CVE-2026-75583: keeper.sh Calendar version prior to 2.18.14 SSRF Guard Bypass via DNS Rebinding
keeper.sh's calendar module version prior to 2.18.14 contains a server-side request forgery (SSRF) guard bypass vulnerability that allows authenticated attackers to reach private network addresses by exploiting a DNS rebinding attack against the two-phase URL validation and connection flow. The SSRF guard validates a hostname's resolved IP addresses but discards them before the actual HTTP connection is opened, allowing an attacker who controls authoritative DNS to return a public address during validation and a private address during the subsequent independent socket-level DNS resolution, causing the guard to pass while the outbound connection reaches internal infrastructure such as cloud instance metadata endpoints.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
keeper.sh Calendar moduleto a version that resolves this vulnerability.Fixed in 2.18.14
Event History
Frequently Asked Questions
What must an attacker control to exploit this issue?
The attacker must be authenticated and control authoritative DNS for a hostname. They use DNS rebinding so the hostname resolves to a public IP address during validation and a private IP address when the server opens the outbound connection.
Which systems are most exposed?
Deployments running a calendar module version earlier than 2.18.14 are affected. The practical impact is greatest where the server can reach private network services, including cloud instance metadata endpoints.
How can I determine whether my deployment is vulnerable?
Check the installed keeper.sh calendar module version. Versions prior to 2.18.14 have the described two-phase validation and connection behavior that can be bypassed through DNS rebinding.