CVE-2026-70367 in stunnel
Summary
by MITRE • 08/04/2026
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.
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Analysis
by VulDB Data Team • 08/04/2026
The vulnerability under discussion represents a critical server-side request forgery flaw in stunnel version 5.79 and earlier when operating in SOCKS proxy mode. This security weakness fundamentally undermines the intended network isolation mechanisms that protect localhost services from external access. The flaw specifically manifests in how the software handles address resolution and connection routing, creating an unexpected pathway for malicious actors to bypass localhost restrictions that are typically enforced by network configuration and operating system level protections.
The technical implementation of this vulnerability relies on the exploitation of IPv4-mapped IPv6 addresses and unspecified address formats to circumvent the normal access controls. When stunnel processes requests in SOCKS proxy mode, it fails to properly validate or sanitize address inputs that contain IPv4-mapped IPv6 representations such as "::ffff:127.0.0.1" or unspecified addresses like "0.0.0.0" and "::". These address formats can be interpreted by the system in ways that allow connections to loopback interfaces despite the intended network restrictions. This behavior violates fundamental security principles of network isolation and demonstrates a failure in input validation mechanisms.
The operational impact of this vulnerability extends beyond simple network access bypass, as it enables attackers to potentially reach services that are explicitly configured to only accept local connections. Loopback-only services such as database servers, administrative interfaces, or internal APIs that should remain inaccessible from external networks become vulnerable to exploitation. This creates a significant risk for systems where stunnel serves as a proxy layer between internal and external network traffic, potentially allowing unauthorized access to sensitive backend systems and data repositories.
Security professionals can reference this vulnerability through CWE-918 which specifically addresses Server-Side Request Forgery vulnerabilities, and it aligns with ATT&CK technique T1071.004 for application layer protocol manipulation. The vulnerability demonstrates how proxy configurations can create unexpected attack vectors when address resolution is not properly validated against security boundaries. Organizations should implement immediate mitigations including upgrading to stunnel versions that address this specific flaw, implementing additional network-level restrictions, and conducting comprehensive audits of all proxy configurations to ensure proper isolation between internal and external network segments.
The root cause analysis reveals that this vulnerability stems from insufficient validation of address formats within the SOCKS proxy implementation. The software fails to properly distinguish between legitimate external addresses and those that should be restricted to local access, creating a path where attackers can manipulate connection requests through carefully crafted address representations. This highlights the importance of thorough input validation in network security applications and demonstrates how seemingly innocuous address format handling can create significant security weaknesses in proxy implementations.
Organizations should consider implementing multiple layers of defense including network segmentation, firewall rules that restrict access to loopback interfaces, and monitoring solutions that can detect unusual connection patterns originating from proxy services. The vulnerability also underscores the necessity of regular security assessments of proxy configurations and the importance of staying current with vendor security patches. Additionally, administrators should review and test their proxy configurations to ensure that localhost restrictions remain effective even when using advanced address formats that may be valid in certain contexts but inappropriate for security purposes.
The remediation approach must include not only updating to patched versions of stunnel but also implementing comprehensive network access controls that prevent unauthorized access to loopback services regardless of the specific address format used. Security teams should establish monitoring protocols that can identify attempts to use IPv4-mapped addresses or unspecified addresses in proxy connections, providing early detection capabilities for potential exploitation attempts. This vulnerability serves as a reminder of the critical importance of validating all network inputs in security-sensitive applications and the potential consequences when such validation is insufficient.