Treat an ID of -1 as invalid since that means "no change".
Fixes CVE-2019-14287.
Found by Joe Vennix from Apple Information Security.

diff -r fcd7a6d8330e lib/util/strtoid.c
--- a/lib/util/strtoid.c	Fri Jan 11 13:31:15 2019 -0700
+++ b/lib/util/strtoid.c	Thu Oct 10 09:52:12 2019 -0600
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2016 Todd C. Miller <Todd.Miller@sudo.ws>
+ * Copyright (c) 2013-2019 Todd C. Miller <Todd.Miller@sudo.ws>
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -47,6 +47,27 @@
 #include "sudo_util.h"
 
 /*
+ * Make sure that the ID ends with a valid separator char.
+ */
+static bool
+valid_separator(const char *p, const char *ep, const char *sep)
+{
+    bool valid = false;
+    debug_decl(valid_separator, SUDO_DEBUG_UTIL)
+
+    if (ep != p) {
+	/* check for valid separator (including '\0') */
+	if (sep == NULL)
+	    sep = "";
+	do {
+	    if (*ep == *sep)
+		valid = true;
+	} while (*sep++ != '\0');
+    }
+    debug_return_bool(valid);
+}
+
+/*
  * Parse a uid/gid in string form.
  * If sep is non-NULL, it contains valid separator characters (e.g. comma, space)
  * If endp is non-NULL it is set to the next char after the ID.
@@ -60,38 +81,35 @@ sudo_strtoid_v1(const char *p, const cha
     char *ep;
     id_t ret = 0;
     long long llval;
-    bool valid = false;
     debug_decl(sudo_strtoid, SUDO_DEBUG_UTIL)
 
     /* skip leading space so we can pick up the sign, if any */
     while (isspace((unsigned char)*p))
 	p++;
-    if (sep == NULL)
-	sep = "";
+
+    /* While id_t may be 64-bit signed, uid_t and gid_t are 32-bit unsigned. */
     errno = 0;
     llval = strtoll(p, &ep, 10);
-    if (ep != p) {
-	/* check for valid separator (including '\0') */
-	do {
-	    if (*ep == *sep)
-		valid = true;
-	} while (*sep++ != '\0');
+    if ((errno == ERANGE && llval == LLONG_MAX) || llval > (id_t)UINT_MAX) {
+	errno = ERANGE;
+	if (errstr != NULL)
+	    *errstr = N_("value too large");
+	goto done;
     }
-    if (!valid) {
+    if ((errno == ERANGE && llval == LLONG_MIN) || llval < INT_MIN) {
+	errno = ERANGE;
+	if (errstr != NULL)
+	    *errstr = N_("value too small");
+	goto done;
+    }
+
+    /* Disallow id -1, which means "no change". */
+    if (!valid_separator(p, ep, sep) || llval == -1 || llval == (id_t)UINT_MAX) {
 	if (errstr != NULL)
 	    *errstr = N_("invalid value");
 	errno = EINVAL;
 	goto done;
     }
-    if (errno == ERANGE) {
-	if (errstr != NULL) {
-	    if (llval == LLONG_MAX)
-		*errstr = N_("value too large");
-	    else
-		*errstr = N_("value too small");
-	}
-	goto done;
-    }
     ret = (id_t)llval;
     if (errstr != NULL)
 	*errstr = NULL;
@@ -106,30 +124,15 @@ sudo_strtoid_v1(const char *p, const cha
 {
     char *ep;
     id_t ret = 0;
-    bool valid = false;
     debug_decl(sudo_strtoid, SUDO_DEBUG_UTIL)
 
     /* skip leading space so we can pick up the sign, if any */
     while (isspace((unsigned char)*p))
 	p++;
-    if (sep == NULL)
-	sep = "";
+
     errno = 0;
     if (*p == '-') {
 	long lval = strtol(p, &ep, 10);
-	if (ep != p) {
-	    /* check for valid separator (including '\0') */
-	    do {
-		if (*ep == *sep)
-		    valid = true;
-	    } while (*sep++ != '\0');
-	}
-	if (!valid) {
-	    if (errstr != NULL)
-		*errstr = N_("invalid value");
-	    errno = EINVAL;
-	    goto done;
-	}
 	if ((errno == ERANGE && lval == LONG_MAX) || lval > INT_MAX) {
 	    errno = ERANGE;
 	    if (errstr != NULL)
@@ -142,28 +145,31 @@ sudo_strtoid_v1(const char *p, const cha
 		*errstr = N_("value too small");
 	    goto done;
 	}
-	ret = (id_t)lval;
-    } else {
-	unsigned long ulval = strtoul(p, &ep, 10);
-	if (ep != p) {
-	    /* check for valid separator (including '\0') */
-	    do {
-		if (*ep == *sep)
-		    valid = true;
-	    } while (*sep++ != '\0');
-	}
-	if (!valid) {
+
+	/* Disallow id -1, which means "no change". */
+	if (!valid_separator(p, ep, sep) || lval == -1) {
 	    if (errstr != NULL)
 		*errstr = N_("invalid value");
 	    errno = EINVAL;
 	    goto done;
 	}
+	ret = (id_t)lval;
+    } else {
+	unsigned long ulval = strtoul(p, &ep, 10);
 	if ((errno == ERANGE && ulval == ULONG_MAX) || ulval > UINT_MAX) {
 	    errno = ERANGE;
 	    if (errstr != NULL)
 		*errstr = N_("value too large");
 	    goto done;
 	}
+
+	/* Disallow id -1, which means "no change". */
+	if (!valid_separator(p, ep, sep) || ulval == UINT_MAX) {
+	    if (errstr != NULL)
+		*errstr = N_("invalid value");
+	    errno = EINVAL;
+	    goto done;
+	}
 	ret = (id_t)ulval;
     }
     if (errstr != NULL)
