diff --git a/src/flexdef.h b/src/flexdef.h
index adf4fd0a7..2f2f93cc1 100644
--- a/src/flexdef.h
+++ b/src/flexdef.h
@@ -849,6 +849,10 @@ extern void dataflush(void);
 /* Report an error message and terminate. */
 extern void flexerror(const char *);
 
+/* Reject `[` / `]` in a %option= value to prevent m4-quote injection. */
+extern void reject_brackets_in_option_value(const char *opt_name,
+					     const char *val);
+
 /* Report a fatal error message and terminate. */
 extern void flexfatal(const char *);
 
diff --git a/src/misc.c b/src/misc.c
index 56d2abab6..39b5e34f3 100644
--- a/src/misc.c
+++ b/src/misc.c
@@ -205,6 +205,30 @@ void flexerror (const char *msg)
 }
 
 
+/* reject_brackets_in_option_value - bail out if a %option= value contains
+ * `[` or `]`.  Such characters break out of the m4 quoting scheme that
+ * flex uses to interpolate option values into m4 input (see PR #81 which
+ * added the same escaping for action / code-block contexts via
+ * ESCAPED_QSTART / ESCAPED_QEND in src/scan.l).  The OPTION-state
+ * quoted-string rule at src/scan.l:480 was never plumbed through that
+ * escape, so without this guard a malicious .l file can inject arbitrary
+ * m4 (e.g. m4_syscmd) into the output pipeline.
+ *
+ * Mirrors the existing prefix= guard that has been in parse.y since
+ * before PR #81.
+ */
+void reject_brackets_in_option_value (const char *opt_name, const char *val)
+{
+	if (val != NULL && (strchr (val, '[') || strchr (val, ']'))) {
+		char    msg[MAXLINE];
+		snprintf (msg, sizeof msg,
+		          _("%%option %s value must not contain '[' or ']'"),
+		          opt_name);
+		flexerror (msg);
+	}
+}
+
+
 /* flexfatal - report a fatal error message and terminate */
 
 void flexfatal (const char *msg)
diff --git a/src/parse.y b/src/parse.y
index 80e7f0977..830002938 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -199,27 +199,33 @@ optionlist	:  optionlist option
 
 option		:  TOK_OUTFILE '=' NAME
 			{
+			reject_brackets_in_option_value("outfile", nmstr);
 			env.outfilename = xstrdup(nmstr);
 			env.did_outfilename = 1;
 			}
 		|  TOK_EXTRA_TYPE '=' NAME
-			{ extra_type = xstrdup(nmstr); }
+			{ reject_brackets_in_option_value("extra-type", nmstr);
+			  extra_type = xstrdup(nmstr); }
 		|  TOK_PREFIX '=' NAME
-			{ ctrl.prefix = xstrdup(nmstr);
-                          if (strchr(ctrl.prefix, '[') || strchr(ctrl.prefix, ']'))
-                              flexerror(_("Prefix must not contain [ or ]")); }
+			{ reject_brackets_in_option_value("prefix", nmstr);
+			  ctrl.prefix = xstrdup(nmstr); }
 		|  TOK_YYCLASS '=' NAME
-			{ ctrl.yyclass = xstrdup(nmstr); }
+			{ reject_brackets_in_option_value("yyclass", nmstr);
+			  ctrl.yyclass = xstrdup(nmstr); }
 		|  TOK_HEADER_FILE '=' NAME
-			{ env.headerfilename = xstrdup(nmstr); }
+			{ reject_brackets_in_option_value("header-file", nmstr);
+			  env.headerfilename = xstrdup(nmstr); }
 		|  TOK_YYLMAX '=' TOK_NUMERIC
 			{ ctrl.yylmax = nmval; }
 		|  TOK_YYDECL '=' NAME
-			{ ctrl.yydecl = xstrdup(nmstr); }
+			{ reject_brackets_in_option_value("yydecl", nmstr);
+			  ctrl.yydecl = xstrdup(nmstr); }
 		|  TOK_PREACTION '=' NAME
-			{ ctrl.preaction = xstrdup(nmstr); }
+			{ reject_brackets_in_option_value("pre-action", nmstr);
+			  ctrl.preaction = xstrdup(nmstr); }
 		|  TOK_POSTACTION '=' NAME
-			{ ctrl.postaction = xstrdup(nmstr); }
+			{ reject_brackets_in_option_value("post-action", nmstr);
+			  ctrl.postaction = xstrdup(nmstr); }
 		|  TOK_BUFSIZE '=' TOK_NUMERIC
 			{ ctrl.bufsize = nmval; }
 		|  TOK_EMIT '=' NAME
@@ -227,7 +233,8 @@ option		:  TOK_OUTFILE '=' NAME
 		|  TOK_USERINIT '=' NAME
 			{ ctrl.userinit = xstrdup(nmstr); }
 		|  TOK_YYTERMINATE '=' NAME
-			{ ctrl.yyterminate = xstrdup(nmstr); }
+			{ reject_brackets_in_option_value("yyterminate", nmstr);
+			  ctrl.yyterminate = xstrdup(nmstr); }
 		|  TOK_TABLES_FILE '=' NAME
         		{ tablesext = true; tablesfilename = xstrdup(nmstr); }
 		;