diff --git a/app/Util/HtmlPurifier/ConfiguredHtmlPurifier.php b/app/Util/HtmlPurifier/ConfiguredHtmlPurifier.php
index 221db6ffc..c4dd91778 100644
--- a/app/Util/HtmlPurifier/ConfiguredHtmlPurifier.php
+++ b/app/Util/HtmlPurifier/ConfiguredHtmlPurifier.php
@@ -3,6 +3,7 @@
 namespace BookStack\Util\HtmlPurifier;
 
 use BookStack\App\AppVersion;
+use BookStack\Util\HtmlPurifier\Filters\UriEnsureScheme;
 use BookStack\Util\HtmlPurifier\Filters\UriLimitFileProtocolToAnchors;
 use BookStack\Util\UrlFilter;
 use HTMLPurifier;
@@ -84,14 +85,18 @@ class ConfiguredHtmlPurifier
         $config->set('Attr.EnableID', true);
         $config->set('Attr.ID.HTML5', true);
         $config->set('Output.FixInnerHTML', false);
-        $config->set('URI.SafeIframeRegexp', '%^(http://|https://|//)%');
 
         $allowedSchemes = UrlFilter::getAllowedSchemes();
         $allowedSchemesSetting = [];
         foreach ($allowedSchemes as $scheme) {
             $allowedSchemesSetting[$scheme] = true;
         }
+        $defaultScheme = str_starts_with(url('/'), 'http:') ? 'http' : 'https';
+        $config->set('URI.SafeIframeRegexp', '%^(http://|https://|//)%');
         $config->set('URI.AllowedSchemes', $allowedSchemesSetting);
+        $config->set('URI.MakeAbsolute', true);
+        $config->set('URI.DefaultScheme', $defaultScheme);
+        $config->set('URI.Base', url('/'));
 
          // $config->set('Cache.DefinitionImpl', null); // Disable cache during testing
     }
@@ -155,7 +160,8 @@ class ConfiguredHtmlPurifier
         // Allow mention-ids on links
         $definition->addAttribute('a', 'data-mention-user-id', 'Number');
 
-        // Set up custom handler for srcset to limit accepted types
+        // Set up custom handler for srcset
+        // To remove once added upstream: https://github.com/xemlock/htmlpurifier-html5/pull/91
         $definition->addAttribute('img', 'srcset', new SrcsetAttrDef());
         $definition->addAttribute('source', 'srcset', new SrcsetAttrDef());
     }
@@ -163,6 +169,7 @@ class ConfiguredHtmlPurifier
     protected function configureUriDefinition(HTMLPurifier_URIDefinition $definition): void
     {
         $definition->registerFilter(new UriLimitFileProtocolToAnchors());
+        $definition->registerFilter(new UriEnsureScheme());
     }
 
     public function purify(string $html): string
diff --git a/app/Util/HtmlPurifier/Filters/UriEnsureScheme.php b/app/Util/HtmlPurifier/Filters/UriEnsureScheme.php
new file mode 100644
index 000000000..6b4651b69
--- /dev/null
+++ b/app/Util/HtmlPurifier/Filters/UriEnsureScheme.php
@@ -0,0 +1,39 @@
+<?php
+
+namespace BookStack\Util\HtmlPurifier\Filters;
+
+use HTMLPurifier_Config;
+use HTMLPurifier_Context;
+use HTMLPurifier_URI;
+use HTMLPurifier_URIFilter;
+
+class UriEnsureScheme extends HTMLPurifier_URIFilter
+{
+    /**
+     * @type string
+     */
+    public $name = 'EnsureScheme';
+
+    /**
+     * @type bool
+     */
+    public $always_load = true;
+
+    /**
+     * @param HTMLPurifier_URI $uri
+     * @param HTMLPurifier_Config $config
+     * @param HTMLPurifier_Context $context
+     * @return bool
+     */
+    public function filter(&$uri, $config, $context): bool
+    {
+        $def = $config->getDefinition('URI');
+        $defaultScheme = $def->defaultScheme ?? '';
+
+        if (empty($uri->scheme) && $defaultScheme) {
+            $uri->scheme = $defaultScheme;
+        }
+
+        return true;
+    }
+}
diff --git a/app/Util/HtmlPurifier/SrcsetAttrDef.php b/app/Util/HtmlPurifier/SrcsetAttrDef.php
index 3b8417a55..2f0ffffab 100644
--- a/app/Util/HtmlPurifier/SrcsetAttrDef.php
+++ b/app/Util/HtmlPurifier/SrcsetAttrDef.php
@@ -3,24 +3,209 @@
 namespace BookStack\Util\HtmlPurifier;
 
 use HTMLPurifier_AttrDef;
+use HTMLPurifier_AttrDef_URI;
 
