CVE-2026-74843

Executive Summary

CVE-2026-74843 is a critical, remotely exploitable stack-based buffer overflow in the Wavlink WN531P3 and WN535M1 routers running firmware version V250922. The vulnerability resides in the internal CGI script /etc/lighttpd/www/cgi-bin/export_pingortrace.cgi, where the strcpy function copies attacker-controlled data from the HTTP_COOKIE environment variable without proper length checking. An unauthenticated remote attacker can trigger memory corruption by sending an oversized Cookie header to the affected CGI endpoint, potentially achieving arbitrary code execution with the privileges of the lighttpd web server. The CVSS v3.1 base score is 10.0, reflecting no required authentication, no user interaction, low attack complexity, and full impact to confidentiality, integrity, and availability, including scope change beyond the vulnerable component.

Technical Analysis

The affected component is the Export Pingortrace CGI application inside the firmware of Wavlink WN531P3 and WN535M1 devices. According to the NVD description, the vulnerable function is strcpy, located in /etc/lighttpd/www/cgi-bin/export_pingortrace.cgi. The argument being manipulated is HTTP_COOKIE.

The strcpy function copies a source string into a destination buffer until a null byte is encountered. It does not perform bounds checking. In this CGI implementation, the value of the HTTP_COOKIE environment variable, populated by lighttpd from the client-supplied Cookie HTTP header, is copied via strcpy into a fixed-size stack buffer. An attacker who sends a Cookie header longer than the allocated stack buffer will cause adjacent memory on the stack to be overwritten. This can corrupt saved stack frames, return addresses, or other critical data, leading to process crash or arbitrary code execution.

The attack surface is the exposed CGI endpoint. Because the web server is lighttpd, the path /etc/lighttpd/www/cgi-bin/export_pingortrace.cgi is likely accessible over HTTP on the LAN side and possibly on the WAN side if remote management is enabled. The CVSS vector AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H confirms that the attacker does not require network-level proximity, does not need credentials, and does not need to trick a user into interacting with the exploit. The scope change (S:C) indicates that exploitation can impact resources beyond the vulnerable CGI process, such as the underlying operating system and other device services.

No additional technical details such as the exact stack buffer size, overwrite offset, or endian-specific exploitation logic are provided in the NVD description. Because no patch has been identified and the vulnerability is unpatched as of the time of this report, the technical analysis is limited to the described vulnerability class and call path.

Exploitation Scenarios

The following attack chain is based strictly on the NVD description and the public disclosure of the vulnerability. No public exploit code is confirmed, so the post-disclosure exploitation details are inferred from the vulnerability class.

1. Reconnaissance / target identification
   An attacker scans for exposed Wavlink WN531P3 or WN535M1 devices, or for lighttpd servers responding to requests for /cgi-bin/export_pingortrace.cgi. Network banners, device web pages, or Shodan-style discovery can be used to identify firmware version V250922. Because the vulnerability is unauthenticated, the attacker can directly target the CGI endpoint without prior access.

2. Triggering the vulnerability class
   The attacker sends an HTTP request to the affected CGI endpoint with an excessively long Cookie header. The lighttpd server passes this header value into the HTTP_COOKIE environment variable. The CGI binary then invokes strcpy to copy this environment variable into a fixed-size stack buffer. The lack of bounds checking in strcpy is the core flaw; no authentication or user interaction is required.

3. Payload / execution method at the vulnerability-class level
   The attacker crafts the Cookie value to overwrite the stack beyond the allocated buffer. In a stack-based buffer overflow, this typically overwrites the saved return address and possibly adjacent local variables. A well-crafted payload can redirect execution to attacker-controlled memory, such as the CMSIS stack or injected shellcode, if the platform is writable and executable. Because the exact memory layout and mitigations are not documented in the NVD data, the exploit chain cannot be described at a firmware-specific level. However, the CVSS score of 10.0 indicates that full compromise of the device is feasible.

4. Post-exploitation impact
   Successful exploitation would allow an unauthenticated remote attacker to execute arbitrary code in the context of the lighttpd process, which on embedded routers often runs with elevated service privileges. This can lead to full device compromise, persistent backdoor installation, network traffic interception, DNS or routing manipulation, credential theft, and lateral movement into the internal LAN. Since the scope is changed (S:C), the attacker may also leverage the compromised router to pivot to other internal hosts.

No public PoC is currently confirmed; the attack chain is inferred from the described vulnerability class and the public disclosure. Administrators should assume that exploitation is possible and treat this as an actively exploitable unpatched flaw.

Impact Assessment

The vulnerability has the maximum CVSS v3.1 base score of 10.0. The individual subcomponents are:

• Attack Vector: Network (AV:N) — Exposed CGI endpoint reachable over the network.
• Attack Complexity: Low (AC:L) — No special conditions or race conditions required.
• Privileges Required: None (PR:N) — No credentials required.
• User Interaction: None (UI:N) — No victim interaction needed.
• Scope: Changed (S:C) — Compromise can affect resources outside the vulnerable component, including the underlying router OS.
• Confidentiality / Integrity / Availability: High (C:H/I:H/A:H) — Full read/write/degrade capability.

The affected systems are Wavlink WN531P3 and WN535M1 routers running firmware version V250922. The blast radius includes:

• Complete compromise of the router.
• Loss of integrity of routing, DNS, and firewall rules.
• Exposure of LAN traffic to an attacker.
• Use of the device as a pivot point for internal network attacks.
• Potential device bricking via memory corruption.

Because the vulnerable CGI is internet-facing in common deployments, and because no patch exists, the business risk is elevated. Any organization or home user using the affected model and firmware should treat the device as compromised if the management interface is exposed to untrusted networks.

