diff --git a/src/Enum/DocsRenderingHistoryStatus.php b/src/Enum/DocsRenderingHistoryStatus.php
index 5a33ebb..edd9746 100644
--- a/src/Enum/DocsRenderingHistoryStatus.php
+++ b/src/Enum/DocsRenderingHistoryStatus.php
@@ -18,6 +18,7 @@ final class DocsRenderingHistoryStatus
     public const NO_COMPOSER_JSON = 'noComposerJson';
     public const INVALID_COMPOSER_JSON = 'invalidComposerJson';
     public const UNKNOWN_REPOSITORY_DOMAIN = 'unknownRepositoryDomain';
+    public const INVALID_COMPOSER_JSON_URL = 'invalidComposerJsonUrl';
     public const PACKAGE_REGISTERED_WITH_DIFFERENT_REPOSITORY = 'packageRegisteredWithDifferentRepository';
     public const NO_RELEVANT_BRANCH_OR_TAG = 'noRelevantBranchOrTag';
     public const MISSING_VALUE_IN_COMPOSER_JSON = 'missingValueInComposerJson';
@@ -35,6 +36,7 @@ final class DocsRenderingHistoryStatus
         self::NO_COMPOSER_JSON,
         self::INVALID_COMPOSER_JSON,
         self::UNKNOWN_REPOSITORY_DOMAIN,
+        self::INVALID_COMPOSER_JSON_URL,
         self::PACKAGE_REGISTERED_WITH_DIFFERENT_REPOSITORY,
         self::NO_RELEVANT_BRANCH_OR_TAG,
         self::MISSING_VALUE_IN_COMPOSER_JSON,
@@ -55,6 +57,7 @@ final class DocsRenderingHistoryStatus
         self::NO_COMPOSER_JSON => 'No composer.json found.',
         self::INVALID_COMPOSER_JSON => 'Invalid composer.json.',
         self::UNKNOWN_REPOSITORY_DOMAIN => 'Unknown repository domain.',
+        self::INVALID_COMPOSER_JSON_URL => 'The composer.json url can not be used.',
         self::PACKAGE_REGISTERED_WITH_DIFFERENT_REPOSITORY => 'Package registered with different repository.',
         self::NO_RELEVANT_BRANCH_OR_TAG => 'No relevant branch or tag found.',
         self::MISSING_VALUE_IN_COMPOSER_JSON => 'Missing value in composer.json.',
diff --git a/src/Service/DocumentationBuildInformationService.php b/src/Service/DocumentationBuildInformationService.php
index ce3ea8c..8e285a0 100644
--- a/src/Service/DocumentationBuildInformationService.php
+++ b/src/Service/DocumentationBuildInformationService.php
@@ -324,6 +324,14 @@ private function assertUrlToComposerFileIsSafe(string $url): void
             throw new InvalidComposerJsonUrlException('URL to composer.json contains disallowed scheme', 1781613532, null, $url);
         }
 
+        // The url is assembled from payload fields, so a '#' or a '?' inside one of
+        // them can push the intended '…/composer.json' suffix out of the path and
+        // leave an arbitrary endpoint on the same host. Every format this
+        // application builds ends in that suffix, so require it.
+        if (!str_ends_with($uri->getPath(), '/composer.json')) {
+            throw new InvalidComposerJsonUrlException('URL to composer.json does not point to a composer.json', 1785816000, null, $url);
+        }
+
         $normalizedHost = RepositoryUrlUtility::getNormalizedDomain($uri);
         $allowedRepositoryDomain = $this->knownRepositoryDomainsRepository->findOneBy([
             'domain' => $normalizedHost,
diff --git a/src/Service/RenderDocumentationService.php b/src/Service/RenderDocumentationService.php
index bdaa161..5b9594b 100644
--- a/src/Service/RenderDocumentationService.php
+++ b/src/Service/RenderDocumentationService.php
@@ -26,6 +26,7 @@
 use App\Exception\DocsPackageDoNotCareBranch;
 use App\Exception\DocsPackageRegisteredWithDifferentRepositoryException;
 use App\Exception\DocumentationRenderingRequestDeclinedException;
+use App\Exception\InvalidComposerJsonUrlException;
 use App\Exception\UnknownComposerJsonUrlException;
 use App\Extractor\DeploymentInformation;
 use App\Extractor\PushEvent;
@@ -90,6 +91,20 @@ public function requestDocumentationRendering(PushEvent $pushEvent, Documentatio
             ));
 
             throw new DocumentationRenderingRequestDeclinedException(sprintf('composer.json\'s host domain %s is disallowed for rendering request', $e->normalizedHost), 1782294348, $e);
+        } catch (InvalidComposerJsonUrlException $e) {
+            $this->historyService->writeHistory(new HistoryEntryDto(
+                type: HistoryEntryType::DOCS_RENDERING,
+                status: DocsRenderingHistoryStatus::INVALID_COMPOSER_JSON_URL,
+                triggeredBy: $trigger->toHistoryEntryTrigger(),
+                data: [
+                    'repository' => $pushEvent->getRepositoryUrl(),
+                    'composerFile' => $pushEvent->getUrlToComposerFile(),
+                    'payload' => $pushEvent->getPayload(),
+                    'user' => $userIdentifier,
+                ]
+            ));
+
+            throw new DocumentationRenderingRequestDeclinedException(sprintf('composer.json url %s can not be used for a rendering request', $e->composerJsonUrl), 1785810600, $e);
         }
 
