diff --git a/mysql-test/suite/maria/maria-recovery-corrupt-add-prefix.result b/mysql-test/suite/maria/maria-recovery-corrupt-add-prefix.result
new file mode 100644
index 0000000000000..9983933d4f03c
--- /dev/null
+++ b/mysql-test/suite/maria/maria-recovery-corrupt-add-prefix.result
@@ -0,0 +1,87 @@
+call mtr.add_suppression("File '.*aria_log.000.*' not found");
+drop database if exists mysqltest;
+create database mysqltest;
+connect  admin, localhost, root,,mysqltest,,;
+connection default;
+use mysqltest;
+connection default;
+connection admin;
+* shut down mysqld, removed logs, restarted it
+connection default;
+create table t1 (a int, b varchar(200), key(b))
+transactional=1 row_format=page engine=aria;
+insert into t1 (a,b)
+select seq, concat('k',lpad(seq*7919%100003,6,'0'),repeat('q',150))
+from seq_1_to_2000;
+set session debug_dbug="+d,corrupt_add_prefix_changed";
+delete from t1 where a % 4 <> 0;
+select count(*) from t1;
+count(*)
+500
+set session debug_dbug="+d,maria_flush_whole_log,maria_crash";
+set global aria_checkpoint_interval=1;
+ERROR HY000: Lost connection to server during query
+# Recovery must refuse the record, not apply it and notice later.
+FOUND 1 /Aria engine: Redo phase failed/ in mdev40497.err
+NOT FOUND /header == header_end|insert_length <= changed_length/ in mdev40497.err
+use mysqltest;
+#
+# Case 2: the logged insert_length pushes the move past the page.
+#
+create table t2 (a int, b varchar(200), key(b))
+transactional=1 row_format=page engine=aria;
+insert into t2 (a,b)
+select seq, concat('k',lpad(seq*7919%100003,6,'0'),repeat('q',150))
+from seq_1_to_2000;
+set session debug_dbug="+d,corrupt_add_prefix_insert";
+delete from t2 where a % 4 <> 0;
+select count(*) from t2;
+count(*)
+500
+set session debug_dbug="+d,maria_flush_whole_log,maria_crash";
+set global aria_checkpoint_interval=1;
+ERROR HY000: Lost connection to server during query
+FOUND 1 /Aria engine: Redo phase failed/ in mdev40497b.err
+NOT FOUND /header == header_end|insert_length <= changed_length/ in mdev40497b.err
+use mysqltest;
+#
+# Case 3: a record that really carries more prefix data than the page
+# can hold. Only the bound on the memcpy destination refuses this one.
+#
+create table t3 (a int, b varchar(200), key(b))
+transactional=1 row_format=page engine=aria;
+insert into t3 (a,b)
+select seq, concat('k',lpad(seq*7919%100003,6,'0'),repeat('q',150))
+from seq_1_to_2000;
+set session debug_dbug="+d,corrupt_add_prefix_big";
+delete from t3 where a % 4 <> 0;
+select count(*) from t3;
+count(*)
+500
+set session debug_dbug="+d,maria_flush_whole_log,maria_crash";
+set global aria_checkpoint_interval=1;
+ERROR HY000: Lost connection to server during query
+FOUND 1 /Aria engine: Redo phase failed/ in mdev40497c.err
+NOT FOUND /header == header_end|insert_length <= changed_length/ in mdev40497c.err
+use mysqltest;
+#
+# Case 4: insert_length larger than changed_length. Every bound above is
+# satisfied, but the record cannot be applied: it would leave the gap
+# between the two lengths holding whatever was on the page.
