	return NULL;
+	goto next_entry;
     *colon++ = '\0';
-    gr.gr_gid = atoi(cp);
+    id = atoid(cp, NULL, NULL, &errstr);
+    if (errstr != NULL)
+	goto next_entry;
+    gr.gr_gid = (gid_t)id;
     len = strlen(colon);
     if (len > 0 && colon[len - 1] == '\n')
 	colon[len - 1] = '\0';
diff --git a/plugins/sudoers/pwutil.c b/plugins/sudoers/pwutil.c
index 48f8fc21d..7d2f3e889 100644
--- a/plugins/sudoers/pwutil.c
+++ b/plugins/sudoers/pwutil.c
@@ -270,10 +270,17 @@ sudo_mkpwent(const char *user, uid_t uid, gid_t gid, const char *home,
 struct passwd *
 sudo_fakepwnam(const char *user, gid_t gid)
 {
+    const char *errstr;
     uid_t uid;
+    debug_decl(sudo_fakepwnam, SUDO_DEBUG_NSS)
 
-    uid = (uid_t) atoi(user + 1);
-    return sudo_mkpwent(user, uid, gid, NULL, NULL);
+    uid = (uid_t) atoid(user + 1, NULL, NULL, &errstr);
+    if (errstr != NULL) {
+	sudo_debug_printf(SUDO_DEBUG_DEBUG|SUDO_DEBUG_DIAG,
+	    "uid %s %s", user, errstr);
+	debug_return_ptr(NULL);
+    }
+    debug_return_ptr(sudo_mkpwent(user, uid, gid, NULL, NULL));
 }
 
 void
@@ -432,6 +439,7 @@ struct group *
 sudo_fakegrnam(const char *group)
 {
     struct cache_item_gr *gritem;
+    const char *errstr;
     struct group *gr;
     struct rbnode *node;
     size_t len, name_len;
@@ -444,9 +452,15 @@ sudo_fakegrnam(const char *group)
     for (i = 0; i < 2; i++) {
 	gritem = ecalloc(1, len);
 	gr = &gritem->gr;
-	gr->gr_gid = (gid_t) atoi(group + 1);
+	gr->gr_gid = (gid_t) atoid(group + 1, NULL, NULL, &errstr);
 	gr->gr_name = (char *)(gritem + 1);
 	memcpy(gr->gr_name, group, name_len + 1);
+	if (errstr != NULL) {
+	    sudo_debug_printf(SUDO_DEBUG_DEBUG|SUDO_DEBUG_DIAG,
+		"gid %s %s", group, errstr);
+	    efree(gritem);
+	    debug_return_ptr(NULL);
+	}
 
 	gritem->cache.refcnt = 1;
 	gritem->cache.d.gr = gr;
@@ -608,6 +622,7 @@ user_in_group(const struct passwd *pw, const char *group)
 {
     struct group_list *grlist;
     struct group *grp = NULL;
+    const char *errstr;
     int i;
     bool matched = false;
     debug_decl(user_in_group, SUDO_DEBUG_NSS)
@@ -617,16 +632,21 @@ user_in_group(const struct passwd *pw, const char *group)
 	 * If it could be a sudo-style group ID check gids first.
 	 */
 	if (group[0] == '#') {
-	    gid_t gid = atoi(group + 1);
-	    if (gid == pw->pw_gid) {
-		matched = true;
-		goto done;
-	    }
-	    for (i = 0; i < grlist->ngids; i++) {
-		if (gid == grlist->gids[i]) {
+	    gid_t gid = (gid_t) atoid(group + 1, NULL, NULL, &errstr);
+	    if (errstr != NULL) {
+		sudo_debug_printf(SUDO_DEBUG_DEBUG|SUDO_DEBUG_DIAG,
+		    "gid %s %s", group, errstr);
+	    } else {
+		if (gid == pw->pw_gid) {
 		    matched = true;
 		    goto done;
 		}
+		for (i = 0; i < grlist->ngids; i++) {
+		    if (gid == grlist->gids[i]) {
+			matched = true;
+			goto done;
+		    }
+		}
 	    }
 	}
 
