Stunnel: ssrf bypass in stunnel socks proxy via ipv4-mapped ipv6 loopback and unspecified addresses allows access to loopback-only services

Overview

A Server-Side Request Forgery (SSRF) bypass vulnerability exists in “stunnel” 5.79 and lower when configured in SOCKS proxy mode. This flaw allows a client to bypass intended localhost restrictions by using IPv4-mapped IPv6 addresses (e.g., “::ffff:127.0.0.1”) or unspecified addresses ("0.0.0.0", "::"), enabling access to loopback-only services on the "stunnel" host that should not be network-reachable.

Affected

• stunnel >= 5.25 < 5.80
• stunnel (all versions)
• stunnel
• stunnel
• stunnel (all versions)
• stunnel (all versions)

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Workarounds

To mitigate this issue, if SOCKS proxying functionality is not required, disable the "protocol = socks" configuration in "stunnel". If SOCKS proxying is necessary, restrict access to the SOCKS listener by binding it to a trusted management network or localhost, and enforce client authentication or network ACL controls. Run "stunnel" in a container or network namespace where no other services are bound to localhost, or add firewall rules to restrict outgoing connections from "stunnel" to localhost.