+#
+create table t4 (a int, b varchar(200), key(b))
+transactional=1 row_format=page engine=aria;
+insert into t4 (a,b)
+select seq, concat('k',lpad(seq*7919%100003,6,'0'),repeat('q',150))
+from seq_1_to_2000;
+set session debug_dbug="+d,corrupt_add_prefix_gap";
+delete from t4 where a % 4 <> 0;
+select count(*) from t4;
+count(*)
+500
+set session debug_dbug="+d,maria_flush_whole_log,maria_crash";
+set global aria_checkpoint_interval=1;
+ERROR HY000: Lost connection to server during query
+FOUND 1 /Aria engine: Redo phase failed/ in mdev40497d.err
+NOT FOUND /header == header_end|insert_length <= changed_length/ in mdev40497d.err
+drop database mysqltest;
diff --git a/mysql-test/suite/maria/maria-recovery-corrupt-add-prefix.test b/mysql-test/suite/maria/maria-recovery-corrupt-add-prefix.test
new file mode 100644
index 0000000000000..913df6d20977c
--- /dev/null
+++ b/mysql-test/suite/maria/maria-recovery-corrupt-add-prefix.test
@@ -0,0 +1,210 @@
+# MDEV-40497: _ma_apply_redo_index() copied KEY_OP_ADD_PREFIX's logged
+# changed_length without checking it against the end of the record, and bounded
+# the page side with a DBUG_ASSERT that release builds compile out and that
+# never bounded changed_length at all.
+
+--source include/not_embedded.inc
+--source include/have_debug.inc
+--source include/have_maria.inc
+--source include/have_sequence.inc
+# the server is killed on purpose below
+--source include/not_valgrind.inc
+
+call mtr.add_suppression("File '.*aria_log.000.*' not found");
+
+let $MYSQLD_DATADIR= `select @@datadir`;
+let $MARIA_LOG=.;
+
+--disable_warnings
+drop database if exists mysqltest;
+--enable_warnings
+create database mysqltest;
+
+# the log is shared with earlier tests, and recovery below replays all of it
+connect (admin, localhost, root,,mysqltest,,);
+--enable_reconnect
+connection default;
+use mysqltest;
+--enable_reconnect
+--source include/maria_empty_logs.inc
+
+# Long keys so the index splits, then a wide delete so pages merge. Only that
+# path logs KEY_OP_ADD_PREFIX; plain inserts never do.
+create table t1 (a int, b varchar(200), key(b))
+transactional=1 row_format=page engine=aria;
+insert into t1 (a,b)
+select seq, concat('k',lpad(seq*7919%100003,6,'0'),repeat('q',150))
+from seq_1_to_2000;
+
+# Case 1: the record claims 100 bytes more prefix data than it carries, so the
+# memcpy reads past the end of the record. The page bound is untouched: the
+# copy still lands inside the page, and both old asserts stay true.
+set session debug_dbug="+d,corrupt_add_prefix_changed";
+delete from t1 where a % 4 <> 0;
+select count(*) from t1;
+
+# Kill without flushing pages, so recovery has to replay the forged records
+--write_line wait $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+set session debug_dbug="+d,maria_flush_whole_log,maria_crash";
+--error 2013
+set global aria_checkpoint_interval=1;
+
+--echo # Recovery must refuse the record, not apply it and notice later.
+# Failed recovery aborts the server, so --bootstrap drives it and exits by
+# itself. A failed redo phase leaves pages pinned, so a debug build then trips
+# the pagecache cleanup assert on the way out; the log below is the assertion,
+# not the exit code.
+--write_file $MYSQLTEST_VARDIR/tmp/mdev40497_boot.sql
+select 1;
+EOF
+
+--error 1,134
+--exec $MYSQLD_CMD --bootstrap --log-error=$MYSQLTEST_VARDIR/tmp/mdev40497.err < $MYSQLTEST_VARDIR/tmp/mdev40497_boot.sql > $MYSQLTEST_VARDIR/tmp/mdev40497.out 2>&1
+
+--let SEARCH_FILE= $MYSQLTEST_VARDIR/tmp/mdev40497.err
+--let SEARCH_PATTERN= Aria engine: Redo phase failed
+--source include/search_pattern_in_file.inc
+
+# Unfixed, the copy runs and the record is only noticed by the debug-only
+# assert at the end of the loop, or by the branch's own assert in case 2.
