CVE-2026-2967

Executive Summary

CVE-2026-2967 is a security advisory affecting Cesanta Mongoose up to version 7.20. The vulnerability resides in the TCP Sequence Number Handler within the getpeer function of /src/net_builtin.c. It involves improper verification of the source of a communication channel, which can be exploited remotely with high complexity. The attack is considered difficult to execute, but a public proof-of-concept (PoC) exists. The vendor has not responded to disclosure attempts, and no official patch is available.

Key Risk: The vulnerability allows an attacker to potentially spoof or hijack TCP connections by manipulating sequence numbers, leading to unauthorized data access or man-in-the-middle attacks. The high attack complexity and low CVSS score (3.7) reduce immediate risk, but the lack of a patch and public exploit details increase long-term exposure.

Supply Chain & Package Security

Cesanta Mongoose is a widely used embedded web server library integrated into many IoT devices, routers, and embedded systems. This vulnerability affects the supply chain of any product using Mongoose <= 7.20. Organizations must inventory their use of this library and assess exposure. No official patch exists, so mitigation relies on workarounds or replacing the library.

CVSS Breakdown & Threat Level

• CVSS Score: 3.7 (Low)
• Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
• Attack Vector (AV): Network – Exploitable remotely.
• Attack Complexity (AC): High – Requires advanced knowledge of TCP sequence numbers and timing.
• Privileges Required (PR): None – No authentication needed.
• User Interaction (UI): None – No user action required.
• Scope (S): Unchanged – Exploit does not affect other components.
• Confidentiality (C): None – No data disclosure.
• Integrity (I): Low – Potential to inject or modify data in transit.
• Availability (A): None – No denial of service.

Threat Level: Low due to high complexity and limited impact, but the public PoC and lack of patch elevate the risk for targeted attacks.

Deep Dive & Mechanics

The vulnerability is in the getpeer function within /src/net_builtin.c. This function is responsible for retrieving the peer address of a TCP connection. The flaw lies in improper verification of the source of a communication channel, specifically in how TCP sequence numbers are handled.

Root Cause: The getpeer function does not properly validate the origin of TCP packets, allowing an attacker to inject packets with crafted sequence numbers. This can lead to:

• TCP Sequence Number Prediction: An attacker can guess or predict sequence numbers to inject malicious data.
• Connection Hijacking: By spoofing the source IP and sequence numbers, an attacker can hijack an existing connection.
• Data Injection: Malicious data can be inserted into the TCP stream.

Attack Flow:

1. Attacker identifies a target using Mongoose <= 7.20.
2. Attacker monitors the TCP handshake to gather sequence numbers.
3. Attacker sends crafted TCP packets with predicted sequence numbers to inject data or hijack the connection.
4. The getpeer function fails to verify the source, accepting the spoofed packets.

Vulnerable vs. Patched Code

Vulnerable Code (Mongoose <= 7.20):

// /src/net_builtin.c - getpeer function (simplified)
int getpeer(struct mg_connection *c, union usa *sa) {
    socklen_t len = sizeof(*sa);
    return getpeername(c->sock, &sa->sa, &len);
}

The vulnerable code does not validate the source of the TCP packet, relying solely on the kernel's getpeername which can be bypassed with crafted packets.

Patched Code (Hypothetical):

// Patched version - adds source verification
int getpeer(struct mg_connection *c, union usa *sa) {
    socklen_t len = sizeof(*sa);
    int ret = getpeername(c->sock, &sa->sa, &len);
    // Additional validation: verify source IP and port match expected values
    if (ret == 0 && !mg_verify_source(c, sa)) {
        return -1; // Reject if source mismatch
    }
    return ret;
}

The patch would add a verification step to ensure the source of the communication channel is legitimate, preventing spoofed packets.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Target running Mongoose <= 7.20 on a known port.
• Network access to the target.
• Ability to sniff TCP traffic (e.g., using tcpdump or Wireshark).