 /**
- * Custom attribute definition to filter out potentially dangerous
- * values from the srcset attribute.
+ * A custom attribute definition for handling Srcset attributes.
+ * Has been provided upstream via:
+ * https://github.com/xemlock/htmlpurifier-html5/pull/91
+ * but awaiting review/merge.
  */
 class SrcsetAttrDef extends HTMLPurifier_AttrDef
 {
     public function validate($string, $config, $context)
     {
-        $lower = strtolower($string);
-        $nonAllowed = ['javascript:', 'vbscript:', 'data:', 'file:'];
+        $sources = $this->parseImageSources($string);
+        if (empty($sources)) {
+            return false;
+        }
 
-        foreach ($nonAllowed as $nonAllowedString) {
-            if (str_contains($lower, $nonAllowedString)) {
-                return false;
+        $uriFilter = new HTMLPurifier_AttrDef_URI(true);
+
+        $filtered = array();
+        foreach ($sources as $source) {
+            $uri = $source['uri'];
+            $descriptor = $source['descriptor'];
+            $validatedUri = $uriFilter->validate($uri, $config, $context);
+            if (is_string($validatedUri)) {
+                if ($descriptor) {
+                    $filtered[] = $validatedUri . ' ' . $source['descriptor'];
+                } else {
+                    $filtered[] = $validatedUri;
+                }
             }
         }
 
-        return $string;
+        if (empty($filtered)) {
+            return false;
+        }
+
+        return implode(', ', $filtered);
+    }
+
+    /**
+     * Parse the image source from srcset attribute text.
+     * Returns false if it's found to be invalid, otherwise
+     * returns an array of uri and descriptor combinations.
+     *
+     * This aims to follow the WHATWG parsing spec as per:
+     * https://html.spec.whatwg.org/multipage/images.html#parsing-a-srcset-attribute
+     *
+     * @param string $string
+     * @return array{uri: string, descriptor: string}[]|false
+     */
+    private function parseImageSources($string)
+    {
+        $imageSources = array();
+        $asciiWhitespace = " \n\r\t\f";
+        $asciiWhiteSpaceComma = $asciiWhitespace . ',';
+        $input = trim($string, $asciiWhiteSpaceComma);
+
+        if ($input === "") {
+            return false;
+        }
+
+        $position = 0;
+        while ($position < strlen($input)) {
+            $position += strspn($input, $asciiWhitespace, $position);
+            $urlEnd = $position + strcspn($input, $asciiWhitespace, $position);
+            $url = substr($input, $position, $urlEnd - $position);
+            $position = $urlEnd;
+            $descriptors = array();
+
+            if (strpos($url, ',') === strlen($url) - 1) {
+                $url = rtrim($url, ',');
+            } else {
+                $position += strspn($input, $asciiWhitespace, $position);
+                $currentDescriptor = '';
+                $state = 'in_descriptor';
+                while (true) {
+                    if ($position < strlen($input)) {
+                        $c = $input[$position];
+                    } else {
+                        $c = null;
+                    }
+
+                    if ($state === 'in_descriptor') {
+                        if ($c !== null && strpos($asciiWhitespace, $c) !== false) {
+                            if ($currentDescriptor !== '') {
+                                $descriptors[] = $currentDescriptor;
+                            }
+                            $state = 'after_descriptor';
+                        } else if ($c === ',') {
+                            $position++;
+                            if ($currentDescriptor !== '') {
+                                $descriptors[] = $currentDescriptor;
+                            }
+                            break;
+                        } else if ($c === '(') {
+                            $currentDescriptor .= $c;
+                            $state = 'in_parens';
+                        } else if ($c === null) {
+                            if ($currentDescriptor !== '') {
+                                $descriptors[] = $currentDescriptor;
+                            }
+                            break;
+                        } else {
+                            $currentDescriptor .= $c;
+                        }
+                    } else if ($state === 'in_parens') {
+                        if ($c === ')') {
+                            $currentDescriptor .= $c;
+                            $state = 'in_descriptor';
+                        } else if ($c === null) {
+                            $descriptors[] = $currentDescriptor;
+                            break;
+                        } else {
+                            $currentDescriptor .= $c;
+                        }
+                    } else if ($state === 'after_descriptor') {
+                        if ($c !== null && strpos($asciiWhitespace, $c) !== false) {
+                            // Stay in this state
+                        } else if ($c === null) {
+                            break;
+                        } else {
+                            $state = 'in_descriptor';
+                            $position--;
+                        }
+                    }
+
+                    $position++;
+                }
+            }
+
+            $descriptor = $this->formatDescriptor($descriptors);
+
+            if ($url && $descriptor !== false) {
+                $imageSources[] = array(
+                    'uri' => $url,
+                    'descriptor' => $descriptor,
+                );
+            }
+        }
+
+        return $imageSources;
+    }
+
+    /**
+     * Parse and format a single descriptor from an array of potential
+     * descriptor strings. Returns empty if valid but no descriptor.
+     * Returns false if invalid.
+     * @param string[] $descriptors
+     * @return false|string
+     */
+    private function formatDescriptor(array $descriptors)
+    {
+        $error = false;
+        $width = '';
+        $density = '';
+        $futureCompatH = '';
+
+        foreach ($descriptors as $descriptor) {
+            $descriptor = trim($descriptor);
+            if ($descriptor === '') {
+                continue;
+            }
+
+            $unit = $descriptor[strlen($descriptor) - 1];
+            $number = trim(substr($descriptor, 0, -1));
+
+            if ($unit === 'w' && filter_var($number, FILTER_VALIDATE_INT) && intval($number) >= 0) {
+                if (!empty($width) || !empty($density) || intval($number) === 0) {
+                    $error = true;
+                }
+                $width = $number;
+            } else if ($unit === 'x' && filter_var($number, FILTER_VALIDATE_FLOAT)) {
+                if (!empty($width) || !empty($density) || !empty($futureCompatH) || floatval($number) < 0) {
+                    $error = true;
+                }
+                $density = $number;
+            } else if ($unit === 'h' && filter_var($number, FILTER_VALIDATE_INT) && intval($number) >= 0) {
+                if (!empty($futureCompatH) || !empty($density)) {
+                    $error = true;
+                }
+                $futureCompatH = $number;
+            } else {
+                $error = true;
+            }
+        }
+
+        if (!empty($futureCompatH) && empty($width)) {
+            $error = true;
+        }
+
+        if ($error) {
+            return false;
+        }
+
+        if ($width) {
+            return $width . 'w';
+        }
+
+        if ($density) {
+            return $density . 'x';
+        }
+
+        return '';
     }
 }
diff --git a/tests/Entity/PageContentFilteringTest.php b/tests/Entity/PageContentFilteringTest.php
index 5cb898e05..68b597387 100644
--- a/tests/Entity/PageContentFilteringTest.php
+++ b/tests/Entity/PageContentFilteringTest.php
@@ -488,6 +488,35 @@ HTML;
         }
     }
 
