# CVE-2026-121617: Linksys E5600 buffer overflow in firmware upload

Source: https://junglewise.ai/threats/cve-2026-121617-a-security-flaw-has-been-discovered-in-linksys-e5600-1-1-0-26  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-04. Updated: 2026-11-04  
License: CC BY 4.0 with a link back

CVE-2026-121617 · Severity: high · CVSS 7.5 · Published 2026-11-04

The Linksys E5600 router firmware upload function contains a memory corruption flaw that allows an attacker to crash the web interface remotely without authentication. A specially crafted firmware upload request can overflow a buffer in the header parser, causing the web service to become unavailable. This is particularly severe because the affected router model is no longer supported with security updates.

Technologies: [Linksys E5600](https://junglewise.ai/threats/technologies/e5600). Vendors: [Linksys](https://junglewise.ai/threats/vendors/linksys).

## Quick answers

**Is CVE-2026-121617 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-121617 being exploited in the wild.

**How severe is CVE-2026-121617?** High severity, with a CVSS score of 7.5, published 4 November 2026.

**Which products and versions are affected?** Linksys E5600 1.1.0.26.

**Is there a fix for CVE-2026-121617?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-121617 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-121617?** Check whether you run Linksys E5600 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is an out-of-bounds write (CWE-787) in the init_multipart function of /cgi-bin/upload_fw.cgi. The code uses unbounded strcat() operations to append attacker-controlled multipart headers into a 1024-byte global buffer, with no size checks; each header block contributes up to 1533 bytes and the parsing loop can be re-entered multiple times, yielding attacker-controlled overflow length. The attack requires no authentication and can be exploited remotely via multipart/form-data POST requests.

## Affected products

- Linksys E5600 1.1.0.26

## Timeline

- 2026-11-04: disclosed
- 2026-11-04: exploited: public exploit released

## References

- [https://github.com/Zimo-666/E5600-cves/tree/main/04-upload-fw-multipart-overflow](https://github.com/Zimo-666/E5600-cves/tree/main/04-upload-fw-multipart-overflow)
- [https://vuldb.com/cve/CVE-2026-121617](https://vuldb.com/cve/CVE-2026-121617)

## Related threats

- [Linksys E5600 unauthenticated information disclosure in factory QC page](https://junglewise.ai/threats/cve-2026-121618-a-weakness-has-been-identified-in-linksys-e5600-1-1-0-26-this)
- [Linksys E5600 buffer overflow in mesh_login.cgi](https://junglewise.ai/threats/cve-2026-121616-a-vulnerability-was-identified-in-linksys-e5600-1-1-0-26)
- [Linksys E5600 authentication bypass in mesh login](https://junglewise.ai/threats/cve-2026-121615-a-vulnerability-was-determined-in-linksys-e5600-1-1-0-26)
- [linksys E5600 V1.1.0.26 is vulnerable to command injection in the function ddnsStatus.](https://junglewise.ai/threats/cve-2025-29229-linksys-e5600-v1-1-0-26-is-vulnerable-to-command-injection-in-the)
- [Linksys E5600 V1.1.0.26 is vulnerable to command injection in the runtime.macClone function via the mc.ip parameter.](https://junglewise.ai/threats/cve-2025-29228-linksys-e5600-v1-1-0-26-is-vulnerable-to-command-injection-in-the)