diff --git a/src/coremods/core_oper.rs b/src/coremods/core_oper.rs
index 6cdf0f3..99e2560 100644
--- a/src/coremods/core_oper.rs
+++ b/src/coremods/core_oper.rs
@@ -131,15 +131,20 @@ impl Command for Oper {
             s.numeric(uid, ERR_PASSWDMISMATCH, ":Password incorrect");
             return CmdResult::Fail;
         };
-        // bcrypt is slow — verify it off the core thread (result arrives as OperAuth).
-        if hash.starts_with("$2") {
-            if !s.spawn_auth(uid, hash, pass, level) {
+        // a KDF password (bcrypt / pbkdf2) is slow — verify it off the core thread
+        // (result arrives as OperAuth), so it can't freeze the server or be a DoS.
+        if crate::modules::password_hash::is_slow(&hash) {
+            let ok = s.spawn_crypto(move || {
+                let ok = crate::modules::password_hash::verify(&hash, &pass);
+                crate::ircd::Event::OperAuth { uid, ok, level }
+            });
+            if !ok {
                 s.numeric(uid, ERR_PASSWDMISMATCH, ":Too many auth attempts, try again");
                 return CmdResult::Fail;
             }
             return CmdResult::Ok; // pending; oper-up happens when the verify returns
         }
-        // fast hashes (plaintext / sha* / pbkdf2) verify inline
+        // fast hashes (plaintext / sha*) verify inline
         if crate::modules::password_hash::verify(&hash, &pass) {
             s.oper_up(uid);
             crate::modules::operlevels::set(s, uid, level); // operlevels: KILL protection
diff --git a/src/ircd.rs b/src/ircd.rs
index acb5bac..598bfe0 100644
--- a/src/ircd.rs
+++ b/src/ircd.rs
@@ -53,13 +53,19 @@ pub enum Event {
         uid: Uid,
         ident: Option<String>,
     },
-    /// A background OPER password verify finished. bcrypt is deliberately slow, so it
-    /// runs on a worker thread (see `Server::spawn_auth`) instead of freezing the core.
+    /// A background OPER password verify finished. KDF hashes are deliberately slow,
+    /// so they run on a worker thread (see `Server::spawn_crypto`), not on the core.
     OperAuth {
         uid: Uid,
         ok: bool,
         level: u32,
     },
+    /// A background MKPASSWD hash finished (KDFs run off the core thread).
+    MkpasswdResult {
+        uid: Uid,
+        algo: String,
+        hash: Option<String>,
+    },
     /// A module's async HTTP request finished. `tag` is `"<module>:<detail>"`
     /// so the core can route the reply back to the module that issued it (e.g.
     /// account registration, captcha verification). `status` is 0 on transport
@@ -219,6 +225,22 @@ impl Ircd {
                         .numeric(uid, ERR_PASSWDMISMATCH, ":Password incorrect");
                 }
             }
+            Event::MkpasswdResult { uid, algo, hash } => {
+                let nick = self
+                    .server
+                    .users
+                    .get(&uid)
+                    .map(|u| u.nick.clone())
+                    .unwrap_or_default();
+                let line = match hash {
+                    Some(h) => format!(
+                        ":{} NOTICE {nick} :{algo} hashed password: {h}",
+                        self.server.name
+                    ),
+                    None => format!(":{} NOTICE {nick} :Could not hash with '{algo}'", self.server.name),
+                };
+                self.server.send(uid, line);
+            }
             Event::HttpResult {
                 uid,
                 tag,
diff --git a/src/modules/password_hash.rs b/src/modules/password_hash.rs
index 9582f59..1b0b602 100644
--- a/src/modules/password_hash.rs
+++ b/src/modules/password_hash.rs
@@ -129,6 +129,18 @@ fn make(algo: &str, plaintext: &str) -> Option<String> {
     Some(format!("{}:{}", algo.to_ascii_lowercase(), hex(&d)))
 }
 
+/// Whether *verifying* this stored credential is a deliberately-slow KDF (bcrypt or
+/// pbkdf2) that should run off the core thread rather than inline.
