diff --git a/lib/libesp32/berry/src/be_lexer.c b/lib/libesp32/berry/src/be_lexer.c
index 90031d3f6..4e41bf8f6 100644
--- a/lib/libesp32/berry/src/be_lexer.c
+++ b/lib/libesp32/berry/src/be_lexer.c
@@ -354,7 +354,9 @@ static btokentype scan_decimal(blexer *lexer)
     if (has_decimal_dots || is_realexp) {
         type = TokenReal;
     }
-    lexer->buf.s[lexer->buf.len] = '\0';
+    /* use save_char to add the null terminator, */
+    /* since it handles expanding the buffer if needed. */
+    save_char(lexer, '\0');
     if (type == TokenReal) {
         setreal(lexer, be_str2real(lexbuf(lexer), NULL));
     } else {
@@ -431,7 +433,7 @@ static btokentype scan_string(blexer *lexer);   /* forward declaration */
 /* scan f-string and transpile it to `format(...)` syntax then feeding the normal lexer and parser */
 static void scan_f_string(blexer *lexer)
 {
-    char ch;
+    char ch = '\0';
     clear_buf(lexer);
     scan_string(lexer);         /* first scan the entire string in lexer->buf */
 
diff --git a/lib/libesp32/berry/src/be_strlib.c b/lib/libesp32/berry/src/be_strlib.c
index 6743c21e2..2ce560910 100644
--- a/lib/libesp32/berry/src/be_strlib.c
+++ b/lib/libesp32/berry/src/be_strlib.c
@@ -549,7 +549,7 @@ static const char* skip2dig(const char *s)
     return s;
 }
 
-static const char* get_mode(const char *str, char *buf)
+static const char* get_mode(const char *str, char *buf, size_t buf_len)
 {
     const char *p = str;
     while (*p && strchr(FLAGES, *p)) { /* skip flags */
@@ -560,8 +560,13 @@ static const char* get_mode(const char *str, char *buf)
         p = skip2dig(++p); /* skip width (2 digits at most) */
     }
     *(buf++) = '%';
-    strncpy(buf, str, p - str + 1);
-    buf[p - str + 1] = '\0';
+    size_t mode_size = p - str + 1;
+    /* Leave 2 bytes for the leading % and the trailing '\0' */
+    if (mode_size > buf_len - 2) { 
+        mode_size = buf_len - 2;
+    }
+    strncpy(buf, str, mode_size);
+    buf[mode_size] = '\0';
     return p;
 }
 
@@ -632,7 +637,7 @@ int be_str_format(bvm *vm)
             }
             pushstr(vm, format, p - format);
             concat2(vm);
-            p = get_mode(p + 1, mode);
+            p = get_mode(p + 1, mode, sizeof(mode));
             buf[0] = '\0';
             if (index > top && *p != '%') {
                 be_raise(vm, "runtime_error", be_pushfstring(vm,
diff --git a/lib/libesp32/berry/tests/lexer.be b/lib/libesp32/berry/tests/lexer.be
index 2114cab54..db2945bc7 100644
--- a/lib/libesp32/berry/tests/lexer.be
+++ b/lib/libesp32/berry/tests/lexer.be
@@ -36,6 +36,10 @@ check(45.1e2, 4510)
 check(45.e2, 4500)
 check(45.e+2, 4500)
 
+# Ensure pathologically long numbers don't crash the lexer (or cause an buffer overflow)
+assert(000000000000000000000000000000000000E0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 == 0.0);
+
+
 test_source('x = 5; 0...x;', 'unexpected symbol near \'.\'')
 test_source('x = 5; 0...x;', 'unexpected symbol near \'.\'')
 test_source('0xg', 'invalid hexadecimal number')
diff --git a/lib/libesp32/berry/tests/string.be b/lib/libesp32/berry/tests/string.be
index 15b403bb5..448ca5215 100644
--- a/lib/libesp32/berry/tests/string.be
+++ b/lib/libesp32/berry/tests/string.be
@@ -149,6 +149,9 @@ assert(string.format("%s", false) == 'false')
 
 assert(string.format("%q", "\ntest") == '\'\\ntest\'')
 
+# corrupt format string should not crash the VM
+string.format("%0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000f", 3.5)
+
 # format is now synonym to string.format
 assert(format == string.format)
 assert(format("%.1f", 3) == '3.0')