From 2c34aec1cf9e67f9e74dfd56f1b7a18cdddaf311 Mon Sep 17 00:00:00 2001
From: Sadie Powell
Date: Wed, 8 Apr 2026 16:04:39 +0100
Subject: Truncate overly-long usernames in USER.

---
 src/coremods/core_user/cmd_user.cpp | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/coremods/core_user/cmd_user.cpp b/src/coremods/core_user/cmd_user.cpp
index b1a337ad1..a8a795858 100644
--- a/src/coremods/core_user/cmd_user.cpp
+++ b/src/coremods/core_user/cmd_user.cpp
@@ -42,10 +42,16 @@ CommandUser::CommandUser(Module* parent)
 CmdResult CommandUser::HandleLocal(LocalUser* user, const Params& parameters)
 {
 	/* A user may only send the USER command once */
-	const auto& newuser = parameters[0];
 	const auto& newreal = parameters[3];
 	if (!(user->connected & User::CONN_USER))
 	{
+		// If the username is too long then we have to truncate it to mimic the
+		// undocumented behaviour of other IRC servers. Failing to do so will
+		// break clients.
+		auto newuser = parameters[0];
+		if (newuser.length() > ServerInstance->Config->Limits.MaxUser)
+			newuser.erase(ServerInstance->Config->Limits.MaxUser);
+
 		if (!ServerInstance->IsUser(newuser))
 		{
 			user->WriteNumeric(ERR_INVALIDUSERNAME, newuser, "Your username is not valid");
-- 
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