diff --git a/plugins/sudoers/regress/iolog_path/check_iolog_path.c b/plugins/sudoers/regress/iolog_path/check_iolog_path.c
index 12fb81ee7..2b0904473 100644
--- a/plugins/sudoers/regress/iolog_path/check_iolog_path.c
+++ b/plugins/sudoers/regress/iolog_path/check_iolog_path.c
@@ -102,6 +102,7 @@ main(int argc, char *argv[])
     char line[2048];
     char *file_in = NULL, *file_out = NULL;
     char *dir_in = NULL, *dir_out = NULL;
+    const char *errstr;
     int state = 0;
     int errors = 0;
     int tests = 0;
@@ -149,7 +150,9 @@ main(int argc, char *argv[])
 	    user_name = strdup(line);
 	    break;
 	case 2:
-	    user_gid = atoi(line);
+	    user_gid = (gid_t)atoid(line, NULL, NULL, &errstr);
+	    if (errstr != NULL)
+		fatalx("group ID %s: %s", line, errstr);
 	    break;
 	case 3:
 	    if (runas_pw->pw_name != NULL)
@@ -157,7 +160,9 @@ main(int argc, char *argv[])
 	    runas_pw->pw_name = strdup(line);
 	    break;
 	case 4:
-	    runas_pw->pw_gid = atoi(line);
+	    runas_pw->pw_gid = (gid_t)atoid(line, NULL, NULL, &errstr);
+	    if (errstr != NULL)
+		fatalx("group ID %s: %s", line, errstr);
 	    break;
 	case 5:
 	    user_shost = strdup(line);
diff --git a/plugins/sudoers/sudoers.c b/plugins/sudoers/sudoers.c
index 1342d9a66..4c092e48a 100644
--- a/plugins/sudoers/sudoers.c
+++ b/plugins/sudoers/sudoers.c
@@ -300,11 +300,15 @@ sudoers_policy_main(int argc, char * const argv[], int pwflag, char *env_add[],
      * Look up the timestamp dir owner if one is specified.
      */
     if (def_timestampowner) {
-	struct passwd *pw;
+	struct passwd *pw = NULL;
 
-	if (*def_timestampowner == '#')
-	    pw = sudo_getpwuid(atoi(def_timestampowner + 1));
-	else
+	if (*def_timestampowner == '#') {
+	    const char *errstr;
+	    uid_t uid = atoid(def_timestampowner + 1, NULL, NULL, &errstr);
+	    if (errstr == NULL)
+		pw = sudo_getpwuid(uid);
+	}
+	if (pw == NULL)
 	    pw = sudo_getpwnam(def_timestampowner);
 	if (pw != NULL) {
 	    timestamp_uid = pw->pw_uid;
@@ -834,17 +838,24 @@ set_fqdn(void)
 static void
 set_runaspw(const char *user)
 {
+    struct passwd *pw = NULL;
     debug_decl(set_runaspw, SUDO_DEBUG_PLUGIN)
 
-    if (runas_pw != NULL)
-	sudo_pw_delref(runas_pw);
     if (*user == '#') {
-	if ((runas_pw = sudo_getpwuid(atoi(user + 1))) == NULL)
-	    runas_pw = sudo_fakepwnam(user, runas_gr ? runas_gr->gr_gid : 0);
-    } else {
-	if ((runas_pw = sudo_getpwnam(user)) == NULL)
+	const char *errstr;
+	uid_t uid = atoid(user + 1, NULL, NULL, &errstr);
+	if (errstr == NULL) {
+	    if ((pw = sudo_getpwuid(uid)) == NULL)
+		pw = sudo_fakepwnam(user, runas_gr ? runas_gr->gr_gid : 0);
+	}
+    }
+    if (pw == NULL) {
+	if ((pw = sudo_getpwnam(user)) == NULL)
 	    log_fatal(NO_MAIL|MSG_ONLY, N_("unknown user: %s"), user);
     }
+    if (runas_pw != NULL)
+	sudo_pw_delref(runas_pw);
+    runas_pw = pw;
     debug_return;
 }
 
@@ -855,17 +866,24 @@ set_runaspw(const char *user)
 static void
 set_runasgr(const char *group)
 {
+    struct group *gr = NULL;
     debug_decl(set_runasgr, SUDO_DEBUG_PLUGIN)
 
