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stunnel CVE-2026-70367

| EUVDEUVD-2026-52687 MEDIUM
Server-Side Request Forgery (SSRF) (CWE-918)
2026-08-04 redhat GHSA-2x69-8qrg-27gr
5.4
CVSS 3.1 · Vendor: redhat
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Severity by source

Vendor (redhat) PRIMARY
5.4 MEDIUM
AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
vuln.today AI
5.4 MEDIUM

Network-reachable SOCKS proxy requires low-privilege client access; limited read/write impact against loopback services; no availability or scope change.

3.1 AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
4.0 AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
SUSE
MEDIUM
qualitative
Red Hat
5.4 MEDIUM
qualitative

Primary rating from Vendor (redhat).

CVSS VectorVendor: redhat

Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

Lifecycle Timeline

2
Analysis Generated
Aug 04, 2026 - 14:36 vuln.today
CVE Published
Aug 04, 2026 - 13:52 nvd
MEDIUM 5.4

DescriptionCVE.org

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.

AnalysisAI

SSRF bypass in stunnel 5.79 and earlier allows authenticated SOCKS proxy clients to circumvent intended localhost access restrictions by supplying IPv4-mapped IPv6 addresses (e.g., ::ffff:127.0.0.1) or unspecified addresses (0.0.0.0, ::) in proxy requests. Exploitation reaches loopback-bound services on the stunnel host - services deliberately isolated from external network access - potentially exposing internal APIs, administrative interfaces, or other localhost-only endpoints. No public exploit code has been identified and this vulnerability is not listed in the CISA KEV catalog at time of analysis.

Technical ContextAI

stunnel is a widely-deployed SSL/TLS tunneling daemon that can operate as a SOCKS proxy, forwarding client connections to upstream targets while enforcing access policies. The flaw resides in stunnel's SOCKS-mode address validation logic, which fails to normalize or reject alternative representations of the loopback address before applying its localhost restriction. CWE-918 (Server-Side Request Forgery) covers this root cause class: the server-side proxy component can be manipulated to initiate connections to unintended internal targets. The dual-stack address ambiguity - where ::ffff:127.0.0.1 is a valid IPv6 representation of 127.0.0.1 but may bypass IPv4-only allow/deny checks, and where 0.0.0.0 or :: resolve to the local stack - is a known pitfall in network address validation. Red Hat reports the issue across stunnel as packaged in RHEL 6 through 10 (CPE: cpe:2.3:a:red_hat:red_hat_enterprise_linux_*), confirming broad distribution-level exposure across the RHEL ecosystem.

RemediationAI

No explicit patched version number is identified in the available references. Consult the Red Hat Security Advisory at https://access.redhat.com/security/cve/CVE-2026-70367 and Red Hat Bugzilla ticket 2462083 for updated RHEL packages as they become available; RHEL users should apply the relevant errata once released. For upstream stunnel, monitor the official stunnel.org release channel for a version superseding 5.79 that addresses SOCKS address normalization. As a compensating control, disable SOCKS proxy mode in stunnel configurations if it is not operationally required - this fully eliminates the attack surface with no functional impact for non-SOCKS deployments. Where SOCKS mode is necessary, enforce network-layer ACLs to restrict which clients can reach the stunnel SOCKS port, reducing the pool of potential abusers. Additionally, audit and harden loopback-bound services on the stunnel host to require authentication independently, ensuring that even if loopback restrictions are bypassed, the internal services are not trivially accessible.

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Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
SUSE Linux Enterprise High Performance Computing 12 Not-Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Affected
SUSE Linux Enterprise Module for Server Applications 15 SP7 Affected
SUSE Linux Enterprise Server 15 SP7 Affected
SUSE Linux Enterprise Server 16.0 Affected

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CVE-2026-70367 vulnerability details – vuln.today

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