Attack Chain

1. An unauthenticated attacker sends an HTTP POST request to the publicly exposed /?api/forms/{formId}/actor/inbox endpoint on a vulnerable YesWiki server.
2. The POST request includes a crafted Signature HTTP header, where the keyId parameter contains an attacker-controlled URL (e.g., http://169.254.169.254/latest/meta-data/iam/security-credentials/<role>).
3. YesWiki's Symfony controller routes the request to the postFormActorInbox() method in tools/bazar/controllers/ApiController.php.
4. If ActivityPub is enabled for the specified form, postFormActorInbox() calls HttpSignatureService::verifySignature() from tools/bazar/services/HttpSignatureService.php.
5. Inside verifySignature(), the Signature header is parsed, and without any prior validation or cryptographic checks, an outbound HTTP GET request is immediately initiated to the URL extracted from the keyId parameter using httpClient->request().
6. The YesWiki server attempts to connect to the attacker-specified internal or external host (e.g., cloud metadata service, intranet web server) and fetches its response.
7. If the fetched content does not contain publicKey.publicKeyPem (which is expected for non-ActivityPub targets), the controller returns an HTTP 500 error.
8. The attacker receives verbose error messages including a stack trace in the JSON response body, which can reveal the attempted URL and provide sufficient information via timing or error content to perform port scanning, service enumeration, or exfiltrate cloud metadata.

Impact

The successful exploitation of CVE-2026-52769 grants an unauthenticated attacker the ability to perform Server-Side Request Forgery, enabling them to initiate arbitrary HTTP GET requests from the vulnerable YesWiki server. This allows for reconnaissance of internal networks, including port scanning and service enumeration, by observing timing differences or explicit error messages that reveal details about unreachable or unparseable responses. Critically, attackers can target cloud metadata endpoints (e.g., 169.254.169.254) to steal sensitive IAM credentials or other instance-specific data. While no specific victim counts are available, any YesWiki instance with ActivityPub enabled and running an affected version is at risk of unauthorized access to its network environment and potential data exfiltration.

Recommendation

• Patch CVE-2026-52769 by upgrading YesWiki to version 4.6.6 or later immediately.
• Deploy the Sigma rule provided in this brief to your SIEM to detect attempts to access the vulnerable API endpoint.
• Monitor webserver logs for HTTP POST requests to /api/forms/*/actor/inbox containing suspicious Signature headers, especially those with internal or cloud metadata IP addresses in the keyId parameter.