CVE-2026-121617 in E5600
Summary
by MITRE • 11/04/2026
A security flaw has been discovered in Linksys E5600 1.1.0.26. Affected by this issue is the function init_multipart of the file /cgi-bin/upload_fw.cgi of the component Header Parser. The manipulation results in out-of-bounds write. The attack may be launched remotely. The exploit has been released to the public and may be used for attacks. The vendor was contacted early about this disclosure but did not respond in any way. This vulnerability only affects products that are no longer supported by the maintainer.
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Analysis
by VulDB Data Team • 11/04/2026
The Linksys E5600 router, specifically running firmware version 1.1.0.26, contains a critical remote code execution vulnerability within its web management interface. The flaw resides in the init_multipart function located in the /cgi-bin/upload_fw.cgi script, which is responsible for parsing multipart form data during firmware upload operations. This component acts as a header parser that processes incoming HTTP requests to extract file metadata and content boundaries. Due to insufficient validation of input parameters related to boundary strings or length fields, an attacker can craft maliciously formatted HTTP POST requests that trigger an out-of-bounds write condition in the device's memory space.
This vulnerability is classified under CWE-787: Out-of-Bounds Write, a category of software errors where a program writes data beyond the allocated buffer boundaries. In the context of embedded devices like routers, such memory corruption can lead to arbitrary code execution if an attacker successfully overwrites critical control structures or function pointers within the device's firmware environment. The attack vector is remote and network-based, meaning that any user with access to the router’s management interface on port 80 or 443 can potentially exploit this flaw without requiring prior authentication in some configurations, or by leveraging existing low-privilege credentials if authenticated access is required for upload functions.
The operational impact of this vulnerability is severe, as it allows an attacker to gain full control over the compromised device. Once executed, malicious code could be used to pivot into internal networks, intercept sensitive traffic, install persistent backdoors, or enlist the router in botnets such as Mirai variants that target IoT devices for distributed denial-of-service attacks. The severity is further exacerbated by the fact that a public exploit has been released, lowering the barrier to entry for opportunistic attackers scanning the internet for vulnerable targets.
Compounding the risk is the lack of vendor support and remediation efforts. Linksys did not respond to disclosure attempts regarding this issue, leaving users with no official patch or firmware update available to mitigate the flaw. This absence of a fix places significant responsibility on network administrators to implement compensating controls. Since the product is end-of-life and no longer supported by the manufacturer, relying solely on vendor-provided security updates is not viable for long-term protection.
To mitigate this risk in environments where such devices remain deployed, organizations should isolate these routers from critical internal networks using VLAN segmentation or firewall rules that restrict management interface access to trusted IP addresses only. Additionally, disabling remote administration features and ensuring that the web interface is not exposed directly to the internet are essential steps. Given the availability of public exploits, proactive monitoring for suspicious HTTP POST requests targeting /cgi-bin/upload_fw.cgi with malformed multipart boundaries can aid in early detection attempts before exploitation occurs. Ultimately, replacing unsupported hardware with modern, actively maintained security appliances remains the most effective long-term solution to eliminate this class of vulnerabilities entirely.