+--let SEARCH_PATTERN= header == header_end|insert_length <= changed_length
+--source include/search_pattern_in_file.inc
+
+# every prefix insert in this log is forged, it can never be replayed
+--error 0,1
+remove_files_wildcard $MYSQLD_DATADIR aria_log.0*;
+
+--write_line restart $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+
+use mysqltest;
+
+--echo #
+--echo # Case 2: the logged insert_length pushes the move past the page.
+--echo #
+
+create table t2 (a int, b varchar(200), key(b))
+transactional=1 row_format=page engine=aria;
+insert into t2 (a,b)
+select seq, concat('k',lpad(seq*7919%100003,6,'0'),repeat('q',150))
+from seq_1_to_2000;
+
+# The record stays length-consistent here, so the record bound passes and the
+# bound on the bmove_upp destination is what refuses it.
+set session debug_dbug="+d,corrupt_add_prefix_insert";
+delete from t2 where a % 4 <> 0;
+select count(*) from t2;
+
+--write_line wait $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+set session debug_dbug="+d,maria_flush_whole_log,maria_crash";
+--error 2013
+set global aria_checkpoint_interval=1;
+
+--error 1,134
+--exec $MYSQLD_CMD --bootstrap --log-error=$MYSQLTEST_VARDIR/tmp/mdev40497b.err < $MYSQLTEST_VARDIR/tmp/mdev40497_boot.sql > $MYSQLTEST_VARDIR/tmp/mdev40497b.out 2>&1
+
+--let SEARCH_FILE= $MYSQLTEST_VARDIR/tmp/mdev40497b.err
+--let SEARCH_PATTERN= Aria engine: Redo phase failed
+--source include/search_pattern_in_file.inc
+
+--let SEARCH_PATTERN= header == header_end|insert_length <= changed_length
+--source include/search_pattern_in_file.inc
+
+--error 0,1
+remove_files_wildcard $MYSQLD_DATADIR aria_log.0*;
+
+--write_line restart $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+
+use mysqltest;
+
+--echo #
+--echo # Case 3: a record that really carries more prefix data than the page
+--echo # can hold. Only the bound on the memcpy destination refuses this one.
+--echo #
+
+create table t3 (a int, b varchar(200), key(b))
+transactional=1 row_format=page engine=aria;
+insert into t3 (a,b)
+select seq, concat('k',lpad(seq*7919%100003,6,'0'),repeat('q',150))
+from seq_1_to_2000;
+
+set session debug_dbug="+d,corrupt_add_prefix_big";
+delete from t3 where a % 4 <> 0;
+select count(*) from t3;
+
+--write_line wait $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+set session debug_dbug="+d,maria_flush_whole_log,maria_crash";
+--error 2013
+set global aria_checkpoint_interval=1;
+
+--error 1,134
+--exec $MYSQLD_CMD --bootstrap --log-error=$MYSQLTEST_VARDIR/tmp/mdev40497c.err < $MYSQLTEST_VARDIR/tmp/mdev40497_boot.sql > $MYSQLTEST_VARDIR/tmp/mdev40497c.out 2>&1
+
+--let SEARCH_FILE= $MYSQLTEST_VARDIR/tmp/mdev40497c.err
+--let SEARCH_PATTERN= Aria engine: Redo phase failed
+--source include/search_pattern_in_file.inc
+
+--let SEARCH_PATTERN= header == header_end|insert_length <= changed_length
+--source include/search_pattern_in_file.inc
+
+--error 0,1
+remove_files_wildcard $MYSQLD_DATADIR aria_log.0*;
+
+--write_line restart $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+
+use mysqltest;
+
+--echo #
+--echo # Case 4: insert_length larger than changed_length. Every bound above is
+--echo # satisfied, but the record cannot be applied: it would leave the gap
+--echo # between the two lengths holding whatever was on the page.
