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The vulnerability exists in the Nextcloud Talk webhook handler, specifically within the signature verification logic. When an external service sends a webhook event, it includes a signature generated using a shared secret configured by the administrator. The application, in extensions/nextcloud-talk/src/monitor.ts, validates this signature to ensure the request is authentic. Prior to the fix, a failed signature verification did not trigger any rate-limiting or throttling mechanism. This oversight allows an attacker with network access to the webhook endpoint to submit an unlimited number of requests. By iterating through possible shared secret values, the attacker can perform an online brute-force attack. If the configured secret is weak or predictable, the attacker can eventually determine the correct secret. Once the secret is compromised, the attacker can forge valid webhook events, potentially leading to unauthorized actions such as triggering notifications, modifying room settings, or injecting malicious content within the Talk application context.
Platform: Nextcloud Talk
Version: <= 2026.3.24
Vulnerability: Unthrottled brute-force
Severity: Medium
date: 2026-04-01
Prediction: Already patched
What Undercode Say:
Simulate a brute-force attempt against the vulnerable webhook endpoint
for secret in $(cat weak_secrets.txt); do
signature=$(echo -n "payload" | hmacsha256 "$secret")
curl -X POST https://target.com/webhook \
-H "X-Nextcloud-Talk-Signature: $signature" \
-d '{"event":"test"}'
done
Exploit:
Attacker repeatedly sends webhook requests with guessed signatures until a 2xx response indicates valid authentication, leveraging the absence of rate-limiting to test thousands of secrets online.
Protection:
Update to Nextcloud Talk version 2026.3.28 or later, which introduces throttling on repeated authentication failures. Additionally, configure a strong, high-entropy shared secret for the webhook integration.
Impact:
Successful exploitation allows an attacker to forge inbound webhook events, potentially enabling unauthorized privilege escalation, data manipulation, or disruption of chat services within the Nextcloud environment.
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Sources:
Reported By: github.com
Extra Source Hub:
Undercode
