CVE-2026-121606 in D621-2
Summary
by MITRE • 11/04/2026
A vulnerability has been found in Qihoo 360 D621-2, D603-2, D600-2, D619, D606-6 and D706 up to 6.4.9. Affected by this issue is the function verify_sign of the file fs2/bin/iscm of the component Firmware Signature Verification. Such manipulation of the argument sign leads to improper verification of cryptographic signature. The attack can be launched remotely. The exploit has been disclosed to the public and may be used. The vendor was contacted early about this disclosure but did not respond in any way.
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Analysis
by VulDB Data Team • 11/04/2026
The identified vulnerability resides within the firmware signature verification component of specific Qihoo 360 router models, including the D621-2, D603-2, D600-2, D619, D606-6, and D706 devices running firmware versions up to 6.4.9. The core technical flaw is located in the verify_sign function within the fs2/bin/iscm binary file. This component is responsible for validating the cryptographic integrity of firmware updates before they are applied to the device. The vulnerability stems from improper handling of the sign argument, which allows an attacker to manipulate this input such that the verification process fails to correctly validate the digital signature. Consequently, the system may accept maliciously crafted or tampered firmware images as legitimate, bypassing critical security checks designed to prevent unauthorized modifications to the device's operating environment.
From a technical perspective, this flaw represents a classic failure in cryptographic validation logic, often categorized under CWE-347: Improper Verification of Cryptographic Signature. When an application fails to properly verify that a digital signature corresponds to the data it claims to sign and was created by a trusted entity using the correct private key, it opens the door for integrity violations. In this specific instance, the manipulation of the sign argument suggests that the verification routine does not strictly enforce the mathematical relationship between the public key embedded in the firmware image and the provided signature. This could result from insufficient length checks, lack of padding validation, or a logical error in how the hash of the payload is compared against the decrypted signature value. Such weaknesses are particularly dangerous in Internet of Things devices where secure boot chains rely heavily on these verification steps to ensure that only authorized code executes with high privileges.
The operational impact of this vulnerability is severe due to its remote exploitability and public disclosure status. Because the attack can be launched remotely, an adversary does not need physical access to the router or prior authentication credentials to initiate a compromise. By exploiting this flaw, an attacker could upload malicious firmware that installs backdoors, rootkits, or botnet agents directly onto the device's file system. Once executed with elevated privileges typical of embedded systems, this malware can be used to hijack the network for distributed denial-of-service attacks, intercept unencrypted traffic through man-in-the-middle techniques, or pivot into other devices on the local area network. The fact that the exploit has been disclosed publicly and is available for use significantly increases the risk landscape, as automated scanning tools and opportunistic attackers can easily target vulnerable instances across the internet without needing to develop custom exploitation code from scratch.
The vendor's lack of response following early contact further exacerbates the security posture of these devices. In vulnerability management frameworks such as MITRE ATT&CK, this scenario aligns with techniques involving supply chain compromise or firmware modification, specifically falling under tactics related to persistence and privilege escalation via system firmware. Without a patch from Qihoo 360, users are left without an official remediation path that addresses the root cause in the binary code. This absence of vendor support forces administrators to rely on workarounds rather than definitive fixes.
To mitigate this risk, immediate action is required at both the network and device configuration levels. First, if remote management interfaces are enabled on these routers, they should be disabled immediately unless absolutely necessary for administrative access. Second, users should restrict inbound traffic from untrusted networks to any ports associated with firmware update mechanisms or web administration panels. Third, since no patch is available, administrators must consider replacing the affected hardware with a model that receives regular security updates and has a transparent vulnerability response policy. For those who cannot replace the devices immediately, monitoring network logs for unusual outbound connections or unauthorized configuration changes can help detect potential compromise early. Additionally, ensuring that any local firmware uploads are strictly controlled by whitelisting allowed IP addresses in router settings adds an extra layer of defense against remote exploitation attempts targeting this signature verification flaw.