diff --git a/Model/StrictImagePayloadValidator.php b/Model/StrictImagePayloadValidator.php
new file mode 100644
index 0000000..981ae58
--- /dev/null
+++ b/Model/StrictImagePayloadValidator.php
@@ -0,0 +1,257 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MarkShust\PolyshellPatch\Model;
+
+use Magento\Framework\Api\Data\ImageContentInterface;
+use Magento\Framework\Exception\LocalizedException;
+use Magento\Framework\Phrase;
+
+/**
+ * OWASP File Upload Cheat Sheet–aligned checks for image payloads:
+ * verify file signatures (magic bytes), confirm decoded content type matches the declared MIME,
+ * cap raw size and pixel dimensions before decode, then re-encode with GD (required) so only
+ * raster data remains (strips appended polyglot bytes, EXIF, etc.).
+ *
+ * Animated GIFs: GD outputs the first frame only — document for merchants if needed.
+ *
+ * @see https://cheatsheetseries.owasp.org/cheatsheets/File_Upload_Cheat_Sheet.html
+ */
+class StrictImagePayloadValidator
+{
+    /**
+     * User-facing invalid-image text (keep in sync with {@see self::getInvalidImagePhrase()}).
+     */
+    public const FAILURE_MESSAGE = 'The image content is invalid. Verify the content and try again.';
+
+    /** Maximum decoded payload size (bytes) before image APIs run — 5 MiB. */
+    public const MAX_DECODED_BYTES = 5242880;
+
+    /**
+     * Reject Base64 input longer than this before base64_decode (no valid payload can exceed MAX_DECODED_BYTES).
+     * Bound: largest L with ⌊L·3/4⌋ ≤ MAX_DECODED_BYTES.
+     */
+    public const MAX_BASE64_INPUT_CHARS = 6990507;
+
+    /** Maximum width/height from getimagesize (IHDR etc.) before imagecreatefromstring — mitigates decompression-bomb headers. */
+    public const MAX_IMAGE_WIDTH = 10000;
+
+    public const MAX_IMAGE_HEIGHT = 10000;
+
+    private const FORMAT_JPEG = 'jpeg';
+    private const FORMAT_PNG = 'png';
+    private const FORMAT_GIF = 'gif';
+
+    /**
+     * Translated phrase for invalid image payload.
+     *
+     * @return Phrase
+     */
+    public function getInvalidImagePhrase(): Phrase
+    {
+        return __('The image content is invalid. Verify the content and try again.');
+    }
+
+    /**
+     * Validate and normalize image payload (magic bytes, MIME, caps, GD re-encode).
+     *
+     * @param string $binary Strict base64-decoded bytes
+     * @param string $declaredMime e.g. image/jpeg
+     * @param ImageContentInterface $imageContent
+     * @return void
+     * @throws LocalizedException
+     */
+    public function assertValidAndNormalize(
+        string $binary,
+        string $declaredMime,
+        ImageContentInterface $imageContent
+    ): void {
+        if ($binary === '') {
+            $this->fail();
+        }
+
+        if (strlen($binary) > self::MAX_DECODED_BYTES) {
+            $this->fail();
+        }
+
+        if (!extension_loaded('gd') || !function_exists('imagecreatefromstring')) {
+            $this->fail();
+        }
+
+        $this->assertMagicBytesAndParserMimeMatchDeclared($binary, $declaredMime);
+        $this->reencodeWithGd($binary, $declaredMime, $imageContent);
+    }
+
+    /**
+     * Throw standardized invalid-image exception.
+     *
+     * @return never
+     * @throws LocalizedException
+     */
+    private function fail(): void
+    {
+        throw new LocalizedException($this->getInvalidImagePhrase());
+    }
+
+    /**
+     * Verify magic bytes and parser MIME match declared type.
+     *
+     * List allowed extensions → verify magic bytes; verify file content matches extension (parser MIME vs declared).
+     * Parallels OWASP: allowlist + verify file signature + verify content matches type.
+     *
+     * @param string $binary
+     * @param string $declaredMime
+     * @return void
+     */
+    private function assertMagicBytesAndParserMimeMatchDeclared(string $binary, string $declaredMime): void
+    {
+        $expectedFormat = $this->declaredMimeToFormat($declaredMime);
+        if ($expectedFormat === null) {
+            $this->fail();
+        }
+
+        $magicFormat = $this->detectFormatFromMagicBytes($binary);
+        if ($magicFormat === null || $magicFormat !== $expectedFormat) {
+            $this->fail();
+        }
+
+        $props = getimagesizefromstring($binary);
+        if ($props === false || empty($props['mime']) || empty($props[0]) || empty($props[1])) {
+            $this->fail();
+        }
+
+        if ($this->declaredMimeToFormat($props['mime']) !== $expectedFormat) {
+            $this->fail();
+        }
+
+        if ($props[0] > self::MAX_IMAGE_WIDTH || $props[1] > self::MAX_IMAGE_HEIGHT) {
+            $this->fail();
+        }
+    }
+
+    /**
+     * Map MIME type to internal format key.
