diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 92982ee0..25aa3d2d 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -1,6 +1,10 @@
 Changelog
 =========
 
+3.3.13 (Unreleased)
+----------------------
+- Fix #512: Validate `from` pagination cursor to prevent unbounded conversation loading
+
 3.3.12 (July 16, 2026)
 ----------------------
 - Fix #504: Message entries overflow horizontally on iOS Safari
diff --git a/models/Message.php b/models/Message.php
index bc124ab6..ddcf10a4 100644
--- a/models/Message.php
+++ b/models/Message.php
@@ -62,10 +62,18 @@ public function getEntryUpdates($from = null)
         $query = $this->hasMany(MessageEntry::class, ['message_id' => 'id']);
         $query->addOrderBy(['created_at' => SORT_ASC]);
 
-        if ($from) {
+        // Normalize $from: only a strictly positive integer counts as a valid cursor.
+        // (Avoids PHP loose-comparison pitfalls, e.g. the string "0" being falsy but != null.)
+        $from = is_numeric($from) ? (int) $from : null;
+
+        if ($from !== null && $from > 0) {
             $query->andWhere(['>', 'message_entry.id', $from]);
         }
 
+        // Always bound the result set, otherwise an invalid/empty cursor (e.g. from=0 or
+        // no cursor at all) would load the entire conversation in one go.
+        $query->limit(Module::getModuleInstance()->conversationUpdatePageSize);
+
         return $query;
     }
 
@@ -78,12 +86,16 @@ public function getEntryPage($from = null)
         $query = $this->getEntries();
         $query->addOrderBy(['created_at' => SORT_DESC]);
 
-        if ($from) {
+        // Normalize $from: only a strictly positive integer counts as a valid cursor.
+        $from = is_numeric($from) ? (int) $from : null;
+        $hasCursor = ($from !== null && $from > 0);
+
+        if ($hasCursor) {
             $query->andWhere(['<', 'message_entry.id', $from]);
         }
 
         $module = Module::getModuleInstance();
-        $limit = $from ? $module->conversationUpdatePageSize : $module->conversationInitPageSize;
+        $limit = $hasCursor ? $module->conversationUpdatePageSize : $module->conversationInitPageSize;
         $query->limit($limit);
 