+--echo #
+
+create table t4 (a int, b varchar(200), key(b))
+transactional=1 row_format=page engine=aria;
+insert into t4 (a,b)
+select seq, concat('k',lpad(seq*7919%100003,6,'0'),repeat('q',150))
+from seq_1_to_2000;
+
+set session debug_dbug="+d,corrupt_add_prefix_gap";
+delete from t4 where a % 4 <> 0;
+select count(*) from t4;
+
+--write_line wait $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+set session debug_dbug="+d,maria_flush_whole_log,maria_crash";
+--error 2013
+set global aria_checkpoint_interval=1;
+
+--error 1,134
+--exec $MYSQLD_CMD --bootstrap --log-error=$MYSQLTEST_VARDIR/tmp/mdev40497d.err < $MYSQLTEST_VARDIR/tmp/mdev40497_boot.sql > $MYSQLTEST_VARDIR/tmp/mdev40497d.out 2>&1
+
+--let SEARCH_FILE= $MYSQLTEST_VARDIR/tmp/mdev40497d.err
+--let SEARCH_PATTERN= Aria engine: Redo phase failed
+--source include/search_pattern_in_file.inc
+
+--let SEARCH_PATTERN= header == header_end|insert_length <= changed_length
+--source include/search_pattern_in_file.inc
+
+--error 0,1
+remove_files_wildcard $MYSQLD_DATADIR aria_log.0*;
+
+--write_line restart $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+
+drop database mysqltest;
+
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497d.err;
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497d.out;
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497.err;
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497.out;
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497b.err;
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497b.out;
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497c.err;
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497c.out;
+remove_file $MYSQLTEST_VARDIR/tmp/mdev40497_boot.sql;
diff --git a/storage/maria/ma_key_recover.c b/storage/maria/ma_key_recover.c
index acec592b9227a..5b0f66ab5e169 100644
--- a/storage/maria/ma_key_recover.c
+++ b/storage/maria/ma_key_recover.c
@@ -363,9 +363,19 @@ my_bool _ma_log_prefix(MARIA_PAGE *ma_page, uint changed_length,
   {
     /* Add prefix */
     DBUG_ASSERT(changed_length >0 && (int) changed_length >= move_length);
+    /* Each keyword forges one operand, so each record trips one check */
+    if (DBUG_IF("corrupt_add_prefix_big"))
+      changed_length= info->s->block_size - info->s->keypage_header;
+    if (DBUG_IF("corrupt_add_prefix_insert"))
+      changed_length= info->s->max_index_block_size - info->s->keypage_header;
     log_pos[0]= KEY_OP_ADD_PREFIX;
-    int2store(log_pos+1, move_length);
-    int2store(log_pos+3, changed_length);
+    int2store(log_pos+1,
+              DBUG_IF("corrupt_add_prefix_insert") ? changed_length :
+              DBUG_IF("corrupt_add_prefix_gap") ? changed_length + 100 :
+              (uint) move_length);
+    int2store(log_pos+3,
+              DBUG_IF("corrupt_add_prefix_changed") ? changed_length + 100
+                                                    : changed_length);
     log_pos+= 5;
   }
 
@@ -1031,8 +1041,15 @@ uint _ma_apply_redo_index(MARIA_HA *info,
       DBUG_PRINT("redo", ("key_op_add_prefix: %u  %u",
                           insert_length, changed_length));
 
-      DBUG_ASSERT(insert_length <= changed_length &&
-                  page_length + insert_length <= max_page_size);
+      if (unlikely((size_t) (header_end - header) < (size_t) changed_length + 4 ||
+                   keypage_header + changed_length > max_page_size ||
+                   page_length + insert_length > max_page_size ||
+                   insert_length > changed_length))
+      {
+        DBUG_ASSERT(!maria_assert_if_crashed_table);
+        result= mark_crashed= 1;
+        goto err;
+      }
 
       bmove_upp(buff + page_length + insert_length, buff + page_length,
                 page_length - keypage_header);