CVE-2026-16027 in Türkiye E-Signature
Summary
by MITRE • 08/07/2026
Server-Side request forgery (SSRF) vulnerability in Revenue Administration Türkiye's E-Signature allows Server Side Request Forgery.
This issue affects Türkiye's E-Signature: from 2.4.4.0 before 2.5.1.0.
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Analysis
by VulDB Data Team • 08/07/2026
Server-side request forgery vulnerabilities represent one of the most critical security flaws in web applications, particularly when they enable unauthorized access to internal systems that should remain isolated from external networks. The vulnerability identified in Revenue Administration Türkiye's E-Signature system demonstrates how a seemingly minor implementation flaw can create significant operational risks. This specific SSRF vulnerability affects versions 2.4.4.0 through 2.5.0.9 of the e-signature solution, creating a pathway for malicious actors to bypass normal network security controls and potentially access sensitive internal resources.
The technical implementation flaw in this e-signature system stems from insufficient validation of user-supplied input that is subsequently used to construct HTTP requests to external services. When the application processes requests containing crafted URLs or parameters, it fails to properly sanitize or validate these inputs before using them to establish connections to backend systems. This allows attackers to manipulate the application's behavior and redirect requests to internal network resources that would normally be inaccessible from the internet. The vulnerability operates at the application layer and can be exploited through various mechanisms including direct URL manipulation, parameter injection, or even through encoded payloads that bypass initial validation checks.
The operational impact of this SSRF vulnerability within the Turkish revenue administration context is particularly concerning given the sensitive nature of tax-related data and financial transactions processed through the e-signature system. Attackers could potentially leverage this flaw to access internal databases containing taxpayer information, financial records, or other confidential administrative data. The vulnerability also poses risks to the broader network infrastructure as it may enable lateral movement within the organization's internal systems, allowing unauthorized access to additional services such as internal APIs, management interfaces, or other backend applications that reside behind network firewalls.
This vulnerability aligns with CWE-918, which specifically addresses server-side request forgery in web applications where an attacker can manipulate a server into making requests to arbitrary destinations. The attack patterns typically follow the techniques outlined in the MITRE ATT&CK framework under the T1071.004 sub-technique for application layer protocol tunneling and T1082 for system information discovery, as attackers often first enumerate internal services before attempting exploitation. The vulnerability's presence in multiple versions of the e-signature software indicates a persistent design flaw that requires immediate remediation.
Mitigation strategies for this SSRF vulnerability must address both the immediate technical issues and implement comprehensive security controls to prevent similar flaws from occurring in future iterations of the system. Organizations should deploy network-level restrictions using firewalls and access control lists to block outbound connections to internal network ranges, implementing strict egress filtering rules that prevent applications from communicating with private IP addresses. Input validation and sanitization mechanisms must be strengthened through proper parameter validation, URL parsing, and content-type checking to ensure that user-supplied inputs cannot be used to construct malicious requests. Additionally, implementing a whitelist approach for allowed external domains and services can significantly reduce the attack surface while maintaining legitimate functionality.
The remediation process should include comprehensive code review and security testing of all application components that handle external requests, particularly focusing on URL parsing functions, HTTP client implementations, and parameter handling mechanisms. Automated security scanning tools should be deployed to identify similar vulnerabilities in other applications within the organization's infrastructure. Regular penetration testing and vulnerability assessments should be conducted to ensure that network security controls remain effective against evolving attack techniques. The implementation of a security incident response plan is also crucial to quickly detect and respond to potential exploitation attempts, including monitoring for unusual outbound network traffic patterns that may indicate successful SSRF attacks.