diff --git a/src/cli/such.c b/src/cli/such.c
index b48995251..61522501f 100644
--- a/src/cli/such.c
+++ b/src/cli/such.c
@@ -1210,6 +1210,9 @@ int main(int argc, char* argv[])
     char* mnemonic_in = 0;
     char* pass = 0;
     char* entropy = 0;
+    /* entropy_size points at a literal, so -e needs somewhere writable.
+       + 1 for the terminator ENTROPY_SIZE_STRING_SIZE does not include. */
+    char entropy_size_buf[ENTROPY_SIZE_STRING_SIZE + 1];
     char* entropy_size = "256";
     MNEMONIC mnemonic = {0};
     SEED seed = {0};
@@ -1252,7 +1255,17 @@ int main(int argc, char* argv[])
                         return showError("Parameter -e cannot be used with -y");
                     entropy = optarg;
                     if (entropy != NULL){
-                        sprintf(entropy_size, "%zu", strlen(entropy) / HEX_CHARS_PER_BYTE * 8);
+                        /* Bound first: unbounded optarg gives four or more
+                           digits and snprintf truncates rather than overflows. */
+                        size_t entropy_hex_len = strlen(entropy);
+                        if (entropy_hex_len > MAX_ENTROPY_STRING_SIZE - 1)
+                            return showError("Parameter -e exceeds the maximum entropy size");
+
+                        /* sprintf through the "256" literal wrote to read-only
+                           memory and crashed on every use of -e. */
+                        snprintf(entropy_size_buf, sizeof(entropy_size_buf), "%zu",
+                                 entropy_hex_len / HEX_CHARS_PER_BYTE * 8);
+                        entropy_size = entropy_size_buf;
                     }
 
                     break;