+     *
+     * @param string $mime
+     * @return string|null
+     */
+    private function declaredMimeToFormat(string $mime): ?string
+    {
+        $mime = strtolower(trim($mime));
+        switch ($mime) {
+            case 'image/jpeg':
+            case 'image/jpg':
+                return self::FORMAT_JPEG;
+            case 'image/png':
+                return self::FORMAT_PNG;
+            case 'image/gif':
+                return self::FORMAT_GIF;
+            default:
+                return null;
+        }
+    }
+
+    /**
+     * Detect image format from magic bytes.
+     *
+     * @param string $binary
+     * @return string|null
+     */
+    private function detectFormatFromMagicBytes(string $binary): ?string
+    {
+        if (strlen($binary) < 8) {
+            return null;
+        }
+
+        if (strncmp($binary, "\xFF\xD8\xFF", 3) === 0) {
+            return self::FORMAT_JPEG;
+        }
+
+        if (strncmp($binary, "\x89PNG\r\n\x1a\n", 8) === 0) {
+            return self::FORMAT_PNG;
+        }
+
+        if (strlen($binary) >= 6
+            && (strncmp($binary, 'GIF87a', 6) === 0 || strncmp($binary, 'GIF89a', 6) === 0)
+        ) {
+            return self::FORMAT_GIF;
+        }
+
+        return null;
+    }
+
+    /**
+     * Re-encode with GD and replace base64 on the DTO.
+     *
+     * @param string $binary
+     * @param string $declaredMime
+     * @param ImageContentInterface $imageContent
+     * @return void
+     */
+    private function reencodeWithGd(
+        string $binary,
+        string $declaredMime,
+        ImageContentInterface $imageContent
+    ): void {
+        // phpcs:disable Magento2.Functions.DiscouragedFunction -- GD decode/destroy required for re-encode pipeline
+        $image = imagecreatefromstring($binary);
+        if ($image === false) {
+            $this->fail();
+        }
+
+        try {
+            $output = $this->encodeGdResourceToDeclaredMime($image, $declaredMime);
+        } finally {
+            imagedestroy($image);
+        }
+        // phpcs:enable Magento2.Functions.DiscouragedFunction
+
+        $imageContent->setBase64EncodedData(base64_encode($output));
+    }
+
+    /**
+     * Encode GD resource to output bytes for the declared MIME type.
+     *
+     * @param \GdImage|resource $image
+     * @param string $declaredMime
+     * @return string
+     */
+    private function encodeGdResourceToDeclaredMime($image, string $declaredMime): string
+    {
+        // phpcs:disable Magento2.Functions.DiscouragedFunction -- GD output capture requires output buffering and image*()
+        ob_start();
+        try {
+            switch ($declaredMime) {
+                case 'image/jpeg':
+                case 'image/jpg':
+                    if (!imagejpeg($image, null, 92)) {
+                        $this->fail();
+                    }
+                    break;
+                case 'image/png':
+                    if (!imagepng($image, null, 6)) {
+                        $this->fail();
+                    }
+                    break;
+                case 'image/gif':
+                    if (!imagegif($image)) {
+                        $this->fail();
+                    }
+                    break;
+                default:
+                    $this->fail();
+            }
+            $buffer = ob_get_contents();
+        } finally {
+            ob_end_clean();
+        }
+        // phpcs:enable Magento2.Functions.DiscouragedFunction
+
+        if ($buffer === false || $buffer === '') {
+            $this->fail();
+        }
+
+        return $buffer;
+    }
+}
diff --git a/Plugin/ImageContentValidatorExtension.php b/Plugin/ImageContentValidatorExtension.php
index a7dd627..2f1614c 100644
--- a/Plugin/ImageContentValidatorExtension.php
+++ b/Plugin/ImageContentValidatorExtension.php
@@ -6,9 +6,8 @@
 
 use Magento\Framework\Api\Data\ImageContentInterface;
 use Magento\Framework\Api\ImageContentValidator;
-use Magento\Framework\Exception\InputException;
+use Magento\Framework\Exception\LocalizedException;
 use Magento\Framework\Filesystem\Io\File as IoFile;
-use Magento\Framework\Phrase;
 
 /**
  * Validate that the uploaded filename has a safe image extension.