+pub fn is_slow(stored: &str) -> bool {
+    stored.starts_with("$2") || stored.starts_with("pbkdf2:")
+}
+
+/// Whether *producing* a hash with this algorithm name is a slow KDF (for MKPASSWD).
+pub fn is_slow_algo(algo: &str) -> bool {
+    let a = algo.to_ascii_lowercase();
+    a == "bcrypt" || a.starts_with("bcrypt:") || a == "pbkdf2"
+}
+
 pub fn commands() -> Vec<Box<dyn Command>> {
     vec![Box::new(MkPasswd)]
 }
@@ -152,13 +164,28 @@ impl Command for MkPasswd {
             );
             return CmdResult::Fail;
         }
-        let (algo, pass) = (&params[0], &params[1]);
+        let (algo, pass) = (params[0].clone(), params[1].clone());
         let nick = s
             .users
             .get(&uid)
             .map(|u| u.nick.clone())
             .unwrap_or_default();
-        match make(algo, pass) {
+        // a KDF (bcrypt / pbkdf2) is slow — hash it off the core thread (result comes
+        // back as MkpasswdResult) so an oper's MKPASSWD can't freeze the whole server.
+        if is_slow_algo(&algo) {
+            let started = s.spawn_crypto(move || {
+                let hash = make(&algo, &pass);
+                crate::ircd::Event::MkpasswdResult { uid, algo, hash }
+            });
+            if !started {
+                s.send(
+                    uid,
+                    format!(":{} NOTICE {nick} :Busy hashing, try again", s.name),
+                );
+            }
+            return CmdResult::Ok;
+        }
+        match make(&algo, &pass) {
             Some(hashed) => {
                 s.send(
                     uid,
diff --git a/src/server.rs b/src/server.rs
index eca7157..39854c6 100644
--- a/src/server.rs
+++ b/src/server.rs
@@ -450,24 +450,36 @@ impl Server {
         });
     }
 
-    /// Verify an OPER password on a worker thread, delivering the result back as
-    /// `Event::OperAuth`. bcrypt is deliberately expensive (tens to hundreds of ms),
-    /// so running it inline would freeze the single-threaded core — and an OPER flood
-    /// against a bcrypt block would be a trivial DoS. Bounded so the flood can't spawn
-    /// unlimited hash threads; returns `false` when at capacity.
-    pub fn spawn_auth(&self, uid: Uid, hash: String, pass: String, level: u32) -> bool {
+    /// Run an expensive credential operation (a KDF: bcrypt / pbkdf2) on a worker
+    /// thread and deliver its result back as the `Event` the closure builds. These
+    /// hashes are deliberately slow (tens to hundreds of ms), so running one inline
+    /// would freeze the single-threaded core — and a flood of them (OPER, TITLE, …)
+    /// against a KDF credential would be a trivial DoS. Bounded so the flood can't
+    /// spawn unlimited threads; returns `false` when at capacity (the caller then
+    /// rejects the attempt). A `Drop` guard keeps the counter correct even if the
+    /// closure panics.
+    pub fn spawn_crypto<F>(&self, f: F) -> bool
+    where
+        F: FnOnce() -> crate::ircd::Event + Send + 'static,
+    {
         use std::sync::atomic::{AtomicUsize, Ordering};
         static ACTIVE: AtomicUsize = AtomicUsize::new(0);
         const MAX_ACTIVE: usize = 16;
+        struct Guard;
+        impl Drop for Guard {
+            fn drop(&mut self) {
+                ACTIVE.fetch_sub(1, Ordering::Relaxed);
+            }
+        }
         if ACTIVE.fetch_add(1, Ordering::Relaxed) >= MAX_ACTIVE {
             ACTIVE.fetch_sub(1, Ordering::Relaxed);
             return false;
         }
         let tx = self.event_tx.clone();
         std::thread::spawn(move || {
-            let ok = crate::modules::password_hash::verify(&hash, &pass);
-            ACTIVE.fetch_sub(1, Ordering::Relaxed);
-            let _ = tx.send(crate::ircd::Event::OperAuth { uid, ok, level });
+            let _guard = Guard; // decrements even on panic
+            let ev = f();
+            let _ = tx.send(ev);
         });
         true
     }