         $composerAsObject = $this->documentationBuildInformationService->getComposerJsonObject($composerJson);
diff --git a/tests/Unit/Service/ComposerJsonUrlShapeTest.php b/tests/Unit/Service/ComposerJsonUrlShapeTest.php
new file mode 100644
index 0000000..c854999
--- /dev/null
+++ b/tests/Unit/Service/ComposerJsonUrlShapeTest.php
@@ -0,0 +1,87 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of the package t3g/intercept.
+ *
+ * For the full copyright and license information, please read the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace App\Tests\Unit\Service;
+
+use App\Entity\KnownRepositoryDomain;
+use App\Enum\RepositoryDomainStatus;
+use App\Exception\InvalidComposerJsonUrlException;
+use App\Repository\DocumentationJarRepository;
+use App\Repository\KnownRepositoryDomainRepository;
+use App\Service\DocumentationBuildInformationService;
+use App\Service\MailService;
+use App\Service\SlackService;
+use Doctrine\ORM\EntityManagerInterface;
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Response;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Filesystem\Filesystem;
+
+class ComposerJsonUrlShapeTest extends TestCase
+{
+    public static function urlsBuiltForTheSupportedServicesDataProvider(): \Iterator
+    {
+        yield 'bitbucket cloud' => ['https://allowed.example/acme/ext/raw/main/composer.json'];
+        yield 'bitbucket server' => ['https://allowed.example/projects/EXT/repos/ext/raw/composer.json?at=refs%2Fheads%2Fmain'];
+        yield 'gitlab' => ['https://allowed.example/acme/ext/raw/main/composer.json'];
+        yield 'github' => ['https://allowed.example/acme/ext/main/composer.json'];
+        yield 'forgejo' => ['https://allowed.example/acme/ext/raw/branch/main/composer.json'];
+    }
+
+    #[DataProvider('urlsBuiltForTheSupportedServicesDataProvider')]
+    public function testUrlsTheServicesActuallyProduceArePassed(string $url): void
+    {
+        $composerJson = $this->buildSubject()->fetchRemoteComposerJson($url);
+
+        $this->assertSame('acme/ext', $composerJson['name']);
+    }
+
+    public static function manipulatedUrlsDataProvider(): \Iterator
+    {
+        // A '#' turns the expected '/…/composer.json' suffix into a fragment,
+        // which is dropped before the request is sent
+        yield 'fragment cuts off the expected path' => ['https://allowed.example/internal/admin#/raw/branch/main/composer.json'];
+        // A '?' in the base url pushes the expected suffix into the query string
+        yield 'query swallows the expected path' => ['https://allowed.example/api/v4/user?a=/raw/branch/main/composer.json'];
+        yield 'path traversal out of the repository' => ['https://allowed.example/acme/ext/raw/branch/../../../../etc/passwd'];
+        yield 'no composer.json at all' => ['https://allowed.example/acme/ext/raw/branch/main/'];
+    }
+
+    #[DataProvider('manipulatedUrlsDataProvider')]
+    public function testUrlsNotPointingAtAComposerJsonAreRejected(string $url): void
+    {
+        $this->expectException(InvalidComposerJsonUrlException::class);
+
+        $this->buildSubject()->fetchRemoteComposerJson($url);
+    }
+
+    private function buildSubject(): DocumentationBuildInformationService
+    {
+        $knownDomain = (new KnownRepositoryDomain())->setDomain('allowed.example')->setStatus(RepositoryDomainStatus::ALLOWED);
+        $knownRepositoryDomainRepository = $this->createMock(KnownRepositoryDomainRepository::class);
+        $knownRepositoryDomainRepository->method('findOneBy')->willReturn($knownDomain);
+
+        return new DocumentationBuildInformationService(
+            '/tmp',
+            'sub',
+            $this->createMock(DocumentationJarRepository::class),
+            $knownRepositoryDomainRepository,
+            $this->createMock(EntityManagerInterface::class),
+            $this->createMock(Filesystem::class),
+            new Client(['handler' => HandlerStack::create(new MockHandler([new Response(200, [], '{"name": "acme/ext"}')]))]),
+            $this->createMock(SlackService::class),
+            $this->createMock(MailService::class),
+        );
+    }
+}