-    if (runas_gr != NULL)
-	sudo_gr_delref(runas_gr);
     if (*group == '#') {
-	if ((runas_gr = sudo_getgrgid(atoi(group + 1))) == NULL)
-	    runas_gr = sudo_fakegrnam(group);
-    } else {
-	if ((runas_gr = sudo_getgrnam(group)) == NULL)
+	const char *errstr;
+	gid_t gid = atoid(group + 1, NULL, NULL, &errstr);
+	if (errstr == NULL) {
+	    if ((gr = sudo_getgrgid(gid)) == NULL)
+		gr = sudo_fakegrnam(group);
+	}
+    }
+    if (gr == NULL) {
+	if ((gr = sudo_getgrnam(group)) == NULL)
 	    log_fatal(NO_MAIL|MSG_ONLY, N_("unknown group: %s"), group);
     }
+    if (runas_gr != NULL)
+	sudo_gr_delref(runas_gr);
+    runas_gr = gr;
     debug_return;
 }
 
diff --git a/plugins/sudoers/testsudoers.c b/plugins/sudoers/testsudoers.c
index c83659b28..936c4c27c 100644
--- a/plugins/sudoers/testsudoers.c
+++ b/plugins/sudoers/testsudoers.c
@@ -127,6 +127,7 @@ main(int argc, char *argv[])
     struct userspec *us;
     char *p, *grfile, *pwfile;
     char hbuf[HOST_NAME_MAX + 1];
+    const char *errstr;
     int match, host_match, runas_match, cmnd_match;
     int ch, dflag, exitcode = 0;
     debug_decl(main, SUDO_DEBUG_MAIN)
@@ -158,7 +159,9 @@ main(int argc, char *argv[])
 		user_host = optarg;
 		break;
 	    case 'G':
-		sudoers_gid = (gid_t)atoi(optarg);
+		sudoers_gid = (gid_t)atoid(optarg, NULL, NULL, &errstr);
+		if (errstr != NULL)
+		    fatalx("group ID %s: %s", optarg, errstr);
 		break;
 	    case 'g':
 		runas_group = optarg;
@@ -173,7 +176,9 @@ main(int argc, char *argv[])
 		trace_print = testsudoers_print;
 		break;
 	    case 'U':
-		sudoers_uid = (uid_t)atoi(optarg);
+		sudoers_uid = (uid_t)atoid(optarg, NULL, NULL, &errstr);
+		if (errstr != NULL)
+		    fatalx("user ID %s: %s", optarg, errstr);
 		break;
 	    case 'u':
 		runas_user = optarg;
@@ -349,36 +354,48 @@ done:
 static void
 set_runaspw(const char *user)
 {
-    debug_decl(main, SUDO_DEBUG_UTIL)
+    struct passwd *pw = NULL;
+    debug_decl(set_runaspw, SUDO_DEBUG_UTIL)
 
-    if (runas_pw != NULL)
-	sudo_pw_delref(runas_pw);
     if (*user == '#') {
-	if ((runas_pw = sudo_getpwuid(atoi(user + 1))) == NULL)
-	    runas_pw = sudo_fakepwnam(user, runas_gr ? runas_gr->gr_gid : 0);
-    } else {
-	if ((runas_pw = sudo_getpwnam(user)) == NULL)
+	const char *errstr;
+	uid_t uid = atoid(user + 1, NULL, NULL, &errstr);
+	if (errstr == NULL) {
+	    if ((pw = sudo_getpwuid(uid)) == NULL)
+		pw = sudo_fakepwnam(user, runas_gr ? runas_gr->gr_gid : 0);
+	}
+    }
+    if (pw == NULL) {
+	if ((pw = sudo_getpwnam(user)) == NULL)
 	    fatalx(U_("unknown user: %s"), user);
     }
-
+    if (runas_pw != NULL)
+	sudo_pw_delref(runas_pw);
+    runas_pw = pw;
     debug_return;
 }
 
 static void
 set_runasgr(const char *group)
 {
-    debug_decl(main, SUDO_DEBUG_UTIL)
+    struct group *gr = NULL;
+    debug_decl(set_runasgr, SUDO_DEBUG_UTIL)
 
-    if (runas_gr != NULL)
-	sudo_gr_delref(runas_gr);
     if (*group == '#') {
-	if ((runas_gr = sudo_getgrgid(atoi(group + 1))) == NULL)
-	    runas_gr = sudo_fakegrnam(group);
-    } else {
-	if ((runas_gr = sudo_getgrnam(group)) == NULL)
+	const char *errstr;
+	gid_t gid = atoid(group + 1, NULL, NULL, &errstr);
+	if (errstr == NULL) {
+	    if ((gr = sudo_getgrgid(gid)) == NULL)
+		gr = sudo_fakegrnam(group);
+	}
+    }
+    if (gr == NULL) {
+	if ((gr = sudo_getgrnam(group)) == NULL)
 	    fatalx(U_("unknown group: %s"), group);
     }
-
+    if (runas_gr != NULL)
+	sudo_gr_delref(runas_gr);
+    runas_gr = gr;
     debug_return;
 }