Tenda A15 UploadCfg Stack Buffer Overflow (CVE-2026-4567)

Executive Summary

A critical stack-based buffer overflow vulnerability (CVE-2026-4567) has been disclosed in the Tenda A15 wireless router, affecting firmware version 15.13.07.13. The flaw resides in the UploadCfg function accessible via the /cgi-bin/UploadCfg endpoint. Manipulation of the File argument triggers an uncontrolled stack overflow, enabling remote code execution. With a CVSS score of 9.8 (Critical), the vulnerability requires no authentication and can be exploited over the network. A public proof-of-concept exploit has already been disclosed.

CVSS Score: 9.8 (Critical) CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Vulnerability Overview

Affected Products

The Tenda A15 is a dual-band 1200 Mbps wireless router widely deployed in home and small-business environments. The /cgi-bin/UploadCfg endpoint is part of the device's web-based management interface used for configuration backup and restore operations.

Technical Details

Vulnerability Root Cause

The UploadCfg function in Tenda A15 firmware 15.13.07.13 fails to validate the size of the File argument before copying it into a fixed-size stack buffer. When a specially crafted request supplies an oversized value, the resulting buffer overflow overwrites adjacent stack memory — including the saved return address — enabling an attacker to redirect execution to arbitrary code.

Attack Chain

1. Attacker discovers Tenda A15 management interface reachable on the network
   (typically port 80 on the LAN-side; misconfigured devices may expose WAN-side)
 
2. Attacker crafts a POST request to /cgi-bin/UploadCfg with an
   oversized File parameter value
 
3. UploadCfg copies attacker-controlled data into a fixed stack buffer
   without bounds checking — overflowing the buffer
 
4. Adjacent stack memory is overwritten, including the saved return address
 
5. On function return, CPU jumps to an attacker-controlled address
   (shellcode or ROP chain)
 
6. Arbitrary code executes with the privileges of the web server process
   (typically root on embedded Linux devices)

Why CVSS 9.8?

The score reflects full network-accessible exploitation requiring no authentication and no user interaction, with low complexity. All three impact categories (Confidentiality, Integrity, Availability) are rated High — an attacker achieves complete control of the device. The score reaches Critical tier due to the combination of unauthenticated remote access and the ubiquity of Tenda A15 deployments.

Impact Assessment

Recommendations

Immediate Actions

1. Restrict management interface access — ensure the Tenda A15 web interface is not reachable from untrusted networks or the WAN-side
2. Check Tenda's support portal for a firmware update addressing CVE-2026-4567; apply immediately if available
3. If no patch is available, place the device behind a separate router or firewall that blocks access to port 80/443 from untrusted segments
4. Change default admin credentials if not already done

Network-Level Mitigations

- Block external access to the Tenda A15 management interface at the
  perimeter firewall (port 80 and 443)
- Apply ACLs restricting management interface access to trusted LAN
  hosts only
- Monitor outbound connections from the router for anomalous patterns
- Enable logging on upstream devices to detect unusual traffic
  originating from the A15's IP address

End-of-Life and Vendor Patch Considerations

Tenda has a mixed track record with security patching for consumer-grade routers. If firmware 15.13.07.13 does not receive a patch:

1. Replace the Tenda A15 with a supported router from a vendor
   with an active security response program
2. Use a separate upstream firewall to compensate for the lack of patch
3. Segment the A15 onto a DMZ or isolated VLAN
4. Monitor Tenda's official security advisory page for any out-of-band fix

Detection Indicators

Example Detection Rule (Suricata)

alert http $EXTERNAL_NET any -> $HOME_NET 80 (
  msg:"CVE-2026-4567 Tenda A15 UploadCfg Stack Overflow Attempt";
  flow:to_server,established;
  content:"/cgi-bin/UploadCfg";
  http_uri;
  http_method;
  content:"POST";
  dsize:>1024;
  classtype:attempted-admin;
  sid:9042026;
  rev:1;
)

Post-Remediation Checklist

1. Verify firmware version — confirm the patched firmware replaces 15.13.07.13 if a fix is released
2. Rotate all credentials stored or configured on the device (Wi-Fi PSKs, admin password, ISP credentials)
3. Audit firewall rules — confirm the management interface is not reachable from untrusted segments
4. Review DHCP leases and ARP tables for unauthorized devices that may have connected during any compromise window
5. Run a network scan to identify any other Tenda A15 devices sharing the same vulnerability
6. Update IDS/IPS signatures to detect exploitation attempts against this endpoint

References

• NIST NVD — CVE-2026-4567
• Tenda Official Support
• CWE-121: Stack-Based Buffer Overflow