Detection Guidance

The following detection rules are intended to detect network or log evidence of exploitation attempts. They should be tuned to the local logging environment and validated for false positives. The most reliable signal is an HTTP request to /cgi-bin/export_pingortrace.cgi carrying an abnormally large Cookie header.

Sigma Rule

title: Wavlink WN531P3/WN535M1 export_pingortrace.cgi Stack Overflow Attempt
id: 2f5ecb3a-0b9e-4b3e-9f07-6a2a4b1c7d01
status: experimental
description: Detects HTTP requests to the vulnerable Wavlink export_pingortrace.cgi endpoint with a very long Cookie header, indicating possible stack buffer overflow exploitation.
logsource:
  category: webserver
  product: lighttpd
detection:
  selection_path:
    http.uri|contains: "/cgi-bin/export_pingortrace.cgi"
  selection_long_cookie:
    http.cookie|length: ">= 1024"
  condition: selection_path and selection_long_cookie
level: high
falsepositives:
  - Legitimate clients with unusual multi-value cookie sets
  - Web application scanners that generate long test cookies

YARA Rule

rule wavlink_export_pingortrace_cookie_overflow
{
    meta:
        author = "Valters IT SOC"
        description = "Hunt for HTTP request artifacts targeting Wavlink CVE-2026-74843 export_pingortrace.cgi endpoint"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-74843"
        cve = "CVE-2026-74843"
        date = "2026-04-01"
    strings:
        $cgi_path = "/cgi-bin/export_pingortrace.cgi" ascii wide nocase
        $cookie_hdr = "Cookie:" ascii wide nocase
        $stack_func = "strcpy" ascii wide nocase
    condition:
        $cgi_path and $cookie_hdr and ($stack_func or #cgi_path > 0)
}

This YARA rule is intended for hunting captured HTTP request payloads or access logs that contain full HTTP headers. It should be paired with a parser that enforces a Cookie value length threshold; the strcpy string is useful for scanning extracted binary artifacts of the CGI file.

Suricata IDS Rule

alert http $EXTERNAL_NET any -> $HOME_NET any (msg:"ET EXPLOIT Wavlink WN531P3/WN535M1 export_pingortrace.cgi HTTP_COOKIE stack buffer overflow attempt"; flow:established,to_server; http.uri; content:"/cgi-bin/export_pingortrace.cgi"; http.cookie; isdataat:1024; reference:cve,2026-74843; classtype:attempted-admin; sid:202674843; rev:1;)

Elastic Detection

any where event.category == "web" and url.path == "/cgi-bin/export_pingortrace.cgi" and length(http.request.headers.cookie) > 1024

Splunk SPL Query

index=web sourcetype=access_combined uri_path="/cgi-bin/export_pingortrace.cgi" Cookie=*
| eval cookie_len = len(Cookie)
| where cookie_len > 1024
| stats count min(_time) max(_time) values(src_ip) values(dest_ip) by uri, cookie_len, source

This query assumes the web access logs include the full Cookie header and a field named Cookie. If the field name differs, adjust accordingly.

Wazuh OSSEC Rule

<group name="wavlink,exploit,">
  <rule id="100321" level="12">
    <decoded>web</decoded>
    <match>/cgi-bin/export_pingortrace.cgi</match>
    <description>Possible exploitation of CVE-2026-74843 - Wavlink WN531P3/WN535M1 export_pingortrace.cgi stack buffer overflow</description>
    <group>wavlink,exploit,</group>
  </rule>
</group>

The Wazuh rule above detects requests to the vulnerable endpoint. For accurate detection of overflow attempts, enrich the rule with a regex or a preprocessor that checks the Cookie header length, as local logs may not expose the full header by default.

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No vendor advisory, patched firmware version, CVE patch commit, or official workaround has been identified from any tracked source. The vendor was contacted early about the disclosure, but no fix is currently available.

1. Immediate Mitigations

• Disable or remove the Export Pingortrace CGI if the functionality is not required for business operations. Where possible, deny access to /cgi-bin/export_pingortrace.cgi from the WAN interface.
• Disable remote administration and the WAN-side management interface on the router. Restrict administration access to trusted LAN IPs or a dedicated management VLAN.
• Apply firewall rules to block inbound connections to the router’s web/management ports from untrusted networks.
• Monitor router logs and network traffic for suspicious requests to the affected CGI path, particularly requests with unusually large Cookie headers.

2. Patch / Upgrade Steps

• No patched firmware is available at the time of writing. Contact Wavlink’s support channel and request firmware status for WN531P3 and WN535M1.
• Subscribe to vendor firmware release announcements and apply an updated build immediately when it is released.
• If no firmware fix is forthcoming, consider replacing the device with a model that receives active security maintenance.

3. Long-Term Hardening

• Segment IoT and embedded devices into a separate network zone so that a compromised router cannot directly reach business-critical systems.
• Implement network-level inspection using Suricata, Snort, or a next-generation firewall capable of decoding HTTP and enforcing Cookie-header length limits.
• Use an external monitoring solution that correlates access to management interfaces with anomaly signals such as abnormally long headers.
• Maintain an asset inventory that includes firmware versions and known CVEs for all network appliances.
• Review the vendor’s security advisories periodically; if a patch becomes available, prioritize deployment given the CVSS 10.0 severity.

References

• NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-74843
• MITRE CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer: https://cwe.mitre.org/data/definitions/119.html
• MITRE ATT&CK Technique T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
• CISA Known Exploited Vulnerabilities Catalog (no CVE-2026-74843 entry at time of writing): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No vendor advisory, researcher blog, or third-party exploit analysis is available for this CVE beyond the NVD description. There is no CISA KEV listing and no confirmed exploit-in-the-wild evidence at the time of writing; however, the public disclosure of the vulnerability means immediate remediation planning is strongly advised.