diff --git a/lib/libc/stdlib/strtoimax.c b/lib/libc/stdlib/strtoimax.c
index 0cb387b64ad9..147fce2b98a3 100644
--- a/lib/libc/stdlib/strtoimax.c
+++ b/lib/libc/stdlib/strtoimax.c
@@ -75,7 +75,11 @@ strtoimax(const char * __restrict nptr, char ** __restrict endptr, int base)
 			c = *s++;
 	}
 	if ((base == 0 || base == 16) &&
-	    c == '0' && (*s == 'x' || *s == 'X')) {
+	    c == '0' && (*s == 'x' || *s == 'X') &&
+	    ((s[1] >= 'a' && s[1] <= 'f') ||
+	     (s[1] >= 'A' && s[1] <= 'F') ||
+	     (s[1] >= '0' && s[1] <= '9'))
+	   ) {
 		c = s[1];
 		s += 2;
 		base = 16;
diff --git a/lib/libc/stdlib/strtol.c b/lib/libc/stdlib/strtol.c
index 658628e370c6..5aae7f9dc0bc 100644
--- a/lib/libc/stdlib/strtol.c
+++ b/lib/libc/stdlib/strtol.c
@@ -76,7 +76,11 @@ strtol(const char * __restrict nptr, char ** __restrict endptr, int base)
 			c = *s++;
 	}
 	if ((base == 0 || base == 16) &&
-	    c == '0' && (*s == 'x' || *s == 'X')) {
+	    c == '0' && (*s == 'x' || *s == 'X') &&
+	    ((s[1] >= 'a' && s[1] <= 'f') ||
+	     (s[1] >= 'A' && s[1] <= 'F') ||
+	     (s[1] >= '0' && s[1] <= '9'))
+	   ) {
 		c = s[1];
 		s += 2;
 		base = 16;
diff --git a/lib/libc/stdlib/strtoll.c b/lib/libc/stdlib/strtoll.c
index 2eb3a509a58c..6cbe76abae99 100644
--- a/lib/libc/stdlib/strtoll.c
+++ b/lib/libc/stdlib/strtoll.c
@@ -75,7 +75,11 @@ strtoll(const char * __restrict nptr, char ** __restrict endptr, int base)
 			c = *s++;
 	}
 	if ((base == 0 || base == 16) &&
-	    c == '0' && (*s == 'x' || *s == 'X')) {
+	    c == '0' && (*s == 'x' || *s == 'X') &&
+	    ((s[1] >= 'a' && s[1] <= 'f') ||
+	     (s[1] >= 'A' && s[1] <= 'F') ||
+	     (s[1] >= '0' && s[1] <= '9'))
+	   ) {
 		c = s[1];
 		s += 2;
 		base = 16;
diff --git a/lib/libc/stdlib/strtoul.c b/lib/libc/stdlib/strtoul.c
index 2146a98b8ac0..a8e37364101b 100644
--- a/lib/libc/stdlib/strtoul.c
+++ b/lib/libc/stdlib/strtoul.c
@@ -73,7 +73,11 @@ strtoul(const char * __restrict nptr, char ** __restrict endptr, int base)
 			c = *s++;
 	}
 	if ((base == 0 || base == 16) &&
-	    c == '0' && (*s == 'x' || *s == 'X')) {
+	    c == '0' && (*s == 'x' || *s == 'X') &&
+	    ((s[1] >= 'a' && s[1] <= 'f') ||
+	     (s[1] >= 'A' && s[1] <= 'F') ||
+	     (s[1] >= '0' && s[1] <= '9'))
+	   ) {
 		c = s[1];
 		s += 2;
 		base = 16;
diff --git a/lib/libc/stdlib/strtoull.c b/lib/libc/stdlib/strtoull.c
index 1720a8f6571c..0e80b0d51295 100644
--- a/lib/libc/stdlib/strtoull.c
+++ b/lib/libc/stdlib/strtoull.c
@@ -73,7 +73,11 @@ strtoull(const char * __restrict nptr, char ** __restrict endptr, int base)
 			c = *s++;
 	}
 	if ((base == 0 || base == 16) &&
-	    c == '0' && (*s == 'x' || *s == 'X')) {
+	    c == '0' && (*s == 'x' || *s == 'X') &&
+	    ((s[1] >= 'a' && s[1] <= 'f') ||
+	     (s[1] >= 'A' && s[1] <= 'F') ||
+	     (s[1] >= '0' && s[1] <= '9'))
+	   ) {
 		c = s[1];
 		s += 2;
 		base = 16;
diff --git a/lib/libc/stdlib/strtoumax.c b/lib/libc/stdlib/strtoumax.c
index ddaee5935a4b..91d5d07ea4c8 100644
--- a/lib/libc/stdlib/strtoumax.c
+++ b/lib/libc/stdlib/strtoumax.c
@@ -73,7 +73,11 @@ strtoumax(const char * __restrict nptr, char ** __restrict endptr, int base)
 			c = *s++;
 	}
 	if ((base == 0 || base == 16) &&
-	    c == '0' && (*s == 'x' || *s == 'X')) {
+	    c == '0' && (*s == 'x' || *s == 'X') &&
+	    ((s[1] >= 'a' && s[1] <= 'f') ||
+	     (s[1] >= 'A' && s[1] <= 'F') ||
+	     (s[1] >= '0' && s[1] <= '9'))
+	   ) {
 		c = s[1];
 		s += 2;
 		base = 16;