+    public function test_media_protocol_relative_urls_are_given_scheme_depending_on_app_url()
+    {
+        $testCasesExpectedByInput = [
+            '<div id="bkmrk-file-img"><img src="//link/to/file" alt="My local image"></div>' => '<div id="bkmrk-file-img"><img src="SCHEME://link/to/file" alt="My local image"></div>',
+            '<iframe src="//link/to/file" id="bkmrk-file-iframe"></iframe>' => '<iframe src="SCHEME://link/to/file" id="bkmrk-file-iframe"></iframe>',
+            '<embed src="//link/to/file" id="bkmrk-file-embed">' => '<embed src="SCHEME://link/to/file" id="bkmrk-file-embed">',
+            '<object data="//link/to/file" id="bkmrk-file-object"></object>' => '<object data="SCHEME://link/to/file" id="bkmrk-file-object"></object>',
+            '<div id="bkmrk-file-img"><img srcset="//link/to/file 2x, //link-to-another 4x" src="//link/to/a/file" alt="My local image"></div>' => '<div id="bkmrk-file-img"><img srcset="SCHEME://link/to/file 2x, SCHEME://link-to-another 4x" src="SCHEME://link/to/a/file" alt="My local image"></div>',
+        ];
+
+        $baseUrls = ['https://example.com' => 'https', 'http://example.com' => 'http'];
+        foreach ($baseUrls as $baseUrl => $expectedScheme) {
+            $this->runWithEnv(['APP_URL' => $baseUrl], function () use ($expectedScheme, $baseUrl, $testCasesExpectedByInput) {
+                config()->set('app.content_filtering', 'a');
+                $page = $this->entities->page();
+                $this->asEditor();
+
+                foreach ($testCasesExpectedByInput as $input => $expected) {
+                    $page->html = $input;
+                    $page->save();
+                    $resp = $this->get($page->getUrl());
+
+                    $resp->assertSee(str_replace('SCHEME', $expectedScheme, $expected), false);
+                    $resp->assertDontSee($input, false);
+                }
+            });
+        }
+    }
+
     public function test_allow_list_does_not_filter_cases()
     {
         $testCasesExpectedByInput = [