CVE-2026-70367: Stunnel: ssrf bypass in stunnel socks proxy via ipv4-mapped ipv6 loopback and unspecified addresses allows access to loopback-only services

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-04 · updated 2026-08-06
Weakness: CWE-918

Description

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 products

Mobi-Com Polska Sp. z o.o. stunnel: from 5.25 before 5.80 — fixed in 5.80
Red Hat Enterprise Linux 10: all versions
Red Hat Enterprise Linux 8: all versions
Red Hat Enterprise Linux 9: all versions

How to fix

Upgrade to a fixed version: Mobi-Com Polska Sp. z o.o. stunnel 5.80. Check the vendor advisory for mitigations.

References

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