mode: cap mode changes per MODE command (modes=, default 20) and advertise it as the MODES= ISUPPORT token — an uncapped modestring like MODE #c +bbbb… dispatched a handler per letter, each fanning out to the whole channel and every S2S link (amplification DoS); matches InspIRCd's MODES limit

src/coremods/core_mode.rs

@@ -119,11 +119,20 @@ pub fn apply_mode(s: &mut Server, uid: Uid, params: &[String]) -> CmdResult {
     let mut echoed: Vec<String> = Vec::new();
     // (sign, letter, displayed-param) per applied change — for hidemode filtering
     let mut changes: Vec<(char, char, Option<String>)> = Vec::new();
+    // Cap mode changes per command (advertised as MODES=, default 20 like InspIRCd):
+    // otherwise `MODE #c +bbbb…` in one line dispatches hundreds of handlers, each
+    // fanning out to the whole channel and every link — a cheap amplification flood.
+    let max_modes = s.conf_num("modes", 20usize).max(1);
+    let mut processed = 0usize;
     for c in modestring.chars() {
         if c == '+' || c == '-' {
             sign = c;
             continue;
         }
+        if processed >= max_modes {
+            break;
+        }
+        processed += 1;
         let adding = sign == '+';
         let Some(handler) = chan_mode(c) else {
             s.numeric(