CVE-2026-13163 Mailerup CVE debrief

Who should care

Defenders managing Mailerup instances, security teams responsible for monitoring and responding to potential phishing attacks, and IT personnel responsible for upgrading software versions should be aware of this vulnerability and take necessary actions to mitigate it.

Why it matters

CVE-2026-13163 is a medium-severity open redirect vulnerability in Mailerup that allows remote unauthenticated attackers to redirect victims to arbitrary external sites and conduct phishing attacks. Defenders should prioritize verifying exposure, especially those managing Mailerup instances, and consider upgrading to version 1.0.0 or later if possible.

• Phishing attacks may be conducted against Mailerup users.
• Victims may be redirected to arbitrary external sites.
• Defenders should verify exposure to this vulnerability and consider upgrading Mailerup instances.
• Additional security measures may be needed to prevent open redirect attacks.

Technical summary

The _safe_redirect function of the click-tracking endpoint in Mailerup has an open redirect vulnerability. This allows remote unauthenticated attackers to redirect victims to arbitrary external sites via a crafted query parameter. The URL scheme is validated, blocking certain types of attacks, but the destination host is not restricted to an allowlist. The vulnerability has a CVSS score of 5.3 and a severity of MEDIUM. Defenders should prioritize verifying exposure to this vulnerability, especially those managing Mailerup instances. They should assess the current version and consider upgrading to version 1.0.0 or later if possible.

Defensive priority

Defenders should prioritize verifying exposure to this vulnerability, especially those managing Mailerup instances. They should assess the current version and consider upgrading to version 1.0.0 or later if possible.

Recommended defensive actions

• Verify the version of Mailerup being used and consider upgrading to version 1.0.0 or later.
• Monitor for potential phishing attacks targeting Mailerup users.
• Implement additional security measures to prevent open redirect attacks.
• Review compensating controls for exposed systems while remediation is scheduled and verified.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.

Evidence notes

The evidence for this vulnerability comes from the CVE Program record and the NVD vulnerability detail page. The CVE description provides details about the open redirect vulnerability in Mailerup. The NVD vulnerability detail page provides additional information about the vulnerability, including its CVSS score and severity. Defenders should verify the version of Mailerup being used and consider upgrading to version 1.0.0 or later. They should also monitor for potential phishing attacks targeting Mailerup users and implement ...

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.