Exploit Steps:

1. Reconnaissance: Identify the target IP and port (e.g., 192.168.1.100:8080).
2. Sniff Traffic: Capture a TCP handshake to obtain initial sequence numbers (ISN).

   tcpdump -i eth0 host 192.168.1.100 and port 8080 -w capture.pcap
   

3. Craft Spoofed Packet: Use a tool like scapy to send a packet with a predicted sequence number.

   from scapy.all import *
   target_ip = "192.168.1.100"
   target_port = 8080
   spoofed_ip = "10.0.0.1"
   # Assume ISN = 1000 from sniffing
   ip = IP(src=spoofed_ip, dst=target_ip)
   tcp = TCP(sport=12345, dport=target_port, seq=1001, flags="PA")
   payload = "GET /admin HTTP/1.1\r\nHost: target\r\n\r\n"
   send(ip/tcp/payload)
   

4. Verify Injection: Check if the target processes the injected data (e.g., logs or response).

Note: This is a simplified example; actual exploitation requires precise sequence number prediction.

Blue Team (Defense)

Detection:

• Monitor for unusual TCP packets with unexpected sequence numbers.
• Use network intrusion detection systems (NIDS) to flag anomalies.

Mitigation:

• Apply network segmentation to limit exposure.
• Use TCP timestamps and randomize ISN to make prediction harder.
• Implement application-layer authentication to verify sources.

Lab Setup:

• Target: Ubuntu 22.04 with Mongoose 7.20 compiled from source.
• Attacker: Kali Linux with Scapy and tcpdump.
• Network: Isolated virtual network (e.g., VirtualBox host-only).

Indicators of Compromise (IOCs)

• Network IOCs:

  • TCP packets with sequence numbers that are out of order or match predicted values.
  • Multiple SYN packets from different source IPs with the same sequence number pattern.
  • Unusual data injection into established TCP streams.

• Host IOCs:

  • Unexpected connections in netstat or ss output.
  • Logs showing getpeer failures or anomalies in /var/log/syslog.
  • Modified Mongoose binaries or libraries.

• File IOCs:

  • Hash of vulnerable libmongoose.so (e.g., SHA256: abc123...).
  • Presence of exploit scripts or tools (e.g., scapy scripts).

Detection Engineering

YARA Rule

rule CVE_2026_2967_Mongoose_TCP_Spoof {
    meta:
        description = "Detects attempts to exploit CVE-2026-2967 in Cesanta Mongoose"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-2967"
    strings:
        $s1 = "getpeer" ascii wide nocase
        $s2 = "net_builtin.c" ascii wide nocase
        $s3 = "TCP Sequence Number" ascii wide nocase
    condition:
        any of ($s*) and filesize < 10MB
}

Sigma Rule

title: CVE-2026-2967 - Cesanta Mongoose TCP Sequence Number Spoofing
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects network traffic indicative of TCP sequence number spoofing against Mongoose
author: Valters IT
date: 2025-04-10
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        event_type: alert
        alert.signature_id: 20262967
    condition: selection
falsepositives:
    - Legitimate TCP retransmissions
level: medium
tags:
    - attack.t1190
    - cve.2026.2967

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2026-2967 - Mongoose TCP Sequence Number Spoofing Attempt"; flow:to_server; content:"|00 00 00 00|"; depth:4; offset:0; reference:cve,2026-2967; classtype:attempted-recon; sid:20262967; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate Affected Systems: Place Mongoose-based devices on segmented networks with strict access controls.
2. Monitor for Exploitation: Deploy the provided detection rules to identify attacks.
3. Apply Workarounds: If possible, disable the getpeer function or restrict its use via firewall rules.

Long-Term Fix:

• Patch: No official patch exists. Monitor Cesanta’s GitHub for updates or consider forking the repository to apply a custom fix.
• Upgrade: Replace Mongoose with an alternative library (e.g., libmicrohttpd, libwebsockets) if feasible.
• Compensating Controls: Use a reverse proxy (e.g., Nginx) to terminate TCP connections and forward to Mongoose, adding an extra layer of verification.

Vendor Communication: Since the vendor did not respond, consider reporting to CERT/CC or other disclosure platforms.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert