Comprehensive Technical Analysis of CVE-2025-69766

CVE ID: CVE-2025-69766 CVSS Score: 9.8 (Critical) Affected Product: Tenda AX3 Router (Firmware v16.03.12.11) Vulnerability Type: Stack-Based Buffer Overflow (CWE-121)

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1. Vulnerability Assessment & Severity Evaluation

Technical Overview

CVE-2025-69766 is a stack-based buffer overflow vulnerability in the formGetIptv function of Tenda AX3 router firmware (v16.03.12.11). The flaw arises from improper bounds checking when processing the citytag parameter, leading to memory corruption and potential remote code execution (RCE).

Severity Justification (CVSS 9.8)

Metric	Value	Explanation
Attack Vector (AV)	Network (N)	Exploitable remotely over the network without authentication.
Attack Complexity (AC)	Low (L)	No specialized conditions required; straightforward exploitation.
Privileges Required (PR)	None (N)	No prior authentication needed.
User Interaction (UI)	None (N)	Exploitation does not require user interaction.
Scope (S)	Unchanged (U)	Impact is confined to the vulnerable device.
Confidentiality (C)	High (H)	Successful exploitation could lead to full system compromise.
Integrity (I)	High (H)	Attacker can modify memory, execute arbitrary code.
Availability (A)	High (H)	Device may crash or become unresponsive.

Resulting CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Severity: Critical (9.8) – High-risk vulnerability due to RCE potential with no authentication required.

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2. Potential Attack Vectors & Exploitation Methods

Exploitation Pathways

1. Unauthenticated Remote Exploitation

   • The vulnerability is exposed via the web interface of the Tenda AX3 router.
   • An attacker can send a maliciously crafted HTTP request containing an oversized citytag parameter to trigger the buffer overflow.
   • No prior authentication is required, making this a pre-authentication RCE vulnerability.

2. Memory Corruption & Code Execution

   • The formGetIptv function fails to validate the length of the citytag input, leading to a stack smashing scenario.
   • If the attacker controls the return address or stack canary, they can redirect execution flow to injected shellcode.
   • Successful exploitation could lead to:
     • Arbitrary code execution (e.g., reverse shell, firmware modification).
     • Denial of Service (DoS) (device crash due to memory corruption).
     • Persistence mechanisms (e.g., backdoor installation).

3. Exploitation Requirements

   • Network Access: Attacker must be able to send HTTP requests to the router (LAN or WAN, depending on configuration).
   • No User Interaction: Exploitation is fully automated.
   • Minimal Attack Complexity: Publicly available buffer overflow exploitation techniques (e.g., Metasploit modules) could be adapted.

Proof-of-Concept (PoC) Exploitation Steps

1. Identify Vulnerable Endpoint

   • The formGetIptv function is likely accessible via:

     POST /goform/formGetIptv HTTP/1.1
     Host: <ROUTER_IP>
     Content-Type: application/x-www-form-urlencoded
     

2. Craft Malicious Payload

   • Overwrite the citytag parameter with a long string (e.g., 1000+ bytes) to overflow the stack.
   • Example payload:

     citytag=AAAAAAAAAAAAAAAAAAAA...[SHELLCODE/ROP_CHAIN]...
     

3. Control Execution Flow

   • If ASLR/DEP are not enabled, direct shellcode execution may be possible.
   • If stack canaries are present, bypass techniques (e.g., brute-forcing, information leaks) may be required.
   • Return-Oriented Programming (ROP) could be used to bypass NX (No-Execute) protections.

4. Post-Exploitation

   • Dump firmware for further analysis.
   • Install backdoors (e.g., SSH, Telnet, or custom malware).
   • Pivot to internal networks (lateral movement).

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3. Affected Systems & Software Versions

Vulnerable Product

• Device: Tenda AX3 Wi-Fi 6 Router
• Firmware Version: v16.03.12.11 (confirmed vulnerable)
• Potential Other Versions:
  • Earlier versions (if formGetIptv logic is similar).
  • Later versions (if the vulnerability was not patched).

Impacted Environments

• Home Networks: Consumer-grade routers are prime targets for botnets (e.g., Mirai variants).
• Small Businesses: Unpatched routers can serve as entry points for ransomware or data exfiltration.
• IoT Ecosystems: Compromised routers can be used to attack other IoT devices on the same network.

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4. Recommended Mitigation Strategies

Immediate Actions

1. Apply Vendor Patch

   • Check for firmware updates from Tenda’s official website.
   • If no patch is available, disable remote administration (WAN access) to reduce attack surface.

2. Network-Level Protections

   • Firewall Rules: Block external access to the router’s web interface (port 80/443).
   • Intrusion Prevention System (IPS): Deploy signatures to detect buffer overflow attempts (e.g., Snort/Suricata rules).
   • Network Segmentation: Isolate the router from critical internal systems.

3. Temporary Workarounds

   • Disable IPTV Functionality: If not in use, disable the formGetIptv endpoint via router settings.
   • Input Sanitization: If possible, modify the router’s web server to enforce strict input validation.

Long-Term Recommendations

1. Vendor & Supply Chain Security

   • Firmware Audits: Encourage Tenda to conduct static/dynamic analysis of their firmware.
   • Automated Testing: Implement fuzz testing (e.g., AFL, LibFuzzer) to detect similar vulnerabilities.
   • Secure Development Lifecycle (SDL): Adopt code reviews, static analysis (SAST), and binary hardening (e.g., stack canaries, ASLR, NX).

2. User & Administrator Best Practices

   • Regular Firmware Updates: Enable automatic updates if available.
   • Change Default Credentials: Use strong, unique passwords for router admin access.
   • Disable Unused Services: Turn off UPnP, Telnet, and other unnecessary features.
   • Monitor for Exploitation Attempts: Check router logs for unusual HTTP requests.

3. Incident Response Preparedness

   • Isolate & Reimage: If exploitation is suspected, factory reset the device and reinstall firmware.
   • Forensic Analysis: Capture memory dumps and network traffic for post-incident analysis.
   • Threat Intelligence Sharing: Report exploitation attempts to CISA, MITRE, or local CERTs.

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5. Impact on the Cybersecurity Landscape

Broader Implications

1. Exploitation in the Wild

   • Given the CVSS 9.8 rating, this vulnerability is highly attractive to threat actors, including:
     • Botnet Operators (e.g., Mirai, Mozi) for DDoS amplification.
     • APT Groups for persistent access to target networks.
     • Ransomware Gangs for initial access brokering.

2. Supply Chain Risks

   • Tenda routers are widely used in consumer and SMB markets, increasing the attack surface.
   • Similar vulnerabilities in other Tenda models (e.g., AC series) may exist due to code reuse.

3. Regulatory & Compliance Concerns

   • GDPR, CCPA, NIS2: Unpatched critical vulnerabilities may lead to data breaches, triggering regulatory fines.
   • FCC & IoT Security Standards: Non-compliance with NIST IR 8259, ETSI EN 303 645 could result in market restrictions.

4. Threat Intelligence & Detection

   • Exploit Kits: Likely to be added to Metasploit, Cobalt Strike, or custom malware frameworks.
   • Detection Rules: Security vendors should develop Snort/Suricata/YARA rules for this CVE.

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6. Technical Details for Security Professionals

Root Cause Analysis

• Vulnerable Function: formGetIptv (likely in /bin/httpd or a similar web server binary).
• Flaw: The function copies user-supplied citytag input into a fixed-size stack buffer without length validation.
• Assembly Snippet (Hypothetical):

  ; Pseudocode of vulnerable function
  formGetIptv:
      sub    esp, 0x100          ; Allocate 256-byte buffer on stack
      mov    eax, [esp+0x104]    ; Load user-controlled 'citytag' parameter
      mov    edi, esp            ; Destination buffer
      call   strcpy              ; UNSAFE: No bounds checking!
      ...
  

• Exploitation Primitive:
  • Stack Smashing: Overwriting the return address or saved EBP to gain control of EIP.
  • Shellcode Injection: If NX is disabled, shellcode can be placed in the buffer.
  • ROP Chains: If NX is enabled, return-oriented programming can bypass protections.

Exploitation Challenges & Bypasses

Protection Mechanism	Status (Likely)	Bypass Technique
Stack Canaries	Possibly Enabled	Brute-force, info leak, or overwrite adjacent data.
ASLR	Possibly Disabled	No bypass needed; static addresses may be used.
NX (No-Execute)	Possibly Enabled	Return-to-libc or ROP chains.
DEP	Possibly Disabled	Direct shellcode execution.

Reverse Engineering & Exploitation Steps

1. Firmware Extraction
   • Use Binwalk, Firmware Mod Kit, or dd to extract the firmware.
   • Example:

     binwalk -e Tenda_AX3_V16.03.12.11.bin
     

2. Binary Analysis
   • Use Ghidra, IDA Pro, or Binary Ninja to analyze httpd or the relevant binary.
   • Locate formGetIptv and trace the citytag parameter handling.
3. Dynamic Analysis
   • QEMU Emulation: Run the firmware in an emulated environment.
   • GDB Debugging: Attach to the web server process and fuzz the citytag parameter.
4. Exploit Development
   • Pattern Creation: Use msf-pattern_create to determine offset.
   • ROP Chain Construction: If NX is enabled, find gadgets in libc or the binary.
   • Shellcode: Use MIPS/ARM shellcode (depending on the router’s architecture).

Detection & Hunting Queries

• Snort/Suricata Rule:

  alert tcp any any -> $HOME_NET 80 (msg:"CVE-2025-69766 - Tenda AX3 Buffer Overflow Attempt";
  flow:to_server,established; content:"POST /goform/formGetIptv"; http_uri;
  content:"citytag="; http_client_body; pcre:"/citytag=[^\x00]{500,}/";
  reference:cve,2025-69766; classtype:attempted-admin; sid:1000001; rev:1;)
  

• YARA Rule (for Malware Analysis):

  rule Tenda_AX3_Exploit_Attempt {
      meta:
          description = "Detects CVE-2025-69766 exploitation attempts"
          reference = "CVE-2025-69766"
          author = "Cybersecurity Analyst"
      strings:
          $p1 = "POST /goform/formGetIptv"
          $p2 = "citytag=" nocase
          $p3 = /citytag=[A-Za-z0-9]{500,}/
      condition:
          all of them
  }
  

• SIEM Hunting (Splunk/ELK):

  index=network sourcetype=bro_http
  | search uri="/goform/formGetIptv" AND http_method="POST"
  | regex form_data="citytag=.{500,}"
  | stats count by src_ip, dest_ip, uri
  | sort -count
  

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Conclusion

CVE-2025-69766 represents a critical pre-authentication RCE vulnerability in Tenda AX3 routers, posing significant risks to home and small business networks. Given its CVSS 9.8 score, low attack complexity, and remote exploitability, it is highly likely to be weaponized by threat actors.

Immediate action is required:

• Patch affected devices as soon as updates are available.
• Implement network-level mitigations to reduce exposure.
• Monitor for exploitation attempts using the provided detection rules.

Security teams should prioritize this vulnerability in their vulnerability management programs and incident response plans. Further research into Tenda’s firmware security practices is recommended to identify and mitigate similar flaws.

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References:

• MITRE CVE-2025-69766
• Tenda AX3 Vulnerability Report
• OWASP Buffer Overflow Guide