Tenda G0 Stack Overflow in formSetPortMirror

Overview

CVE-2026-19790 is a remotely exploitable stack-based buffer overflow residing in the formSetPortMirror function within the httpd web management interface of Tenda G0 routers running firmware up to build 20260625. The vulnerable code path is reached via a crafted HTTP POST request to /goform/module, where the portMirrorMirroredPorts parameter is copied into a fixed-size stack buffer without adequate length validation. Because no authentication barrier is described as reliably standing between an attacker and this endpoint in the default configuration, the attack surface is effectively the router’s LAN-facing — and in many deployments, WAN-facing — management interface.

The vulnerability was identified and documented by independent security researchers who published a proof-of-concept demonstrating reliable crash reproduction and the potential for control-flow hijack. Tenda G0 devices are commonly deployed as SMB and branch-office network gateways, meaning successful exploitation can give an attacker a persistent foothold at the network perimeter — an exceptionally high-value position for lateral movement, traffic interception, or command-and-control relay.

From a CWE taxonomy perspective, this is a textbook CWE-121 (Stack-Based Buffer Overflow), compounded by CWE-20 (Improper Input Validation). The CVSS 3.1 score of 8.8 reflects the network-accessible attack vector, low attack complexity, no required privileges, and the full confidentiality, integrity, and availability impact achievable upon successful exploitation.

Technical Analysis

Tenda router firmware is typically built around a uClibc-linked, MIPS or ARM ELF binary (httpd) that handles all web management traffic. CGI-style form handlers are registered by name and dispatched when a matching /goform/ URI is requested. The formSetPortMirror handler processes port mirroring configuration submitted by the administrator UI.

The root cause is a classic unsafe string copy pattern. Inside formSetPortMirror, the firmware retrieves the value of portMirrorMirroredPorts from the HTTP request body using an internal helper — functionally equivalent to websGetVar or a thin wrapper around getenv/nvram_get — and then copies that value directly into a local stack buffer using strcpy (or an equivalent unbounded copy primitive such as sprintf without a width specifier).

/* Simplified reconstruction of the vulnerable pattern in formSetPortMirror */
int formSetPortMirror(webs_t wp, char_t *path, char_t *query)
{
    char mirroredPorts[64];   /* Fixed-size stack buffer */
    char *val;

    /* Retrieve attacker-controlled input from POST body */
    val = websGetVar(wp, "portMirrorMirroredPorts", "");

    /*
     * VULNERABLE: strcpy performs no length check.
     * If strlen(val) >= 64, adjacent stack frames are overwritten,
     * including the saved return address.
     */
    strcpy(mirroredPorts, val);

    /* ... further processing ... */
    return 0;
}

When portMirrorMirroredPorts carries a payload longer than the buffer (64 bytes in this reconstruction — the actual boundary is firmware-specific), strcpy walks past the end of mirroredPorts and overwrites whatever sits above it on the stack: other local variables, the saved frame pointer, and critically the return address. On MIPS targets the return address lives in the saved $ra register slot pushed during the function prologue; on ARM it is the saved LR/PC on the stack. Either way, the attacker controls where execution resumes when formSetPortMirror returns.

Because many Tenda firmware builds ship without stack canaries (-fno-stack-protector is a common cost-cutting choice for embedded targets), without ASLR (or with extremely low-entropy ASLR on 32-bit MIPS), and without NX enforcement backed by hardware, a classic ret2shellcode or ret2libc ROP chain is straightforward to construct — especially given the public proof-of-concept that anchors the overflow offset.

The HTTP endpoint requires only a POST request with a Content-Type: application/x-www-form-urlencoded body. No session token or authenticated cookie is needed to reach formSetPortMirror in the default firmware configuration, which aligns with the CVSS “Privileges Required: None” scoring.

Impact

An unauthenticated remote attacker who can reach the Tenda G0 management interface — typically TCP/80 or TCP/443 on the LAN side, and frequently the WAN side when remote management is enabled — can achieve arbitrary code execution as the process running httpd, which in embedded Linux firmware almost universally runs as root.

Concrete consequences include:

• Full device compromise: The attacker gains a root shell, enabling persistent backdoors via modified nvram settings or injected startup scripts that survive soft reboots.
• Network interception: A compromised gateway can silently redirect, clone, or drop traffic for all connected hosts — enabling credential harvesting, session hijacking, or DNS poisoning at scale.
• Lateral movement pivot: The router’s privileged network position allows the attacker to probe and attack internal hosts that would otherwise be unreachable from the internet.
• Botnet recruitment: IoT routers with publicly reachable management planes are high-value targets for Mirai-variant campaigns; this vulnerability class maps directly to how such botnets are built.
• Denial of Service: Even without successful code execution, a malformed payload crashes the httpd process, disabling web management and, on some firmware variants, triggering a watchdog reboot loop.

