diff --git a/api/src/lib.rs b/api/src/lib.rs
index b075c67..d4b312b 100644
--- a/api/src/lib.rs
+++ b/api/src/lib.rs
@@ -1858,6 +1858,7 @@ pub enum CertError {
     Invalid,    // not a plausible fingerprint
     InUse,      // already registered (to any account)
     NoAccount,  // target account does not exist
+    Full,       // the account's fingerprint list is at its cap
     Internal,   // persistence failed
 }
 
@@ -1866,6 +1867,7 @@ pub enum ChanError {
     Exists,         // channel already registered
     NoChannel,      // channel is not registered
     InvalidPattern, // a badword regex didn't compile
+    Full,           // the list (access / akick) is at its cap
     Internal,       // persistence failed
 }
 
diff --git a/modules/nickserv/src/cert.rs b/modules/nickserv/src/cert.rs
index 8aeaf21..e224ee7 100644
--- a/modules/nickserv/src/cert.rs
+++ b/modules/nickserv/src/cert.rs
@@ -33,6 +33,7 @@ pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, db:
                 Ok(()) => ctx.notice(me, from.uid, format!("Added fingerprint \x02{fp}\x02. You can now log in with SASL EXTERNAL.")),
                 Err(CertError::InUse) => ctx.notice(me, from.uid, "That fingerprint is already registered."),
                 Err(CertError::Invalid) => ctx.notice(me, from.uid, "That does not look like a certificate fingerprint."),
+                Err(CertError::Full) => ctx.notice(me, from.uid, "You have too many fingerprints registered. Remove one first."),
                 Err(CertError::NoAccount | CertError::Internal) => ctx.notice(me, from.uid, "Could not add the fingerprint, please try again later."),
             }
         }
diff --git a/src/engine/db/account.rs b/src/engine/db/account.rs
index 7f61bf9..f5060dc 100644
--- a/src/engine/db/account.rs
+++ b/src/engine/db/account.rs
@@ -632,8 +632,14 @@ impl Db {
             return Err(CertError::InUse);
         }
         let k = key(account);
-        if !self.accounts.contains_key(&k) {
+        let Some(acct) = self.accounts.get(&k) else {
             return Err(CertError::NoAccount);
+        };
+        // Cap the list so an account can't grow certfps (and the replicated log)
+        // without bound by looping CERT ADD with fabricated fingerprints.
+        const MAX_CERTFP: usize = 25;
+        if acct.certfps.len() >= MAX_CERTFP {
+            return Err(CertError::Full);
         }
         self.log.append(Event::CertAdded { account: account.to_string(), fp: fp.clone() }).map_err(|_| CertError::Internal)?;
         self.accounts.get_mut(&k).unwrap().certfps.push(fp);
diff --git a/src/engine/db/channel.rs b/src/engine/db/channel.rs
index 04ce736..011f270 100644
--- a/src/engine/db/channel.rs
+++ b/src/engine/db/channel.rs
@@ -1,5 +1,9 @@
 use super::*;
 
+// Per-channel cap on the access and akick lists, bounding the event-log growth an
+// op can drive (an event-sourced log is never reclaimed, even by later DEL).
+const MAX_CHAN_LIST: usize = 256;
+
 impl Db {
     /// Register `name` to `founder` (an account name).
     pub fn register_channel(&mut self, name: &str, founder: &str) -> Result<(), ChanError> {
@@ -68,8 +72,12 @@ impl Db {
     /// Grant `account` a level ("op"/"voice") on `channel`.
     pub fn access_add(&mut self, channel: &str, account: &str, level: &str) -> Result<(), ChanError> {
         let k = key(channel);
-        if !self.channels.contains_key(&k) {
+        let Some(c0) = self.channels.get(&k) else {
             return Err(ChanError::NoChannel);
+        };
+        // Same unbounded-growth cap as akick; updating an existing entry is exempt.
+        if c0.access.len() >= MAX_CHAN_LIST && !c0.access.iter().any(|a| a.account.eq_ignore_ascii_case(account)) {
+            return Err(ChanError::Full);
         }
         self.log
             .append(Event::ChannelAccessAdd { channel: channel.to_string(), account: account.to_string(), level: level.to_string() })
@@ -99,8 +107,13 @@ impl Db {
     /// Add an auto-kick `mask` (with `reason`) to `channel`.
     pub fn akick_add(&mut self, channel: &str, mask: &str, reason: &str) -> Result<(), ChanError> {
         let k = key(channel);
-        if !self.channels.contains_key(&k) {
+        let Some(c0) = self.channels.get(&k) else {
             return Err(ChanError::NoChannel);
+        };
+        // Cap the list (and the append-only log) so an op can't grow it without
+        // bound; a replace of an existing mask doesn't count toward the limit.
+        if c0.akick.len() >= MAX_CHAN_LIST && !c0.akick.iter().any(|a| a.mask.eq_ignore_ascii_case(mask)) {
+            return Err(ChanError::Full);
         }
         self.log
             .append(Event::ChannelAkickAdd { channel: channel.to_string(), mask: mask.to_string(), reason: reason.to_string() })