CVE-2026-78861 in AC12
Summary
by MITRE • 10/06/2026
An issue in Mercusys AC12 V2 allows a local attacker to execute arbitrary code via a hardcoded 512-bit RSA Private Key
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Analysis
by VulDB Data Team • 10/06/2026
The vulnerability identified in the Mercusys AC12 V2 wireless router represents a critical failure in cryptographic key management, specifically involving a hardcoded 512-bit RSA private key. This flaw allows any local attacker with network access to potentially execute arbitrary code on the device or compromise its integrity and confidentiality. The presence of a static, pre-generated private key embedded directly into the firmware is a severe deviation from established security best practices. In secure systems, cryptographic keys must be unique per device instance, generated using high-entropy random sources during manufacturing or first boot, and protected against extraction. By utilizing a hardcoded key that is likely shared across all units of this model, Mercusys has created a single point of failure where compromise of the key leads to total loss of security for every affected device in the field.
From a technical perspective, RSA keys are designed to be asymmetric, meaning they consist of a public key used for encryption or signature verification and a private key used for decryption or signing. The security relies entirely on the secrecy of the private key. When this key is hardcoded into the firmware image, it becomes accessible to anyone who can reverse-engineer the device's software. A 512-bit RSA key is also cryptographically weak by modern standards; while not immediately breakable via brute force due to its size, it offers significantly less security margin than current recommendations of at least 2048 bits. More critically, because the same private key is used across all devices, an attacker who obtains this key can impersonate the device in any communication channel that relies on mutual authentication or encrypted sessions protected by this specific key. This undermines Transport Layer Security (TLS) and other protocol-level protections intended to secure management interfaces and data transmission between the router and connected clients or cloud services.
The operational impact of this vulnerability is profound, particularly regarding remote code execution potential if combined with other flaws in the device's web interface or administrative API. If an attacker can leverage their knowledge of the private key to bypass authentication mechanisms or decrypt sensitive configuration data, they may gain unauthorized access to the router’s command line interface or embedded services. This level of access enables the injection of malicious scripts, modification of DNS settings for phishing campaigns, interception of unencrypted traffic via man-in-the-middle attacks, and use of the compromised device as a pivot point within the local network. Furthermore, because the key is static, patching this issue requires not only updating the firmware to remove or randomize the key but also ensuring that any existing certificates issued with the old key are revoked and replaced across all deployed units, which poses significant logistical challenges for both the vendor and end-users who may have outdated software versions.
This vulnerability aligns closely with CWE-798: Use of Hard-coded Credentials, as it involves storing sensitive authentication information directly within the source code or firmware binary rather than deriving it dynamically. It also relates to CWE-326: Inadequate Encryption Strength, given that 512-bit RSA is considered obsolete and insufficient for protecting modern communications against determined adversaries with sufficient computational resources. From an offensive security perspective, this flaw facilitates actions categorized under MITRE ATT&CK techniques such as T1078: Valid Accounts, where attackers use legitimate credentials to maintain persistence, or T1553: Subvert Trust Controls by bypassing signature verification mechanisms that rely on the compromised private key. The ability to decrypt traffic signed with this key also touches upon T1429: Unauthorized API Access if the router exposes APIs protected solely by certificate-based authentication.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. For end-users, the primary defense is to immediately update the firmware on all Mercusys AC12 V2 devices to a version that implements unique key generation per device. Users should also change default administrative passwords and disable remote management features if not strictly necessary, reducing the attack surface available to local or external actors. Network segmentation can help limit the blast radius of a compromised router by isolating IoT devices from critical network resources. For manufacturers, this incident highlights the necessity of implementing secure boot processes that verify firmware signatures using unique device-specific keys stored in hardware security modules (HSMs) rather than software-stored hardcoded values. Additionally, adopting automated key rotation policies and conducting regular third-party security audits can prevent similar vulnerabilities from reaching production environments. The industry standard requires moving away from static cryptographic material toward dynamic, per-device identity management to ensure that compromise of one unit does not lead to systemic failure across the entire product line.