Executive Summary

CVE-2026-47684 is a Server-Side Request Forgery (SSRF) protection bypass vulnerability in the Sync-in Server software versions up to 2.2.1.

The issue occurs because the private IP blocklist regex used in the URL download feature does not correctly match IPv4-mapped IPv6 addresses (for example, ::ffff:127.0.0.1).

This flaw allows attackers on dual-stack systems (supporting both IPv4 and IPv6) to bypass SSRF protections by crafting URLs that point to internal IP addresses using IPv4-mapped IPv6 notation.

As a result, attackers can access restricted internal resources that should normally be protected.

The vulnerability was fixed in version 2.3.0 of Sync-in Server.

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Impact Analysis

This vulnerability can have significant security impacts by allowing an attacker to bypass SSRF protections and access internal network resources that are normally restricted.

• Attackers can exploit this to access sensitive internal services or data.
• It can lead to unauthorized internal network reconnaissance or data exfiltration.
• The attack requires low privileges and no user interaction, making it easier to exploit.

Overall, this vulnerability poses a high risk to the confidentiality of internal systems and data.

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Detection Guidance

Detection of this vulnerability involves identifying if your system is running a vulnerable version of Sync-in Server (version 2.2.1 or earlier) and monitoring for suspicious SSRF attempts that exploit IPv4-mapped IPv6 addresses.

You can check the installed version of Sync-in Server by running commands such as:

• npm list @sync-in/server
• docker images | grep sync-in/server

To detect exploitation attempts on your network, monitor logs for URL download requests containing IPv4-mapped IPv6 addresses (e.g., ::ffff:127.0.0.1 or ::ffff:10.x.x.x). You can use commands like:

• grep -E '::ffff:(127\.0\.0\.1|10\.[0-9]+\.[0-9]+\.[0-9]+)' /var/log/sync-in/*.log
• tcpdump or Wireshark filters to capture traffic with IPv4-mapped IPv6 addresses targeting internal resources.

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Mitigation Strategies

The immediate mitigation step is to upgrade Sync-in Server to version 2.3.0 or later, where the SSRF protection bypass issue has been fixed.

Until the upgrade can be applied, consider restricting access to the URL download feature or implementing additional network-level controls to block requests containing IPv4-mapped IPv6 addresses.

Review and harden firewall rules to prevent unauthorized internal network access from the Sync-in Server.

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Compliance Impact

The vulnerability CVE-2026-47684 allows attackers to bypass SSRF protections and potentially access restricted internal resources by exploiting weaknesses in the private IP blocklist regex. This could lead to unauthorized internal network access or data exfiltration through manipulated download requests.

Such unauthorized access or data exposure could impact compliance with common standards and regulations like GDPR and HIPAA, which require strict controls to protect sensitive data and prevent unauthorized access.

Therefore, if exploited, this vulnerability may lead to violations of data protection requirements mandated by these regulations.

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