CVE-2026-36778 Overview
CVE-2026-36778 is a stack-based buffer overflow [CWE-121] in the Shenzhen Tenda Technology Tenda O3 Wireless Router, firmware version v1.0.0.5(4180). The flaw resides in the R7WebsSecurityHandler function, which improperly handles the username parameter from incoming HTTP requests. An authenticated attacker can send a crafted HTTP request to trigger the overflow and crash the device, resulting in a denial of service (DoS). The issue is tracked with an EPSS probability of 0.149%.
Critical Impact
Authenticated attackers on the network can crash the router by sending an oversized username field, disrupting wireless connectivity for all clients.
Affected Products
- Shenzhen Tenda Technology Co., Ltd Tenda O3 Wireless Router
- Firmware version v1.0.0.5(4180)
- Web management interface component handling the R7WebsSecurityHandler function
Discovery Timeline
- 2026-06-09 - CVE-2026-36778 published to NVD
- 2026-06-10 - Last updated in NVD database
Technical Details for CVE-2026-36778
Vulnerability Analysis
The vulnerability exists in the R7WebsSecurityHandler function within the Tenda O3 router's embedded web server. The function processes login-related HTTP parameters, including username, without validating the length of the input before copying it into a fixed-size stack buffer. When the supplied username value exceeds the buffer capacity, the write operation corrupts adjacent stack memory, including saved registers and return addresses.
The corruption causes the web server process to crash. On embedded routers running a single-process httpd, this terminates the management plane and can destabilize networking services that share the runtime. The classified impact is loss of availability, as confidentiality and integrity remain unaffected per the assigned vector.
Root Cause
The root cause is missing bounds checking on the username parameter inside R7WebsSecurityHandler. The function uses unsafe string handling routines such as strcpy or equivalent fixed-buffer copies without enforcing maximum input length. This pattern aligns with [CWE-121: Stack-based Buffer Overflow] and is common in MIPS-based SOHO router firmware that reuses legacy GoAhead-derived web stacks.
Attack Vector
Exploitation requires network access to the router's web management interface and valid authentication credentials. An attacker submits an HTTP request, typically a POST to a login or security endpoint, containing an overlong username parameter. When the firmware processes the request, the stack overflow corrupts execution state and crashes the daemon. The crash forces administrators to reboot the device to restore management access.
For additional technical context, see the GitHub PoC Repository.
Detection Methods for CVE-2026-36778
Indicators of Compromise
- Repeated crashes or restarts of the router's HTTP management daemon without a corresponding administrator action
- HTTP requests to authentication endpoints containing abnormally long username field values
- Loss of web UI availability while the device remains reachable on the network briefly before reboot
Detection Strategies
- Inspect HTTP traffic destined for the router management interface and flag requests where the username parameter exceeds expected length thresholds, such as 64 bytes
- Correlate router availability alerts with preceding HTTP POST traffic to the security or login handler
- Review router syslog entries, if forwarded, for httpd segmentation faults or watchdog-triggered restarts
Monitoring Recommendations
- Forward router logs to a centralized log platform and alert on repeated httpd restarts
- Apply network intrusion detection signatures that match overlong parameter values in HTTP POST bodies targeting /goform/ or equivalent management URIs
- Track management-plane reachability with active probes to detect short outages caused by exploitation attempts
How to Mitigate CVE-2026-36778
Immediate Actions Required
- Restrict access to the router web management interface to trusted management VLANs or specific administrative hosts
- Rotate administrative credentials, since exploitation requires authentication, and remove any unused accounts
- Disable remote WAN-side management if it is currently enabled
Patch Information
No vendor advisory or firmware patch has been published for Tenda O3 firmware v1.0.0.5(4180) at the time of writing. Monitor the Tenda support portal for an updated firmware release that addresses the R7WebsSecurityHandler input validation flaw.
Workarounds
- Place the affected router behind a network segment that blocks untrusted clients from reaching its HTTP management port
- Use ACLs on upstream switches to permit management traffic only from designated administrator IP addresses
- Replace the device with a supported model if a vendor patch is not released within an acceptable risk window
# Example ACL restricting router management interface to a single admin host
# Replace 192.0.2.10 with the administrator workstation IP
# Replace 10.0.0.1 with the Tenda O3 management IP
iptables -A FORWARD -s 192.0.2.10 -d 10.0.0.1 -p tcp --dport 80 -j ACCEPT
iptables -A FORWARD -d 10.0.0.1 -p tcp --dport 80 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