@@ -41,7 +40,7 @@ public function __construct(IoFile $ioFile)
      * @param bool $result
      * @param ImageContentInterface $imageContent
      * @return bool
-     * @throws InputException
+     * @throws LocalizedException
      *
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
@@ -55,8 +54,8 @@ public function afterIsValid(
         $extension = strtolower($pathInfo['extension'] ?? '');
 
         if ($extension && !in_array($extension, self::ALLOWED_EXTENSIONS, true)) {
-            throw new InputException(
-                new Phrase('The image file extension "%1" is not allowed.', [$extension])
+            throw new LocalizedException(
+                __('The image file extension "%1" is not allowed.', $extension)
             );
         }
 
diff --git a/Plugin/ImageProcessorStrictBinaryValidation.php b/Plugin/ImageProcessorStrictBinaryValidation.php
new file mode 100644
index 0000000..8d6d927
--- /dev/null
+++ b/Plugin/ImageProcessorStrictBinaryValidation.php
@@ -0,0 +1,74 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MarkShust\PolyshellPatch\Plugin;
+
+use Magento\Framework\Api\Data\ImageContentInterface;
+use Magento\Framework\Api\ImageProcessor;
+use Magento\Framework\Exception\LocalizedException;
+use MarkShust\PolyshellPatch\Model\StrictImagePayloadValidator;
+
+/**
+ * Runs strict binary validation and GD re-encode before media write.
+ *
+ * Size limits: {@see StrictImagePayloadValidator::MAX_BASE64_INPUT_CHARS} and
+ * {@see StrictImagePayloadValidator::MAX_DECODED_BYTES} (reject oversized input before full decode).
+ */
+class ImageProcessorStrictBinaryValidation
+{
+    /**
+     * @var StrictImagePayloadValidator
+     */
+    private StrictImagePayloadValidator $strictImagePayloadValidator;
+
+    /**
+     * @param StrictImagePayloadValidator $strictImagePayloadValidator
+     */
+    public function __construct(StrictImagePayloadValidator $strictImagePayloadValidator)
+    {
+        $this->strictImagePayloadValidator = $strictImagePayloadValidator;
+    }
+
+    /**
+     * Validate payload before ImageProcessor writes to media.
+     *
+     * @param ImageProcessor $subject
+     * @param callable $proceed
+     * @param string $entityType
+     * @param ImageContentInterface $imageContent
+     * @return string
+     * @throws LocalizedException
+     *
+     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
+     */
+    public function aroundProcessImageContent(
+        ImageProcessor $subject,
+        callable $proceed,
+        $entityType,
+        ImageContentInterface $imageContent
+    ) {
+        $encoded = $imageContent->getBase64EncodedData();
+        if ($encoded === null || $encoded === '') {
+            throw new LocalizedException(__('The image content is invalid. Verify the content and try again.'));
+        }
+
+        if (strlen($encoded) > StrictImagePayloadValidator::MAX_BASE64_INPUT_CHARS) {
+            throw new LocalizedException(__('The image content is invalid. Verify the content and try again.'));
+        }
+
+        // phpcs:ignore Magento2.Functions.DiscouragedFunction -- strict mode; invalid base64 returns false
+        $binary = base64_decode($encoded, true);
+        if ($binary === false || $binary === '') {
+            throw new LocalizedException(__('The image content is invalid. Verify the content and try again.'));
+        }
+
+        $this->strictImagePayloadValidator->assertValidAndNormalize(
+            $binary,
+            (string) $imageContent->getType(),
+            $imageContent
+        );
+
+        return $proceed($entityType, $imageContent);
+    }
+}
diff --git a/composer.json b/composer.json
index 1dc0e06..687ed5b 100644
--- a/composer.json
+++ b/composer.json
@@ -3,7 +3,8 @@
     "description": "The PolyshellPatch module mitigates the PolyShell vulnerability (APSB25-94) — an unrestricted file upload in the Magento REST API that allows attackers to upload executable files via cart item custom option file uploads.",
     "require": {
         "php": "^8.1",
-        "magento/framework": "^103"
+        "magento/framework": "^103",
+        "ext-gd": "*"
     },
     "require-dev": {
         "roave/security-advisories": "dev-latest"
diff --git a/etc/di.xml b/etc/di.xml
index 604c99d..fd3209c 100644
--- a/etc/di.xml
+++ b/etc/di.xml
@@ -2,8 +2,12 @@
 <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
     <type name="Magento\Framework\Api\ImageProcessor">
+        <plugin name="markshust_polyshell_strict_binary_validation"
+                type="MarkShust\PolyshellPatch\Plugin\ImageProcessorStrictBinaryValidation"
+                sortOrder="5"/>
         <plugin name="markshust_polyshell_restrict_upload_extensions"
-                type="MarkShust\PolyshellPatch\Plugin\ImageProcessorRestrictExtensions"/>
+                type="MarkShust\PolyshellPatch\Plugin\ImageProcessorRestrictExtensions"
+                sortOrder="10"/>
     </type>
     <type name="Magento\Framework\Api\ImageContentValidator">
         <plugin name="markshust_polyshell_validate_file_extension"