         return array_reverse($query->all());
diff --git a/module.json b/module.json
index 211a8abe..1660bf24 100644
--- a/module.json
+++ b/module.json
@@ -8,7 +8,7 @@
     "messenger",
     "communication"
   ],
-  "version": "3.3.12",
+  "version": "3.3.13",
   "humhub": {
     "minVersion": "1.18.1",
     "maxVersion": "1.18"
diff --git a/tests/codeception/unit/MessageEntryPaginationTest.php b/tests/codeception/unit/MessageEntryPaginationTest.php
new file mode 100644
index 00000000..2f1baff5
--- /dev/null
+++ b/tests/codeception/unit/MessageEntryPaginationTest.php
@@ -0,0 +1,162 @@
+<?php
+
+use humhub\modules\mail\models\forms\CreateMessage;
+use humhub\modules\mail\models\Message;
+use humhub\modules\mail\models\MessageEntry;
+use humhub\modules\user\models\User;
+use tests\codeception\_support\HumHubDbTestCase;
+
+/**
+ * Regression tests for the pagination cursor handling in Message::getEntryUpdates()
+ * and Message::getEntryPage().
+ *
+ * Background: both methods used `if ($from)` to decide whether a pagination cursor
+ * was given. Because PHP casts the string "0" to false, a request with `from=0`
+ * was silently treated the same as "no cursor at all". In getEntryUpdates() this
+ * additionally meant no limit() was ever applied, so the entire conversation
+ * history was loaded in a single query - a self-inflicted DoS on large
+ * conversations. See cipher-lab.org advisory, 2026-08-08.
+ */
+class MessageEntryPaginationTest extends HumHubDbTestCase
+{
+    /**
+     * Creates a fresh conversation between User1 (id=2) and User2 (id=3), logged in
+     * as User1. The conversation already contains exactly one entry afterwards,
+     * with a deterministic created_at (offset 0).
+     */
+    private function createConversation(): Message
+    {
+        $this->becomeUser('User1');
+
+        $form = new CreateMessage([
+            'title' => 'Pagination Test',
+            'message' => 'Entry 0',
+            'recipient' => [User::findOne(['id' => 3])->guid],
+        ]);
+
+        $this->assertTrue($form->save(), 'Message creation failed: ' . json_encode($form->getErrors()));
+
+        $message = $form->messageInstance;
+        $this->setEntryCreatedAt($message->getLastEntry(), 0);
+
+        return $message;
+    }
+
+    /**
+     * Appends $count additional entries to $message, as the currently logged in
+     * user, each one second apart (starting one second after the initial entry)
+     * so their relative order is always unambiguous.
+     */
+    private function addEntries(Message $message, int $count): void
+    {
+        for ($i = 0; $i < $count; $i++) {
+            $entry = MessageEntry::createForMessage($message, Yii::$app->user->getIdentity(), 'Extra entry ' . $i);
+            $this->assertTrue($entry->save(), 'Entry creation failed: ' . json_encode($entry->getErrors()));
+            $this->setEntryCreatedAt($entry, $i + 1);
+        }
+    }
+
+    /**
+     * Forces a deterministic created_at on an already-saved entry, $secondsOffset
+     * seconds after a fixed base time.
+     */
+    private function setEntryCreatedAt(MessageEntry $entry, int $secondsOffset): void
+    {
+        $entry->created_at = (new DateTime('2026-01-01 00:00:00'))
+            ->modify("+{$secondsOffset} seconds")
+            ->format('Y-m-d H:i:s');
+
+        $this->assertTrue($entry->save(false), 'Failed to set deterministic created_at on entry ' . $entry->id);
+    }
+
+    public function testGetEntryUpdatesIsAlwaysBoundedByPageSize(): void
+    {
+        $module = Yii::$app->getModule('mail');
+        $module->conversationUpdatePageSize = 3;
+
+        $message = $this->createConversation();
+        $this->addEntries($message, 10); // 11 entries total, well above the page size of 3
+
+        // Every one of these must be treated as "no valid cursor" and still be
+        // capped at conversationUpdatePageSize - none of them must load everything.
+        foreach ([null, '0', 0, '', false] as $from) {
+            $entries = $message->getEntryUpdates($from)->all();
+            $this->assertLessThanOrEqual(
+                3,
+                count($entries),
+                'getEntryUpdates(' . var_export($from, true) . ') must never exceed conversationUpdatePageSize'
+            );
+        }
+    }
+
+    public function testGetEntryUpdatesFromZeroBehavesLikeNoCursor(): void
+    {
+        $module = Yii::$app->getModule('mail');
+        $module->conversationUpdatePageSize = 50; // large enough to not truncate any of these results
+
+        $message = $this->createConversation();
+        $this->addEntries($message, 4); // 5 entries total
+
+        $withNull = array_map(fn($e) => $e->id, $message->getEntryUpdates(null)->all());
+        $withStringZero = array_map(fn($e) => $e->id, $message->getEntryUpdates('0')->all());
+        $withIntZero = array_map(fn($e) => $e->id, $message->getEntryUpdates(0)->all());
+
+        $this->assertSame($withNull, $withStringZero, 'from="0" must be treated the same as no cursor at all');
+        $this->assertSame($withNull, $withIntZero, 'from=0 (int) must be treated the same as no cursor at all');
+    }
+
+    public function testGetEntryUpdatesRespectsARealPositiveCursor(): void
+    {
+        $module = Yii::$app->getModule('mail');
+        $module->conversationUpdatePageSize = 50;
+
+        $message = $this->createConversation();
+        $this->addEntries($message, 4); // 5 entries total, ascending ids
+
+        $allEntries = $message->getEntryUpdates(null)->all();
+        $this->assertCount(5, $allEntries);
+
+        $cursor = $allEntries[1]->id; // second oldest entry
+        $expectedRemainingIds = array_map(fn($e) => $e->id, array_slice($allEntries, 2));
+
+        $filtered = array_map(fn($e) => $e->id, $message->getEntryUpdates($cursor)->all());
+
+        $this->assertSame($expectedRemainingIds, $filtered);
+    }
+
+    public function testGetEntryPageFromZeroBehavesLikeNoCursor(): void
+    {
+        $module = Yii::$app->getModule('mail');
+        $module->conversationInitPageSize = 3;
+        $module->conversationUpdatePageSize = 3;
+
+        $message = $this->createConversation();
+        $this->addEntries($message, 10); // 11 entries total
+
+        $withNull = array_map(fn($e) => $e->id, $message->getEntryPage(null));
+        $withStringZero = array_map(fn($e) => $e->id, $message->getEntryPage('0'));
+
+        $this->assertSame($withNull, $withStringZero, 'getEntryPage("0") must behave like getEntryPage(null)');
+        $this->assertCount(3, $withStringZero);
+    }
+
+    public function testGetEntryPageRespectsARealPositiveCursor(): void
+    {
+        $module = Yii::$app->getModule('mail');
+        $module->conversationInitPageSize = 50;
+        $module->conversationUpdatePageSize = 50;
+
+        $message = $this->createConversation();
+        $this->addEntries($message, 4); // 5 entries total, ascending ids
+
+        $allEntries = $message->getEntryUpdates(null)->all();
+        $this->assertCount(5, $allEntries);
+
+        $cursor = $allEntries[3]->id; // fourth oldest entry (index 3)
+        $expectedOlderIds = array_map(fn($e) => $e->id, array_slice($allEntries, 0, 3));
+
+        $page = array_map(fn($e) => $e->id, $message->getEntryPage($cursor));
+
+        $this->assertSame($expectedOlderIds, $page);
+    }
+}