CVE-2026-55824: Contao crawler leaks auth credentials to external hosts

Summary Contao's crawler tries to prevent confidential HTTP client options from being sent to external domains by creating a scoped client: full options for root page origins, cleaned options for everything else. The cleaner removes Cookie and Authorization headers, but it removes the non-Symfony option names basicauth and bearerauth instead of Symfony HttpClient's real authbasic and authbearer options.

When contao.crawl.defaulthttpclientoptions contains Basic or Bearer authentication for a protected staging/production site, those credentials remain in the "clean" client used for external links or configured additional URIs. An attacker who can get an external URL crawled, for example through a link on a crawled page while the broken-link checker is enabled, can receive the crawler credentials.

Technical Detail

Root Cause

php // core-bundle/src/Crawl/Escargot/Factory.php:175-209 @ e550b92a01ef625bd546e6c3956dd200af05ebf0 private function createHttpClient(array $options = []): HttpClientInterface { $options = arraymergerecursive( [ 'headers' => [ 'accept' => 'text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8', 'user-agent' => self::USERAGENT, ], 'maxduration' => 10, ], arraymergerecursive($this->getDefaultHttpClientOptions(), $options), );

$cleanOptions = $this->cleanOptionsFromConfidentialData($options);

if ($options === $cleanOptions) { return ($this->httpClientFactory)($options); }

$scopedOptionsByRegex = [];

foreach ($this->getRootPageUriCollection()->all() as $rootPageUri) { $scopedOptionsByRegex[pregquote($this->getOriginFromUri($rootPageUri))] = $options; }

return new ScopingHttpClient(($this->httpClientFactory)($cleanOptions), $scopedOptionsByRegex); }

php // core-bundle/src/Crawl/Escargot/Factory.php:226-247 @ e550b92a01ef625bd546e6c3956dd200af05ebf0 foreach ($options as $k => $v) { if ('headers' === $k) { foreach ($v as $header => $value) { if (\inarray(strtolower($header), ['authorization', 'cookie'], true)) { continue; }

$cleanOptions['headers'][$header] = $value; }

continue; }

if ('basicauth' === $k || 'bearerauth' === $k) { continue; }

$cleanOptions[$k] = $v; }

Symfony HttpClient authentication options are authbasic and authbearer; Contao's own manual documents authbasic for crawler Basic Authentication. Because the cleaner only strips basicauth and bearerauth, the "clean" default client for non-root-page hosts still carries the real auth options. The existing factory test intends to assert that Authorization is not sent to www.foreign-domain.com, but its mock client factory ignores the $defaultOptions argument, so it does not catch auth options that survive into HttpClient::create($cleanOptions).

Suggested Mitigation

Strip the actual Symfony HttpClient authentication option keys from the clean client. Include NTLM as a defensive extension because Symfony documents it as another auth option.

diff - if ('basicauth' === $k || 'bearerauth' === $k) { + if (\inarray($k, ['authbasic', 'authbearer', 'authntlm', 'basicauth', 'bearerauth'], true)) { continue; }

Also update the factory test so the mock factory records or preserves $defaultOptions; otherwise the test does not verify what HttpClient::create($cleanOptions) receives in production.

Impact

- Direct primitive: disclosure of crawler Basic/Bearer credentials to an external host reached by the crawler. - Chain potential: if those credentials protect a staging or pre-publication environment, an attacker can use them to access that environment. The impact depends on what the leaked credential unlocks. - Realistic exploitation: a content editor adds a link to https://attacker.example/probe on a page that the crawler visits. When an administrator or scheduled maintenance run starts the broken-link checker with crawler Basic/Bearer authentication configured, the request to the attacker URL includes the generated Authorization header.