CVE-2025-5963 in Postbox
Summary
by MITRE • 06/20/2025
The Postbox's configuration on macOS, specifically the presence of entitlements: "com.apple.security.cs.allow-dyld-environment-variables" and "com.apple.security.cs.disable-library-validation" allows for Dynamic Library (Dylib) injection. A local attacker with unprivileged access can use environment variables like DYLD_INSERT_LIBRARIES to successfully inject code in application's context and bypass Transparency, Consent, and Control (TCC). Acquired resource access is limited to previously granted permissions by the user. Access to other resources beyond granted-permissions requires user interaction with a system prompt asking for permission.
The original company behind Postbox is no longer operational, the software will no longer receive updates. The acquiring company (em Client) did not cooperate in vulnerability disclosure.
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Analysis
by VulDB Data Team • 06/24/2025
CVE-2025-5963 represents a critical privilege escalation vulnerability within the Postbox email client on macOS systems, leveraging improper entitlement configuration that fundamentally undermines the operating system's security model. The vulnerability stems from the application's inclusion of two dangerous entitlements: "com.apple.security.cs.allow-dyld-environment-variables" and "com.apple.security.cs.disable-library-validation" which together create an exploitable attack surface. These entitlements effectively disable macOS's dynamic library validation mechanisms and allow arbitrary dylib loading through environment variables, creating a pathway for code injection that bypasses the system's Transparency, Consent, and Control (TCC) framework. The flaw operates at the core of macOS security architecture, specifically targeting the System Integrity Protection (SIP) and code signing enforcement mechanisms that are designed to prevent unauthorized code execution within application contexts.
The technical exploitation of this vulnerability follows a well-established pattern of dynamic library injection that aligns with CWE-426 and ATT&CK technique T1546.008, where an attacker manipulates environment variables to inject malicious libraries into the target process. When a local attacker with unprivileged access sets the DYLD_INSERT_LIBRARIES environment variable to point to a malicious dylib, the system loads this library in the context of the Postbox process. This injection bypasses TCC protections because the malicious code executes within the legitimate application's security context, inheriting the application's granted permissions. The vulnerability is particularly concerning because it operates below the level of user awareness, allowing code execution without the typical system prompts that would normally require user consent for new permissions.
The operational impact of CVE-2025-5963 extends beyond simple code injection to represent a complete bypass of macOS's application sandboxing model. While the acquired access is limited to permissions previously granted by the user, the vulnerability allows an attacker to escalate privileges within the application's existing permission scope. This creates a significant risk for users who have granted Postbox access to sensitive resources such as email accounts, calendar data, or file system access. The vulnerability's persistence is enhanced by the fact that Postbox is no longer receiving updates, meaning users cannot benefit from security patches or remediations. The lack of cooperation from em Client, the acquiring company, compounds the issue by preventing coordinated vulnerability disclosure and the development of proper mitigations. The attack vector requires only local access and does not rely on network connectivity, making it particularly dangerous in environments where physical access to systems is possible.
Mitigation strategies for CVE-2025-5963 should focus on immediate defensive measures given the lack of vendor support for patching. System administrators should implement strict monitoring of Postbox processes and their environment variable configurations to detect potential exploitation attempts. The most effective immediate solution involves removing or disabling Postbox from affected systems until a secure alternative is implemented, as the vulnerability cannot be patched due to the software's discontinued support status. Organizations should also implement endpoint detection and response solutions that can monitor for suspicious environment variable modifications and dylib loading patterns. The vulnerability's classification under CWE-276 and its alignment with ATT&CK techniques T1548.001 and T1059.003 highlight the need for comprehensive endpoint security controls. Additionally, users should be educated about the risks of running outdated software and the importance of maintaining current security practices, particularly in environments where legacy applications continue to operate without vendor support. The vulnerability demonstrates the critical importance of proper entitlement management in macOS applications and serves as a reminder of the long-term security implications of discontinued software support.