Description

Mailpit's IsInternalIP deny list function fails to block the Azure WireServer address 168.63.129.16 and the RFC 2765/6145 IPv4-translated IPv6 prefix, allowing server-side request forgery to internal destinations. Attackers can supply hostnames resolving to these addresses in message content to reach the link check API and proxy endpoint for accessing internal resources.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.3",
  "pubDate": "2026-09-02T01:17:24.703Z",
  "pubdate": "2026-09-02T01:17:24.703Z",
  "executiveSummary": "This vulnerability is an Improper Input Validation issue leading to Server-Side Request Forgery (SSRF) within Mailpit. The vulnerability exists in the IsInternalIP deny list function, which fails to properly restrict access to critical internal IP addresses, specifically the Azure WireServer address (168.63.129.16) and RFC 2765/6145 IPv4-translated IPv6 prefixes.\nBy supplying attacker-controlled hostnames that resolve to these blocked or restricted address spaces, a remote attacker can bypass existing security controls. This allows the Mailpit server to act as a proxy, facilitating unauthorized requests to internal resources that should be inaccessible to the public or external entities.\nThe risk implication is significant for deployments within cloud environments like Azure, where access to the WireServer endpoint can lead to the retrieval of sensitive metadata or interaction with infrastructure management services. Exploitation does not necessarily require prior authentication, as the mechanism is exposed through message content processing, which is often reachable by external senders or via malicious email injection. This facilitates post-exploitation activities including reconnaissance and potential lateral movement within the hosting infrastructure.",
  "technicalDetails": "The root cause of this vulnerability is an incomplete implementation of the IsInternalIP validation logic within the Mailpit application. While Mailpit maintains a deny list intended to prevent SSRF by restricting outbound requests to internal IP address ranges, the current implementation fails to include specific high-risk addresses used in cloud environments and protocol-translated address spaces.\nSpecifically, the check omits the Azure WireServer address (168.63.129.16), a critical infrastructure endpoint that provides DHCP, DNS, and metadata services to Azure virtual machines. Furthermore, the logic does not account for RFC 2765/6145 IPv4-translated IPv6 prefixes, which allow for the representation of IPv4 addresses within an IPv6 space, effectively obfuscating restricted internal IPs to bypass filters relying on standard IPv4 address matching.\nThe attack flow begins when an attacker crafts a malicious message containing a hostname under their control. When the Mailpit server processes this content, it triggers the link check API or the internal proxy endpoint to resolve the hostname. Because the underlying validation function (IsInternalIP) considers the resolved address as legitimate (i.e., not within the restricted/internal range), the application proceeds to initiate a request to the resolved destination.\nBy manipulating DNS resolution, an attacker can ensure the hostname maps to 168.63.129.16 or an IPv6-translated equivalent. The application then performs the request on behalf of the attacker, effectively proxying the connection through the Mailpit service. This grants the attacker the ability to interact with the Azure WireServer, potentially allowing them to exfiltrate information, retrieve instance identity documents, or probe other internal services that the Mailpit process has network connectivity to. The vulnerability exists in the core network request handling logic, and successful exploitation effectively turns the Mailpit instance into a pivot point for internal network scanning and unauthorized interaction with sensitive infrastructure APIs, bypassing the intended service-level security policies."
}