The CVSS 3.1 vector AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H accurately reflects these consequences.

How to Fix It

For end users and network administrators:

1. Apply firmware updates immediately once Tenda releases a patched build. Monitor https://www.tenda.com.cn/ for security advisories and updated firmware images for the G0 product line.
2. Disable remote (WAN-side) management until a patch is available. On the G0 this is typically under System → Remote Management.
3. Restrict LAN access to the management interface via firewall ACLs or VLAN segmentation so only trusted administrator hosts can reach TCP/80 and TCP/443.
4. Monitor for exploitation attempts by alerting on anomalously long values in POST bodies destined for /goform/module.

For firmware developers (and Tenda’s engineering team specifically):

Replace all unbounded copy operations on attacker-controlled data with length-limited equivalents and explicit input validation:

/* Fixed version of the vulnerable handler */
#define MIRRORED_PORTS_MAX 63   /* Max legitimate port-list length */

int formSetPortMirror(webs_t wp, char_t *path, char_t *query)
{
    char mirroredPorts[64];
    char *val;
    size_t valLen;

    val = websGetVar(wp, "portMirrorMirroredPorts", "");
    valLen = strlen(val);

    /* Reject inputs that exceed the maximum expected length */
    if (valLen > MIRRORED_PORTS_MAX) {
        websError(wp, 400, "Invalid portMirrorMirroredPorts value");
        return -1;
    }

    /* Safe bounded copy */
    strncpy(mirroredPorts, val, MIRRORED_PORTS_MAX);
    mirroredPorts[MIRRORED_PORTS_MAX] = '\0';

    /* ... further processing ... */
    return 0;
}

Additionally, the firmware build pipeline should enable:

• Stack canaries (-fstack-protector-strong)
• NX/XN bit enforcement for data pages
• Full RELRO and PIE where the toolchain supports it
• Input validation at the HTTP parsing layer before values ever reach form handlers

Our Take

This vulnerability is not unusual — it is the norm for a wide swath of consumer and SMB IoT firmware. The strcpy-into-fixed-buffer pattern has been the root cause of exploitable memory corruption in embedded devices for decades, and it persists because embedded firmware development frequently prioritizes code size and compilation speed over security hygiene. Toolchain defaults that omit stack canaries and ASLR make exploitation trivially reliable once a buffer boundary is known, and the public proof-of-concept here removes even that barrier.

For enterprises operating branch offices or remote sites with Tenda or similar commodity routers, this CVE should serve as a prompt to audit the attack surface of every network perimeter device. “It’s just a router” is not a risk mitigation strategy when that router runs a root-privileged HTTP daemon reachable from the internet.

The broader lesson for developers: input handling at trust boundaries must be treated as adversarial. Every parameter arriving over HTTP is attacker-controlled data. Length validation must happen before — not after — that data is placed in a bounded buffer.

Detection with SAST

Static analysis catches this vulnerability class by modeling data flow from HTTP input sources to unsafe sink functions. Offensive360’s SAST engine flags this pattern under CWE-121: Stack-Based Buffer Overflow and CWE-20: Improper Input Validation, using the following detection strategy:

• Source identification: Functions that retrieve HTTP request parameters (websGetVar, getenv, nvram_get, custom wrappers) are tagged as taint sources returning untrusted, attacker-controlled strings.
• Sink identification: Functions that perform unbounded memory writes — strcpy, strcat, sprintf without a width specifier, gets, scanf("%s", ...) — are flagged as dangerous sinks.
• Taint propagation: The engine traces data flow across assignments, function calls, and pointer dereferences. If a tainted value reaches a dangerous sink operating on a fixed-size stack buffer without an intervening length check, a HIGH severity finding is raised.
• Buffer size modeling: Where the compiler-emitted metadata or source annotations allow stack frame layout inference, the engine estimates the overflowable distance and classifies exploitability.
• Sanitizer detection: Calls to strnlen, strlen-with-comparison, or strncpy/snprintf-with-correct-size are recognized as sanitizers that break the taint path — reducing false positives on correctly guarded copies.

In DAST mode, this class of vulnerability is detected by fuzzing form parameters with progressively longer payloads and monitoring for process crashes, watchdog resets, or anomalous HTTP 500 responses — all indicators of stack corruption.

References

• https://github.com/lipenghai/iot_bug/blob/main/Tenda/G0/1.md
• https://vuldb.com/cve/CVE-2026-19790
• https://vuldb.com/submit/868924
• https://vuldb.com/vuln/389757
• [https://vuldb.com/vuln/389757/cti](https://vuldb.com/vuln/389757/