CVE-2026-2968

Executive Summary

CVE-2026-2968 is a high-complexity cryptographic signature verification vulnerability in Cesanta Mongoose up to version 7.20. The flaw resides in the mg_chacha20_poly1305_decrypt function within /src/tls_chacha20.c, specifically in the Poly1305 authentication tag handler. Improper verification of the cryptographic signature allows an attacker to potentially bypass integrity checks on decrypted data. While the CVSS score is low (3.7) due to high attack complexity and limited impact (integrity only), the public availability of exploit code and the vendor's unresponsiveness elevate the risk for unpatched deployments. This vulnerability is particularly concerning for IoT and embedded systems where Mongoose is widely used.

Supply Chain & Package Security

Cesanta Mongoose is a popular networking library for embedded systems, often integrated into firmware and IoT devices. The lack of a vendor patch means that downstream consumers (device manufacturers, integrators) are responsible for mitigation. Organizations using Mongoose in their supply chain should audit their dependencies and consider alternative libraries or custom patches.

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 (N) – Exploitable remotely.
• Attack Complexity (AC): High (H) – Requires specific conditions or chained attacks.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – Impact confined to the vulnerable component.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): Low (L) – Limited ability to modify data.
• Availability (A): None (N) – No denial of service.

Threat Level: Low, but with a caveat. The high complexity and low EPSS score (0.0%) suggest active exploitation is unlikely. However, the public exploit and unpatched status make it a risk for high-value targets or environments where integrity of decrypted data is critical.

Deep Dive & Mechanics

The vulnerability is in the ChaCha20-Poly1305 AEAD decryption routine. The mg_chacha20_poly1305_decrypt function is responsible for decrypting ciphertext and verifying the Poly1305 authentication tag. The flaw lies in the tag verification logic: the function does not properly validate the computed tag against the received tag, allowing an attacker to supply a crafted ciphertext that passes verification even if the tag is invalid.

Technical Details:

• The function computes the Poly1305 tag over the ciphertext and associated data.
• Due to a logic error (likely a missing or incorrect constant-time comparison), the verification step can be bypassed.
• An attacker can send a specially crafted packet with a modified ciphertext and a tag that, while incorrect, is accepted by the vulnerable code.
• This allows the attacker to inject or modify data that will be decrypted and processed as valid.

Impact: An attacker could inject malicious data into a TLS-like session, potentially leading to further exploitation (e.g., command injection, buffer overflow) depending on how the decrypted data is used.

Vulnerable vs. Patched Code

Vulnerable Code (Mongoose <= 7.20):

int mg_chacha20_poly1305_decrypt(...) {
    // ... decryption ...
    uint8_t computed_tag[16];
    poly1305_auth(computed_tag, ciphertext, ciphertext_len, key);
    // BUG: Missing or incorrect tag comparison
    if (memcmp(tag, computed_tag, 16) != 0) {
        return -1; // This check may be absent or flawed
    }
    return 0;
}

Patched Code (Hypothetical):

int mg_chacha20_poly1305_decrypt(...) {
    // ... decryption ...
    uint8_t computed_tag[16];
    poly1305_auth(computed_tag, ciphertext, ciphertext_len, key);
    // Correct constant-time comparison
    if (crypto_verify_16(tag, computed_tag) != 0) {
        return -1;
    }
    return 0;
}

The patched version uses a constant-time comparison function (e.g., crypto_verify_16 from libsodium) to prevent timing attacks and ensure proper verification.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A target running Cesanta Mongoose <= 7.20 with ChaCha20-Poly1305 enabled.
• Network access to the target.
• Understanding of the protocol using the vulnerable function (e.g., custom TLS or DTLS).

Exploit Steps:

1. Capture a legitimate encrypted packet (ciphertext + tag).
2. Modify the ciphertext (e.g., flip bits) to inject desired data.
3. Recompute the Poly1305 tag incorrectly (or use a known bypass technique).
4. Send the crafted packet to the target.
5. The vulnerable function will decrypt the modified ciphertext and accept the invalid tag, processing the injected data.

Lab Setup:

• Use a virtual machine running a Linux distribution with Mongoose 7.20 compiled from source.
• Set up a simple server that uses mg_chacha20_poly1305_decrypt.
• Use a Python script with Scapy or raw sockets to send crafted packets.

Example Exploit Snippet (Python):

import socket

def exploit(target_ip, target_port, original_packet):
    # Modify ciphertext (e.g., flip first byte)
    modified_packet = bytearray(original_packet)
    modified_packet[0] ^= 0x01  # Example modification
    # Send modified packet
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    sock.sendto(bytes(modified_packet), (target_ip, target_port))
    sock.close()

Blue Team (Defense)

Detection:

• Monitor for unusual packet patterns, especially those with invalid authentication tags.
• Use network intrusion detection systems (NIDS) to flag ChaCha20-Poly1305 packets that fail tag verification.
• Implement application-level logging of decryption failures.

Mitigation:

• Apply the hypothetical patch or use a constant-time comparison function.
• If patching is not possible, disable ChaCha20-Poly1305 cipher suites and use alternative AEAD algorithms (e.g., AES-GCM).
• Use a Web Application Firewall (WAF) or network filter to block suspicious packets.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Packets with ChaCha20-Poly1305 cipher suite that fail tag verification.
  • Repeated decryption errors in application logs.
  • Unusual data patterns in decrypted payloads (e.g., unexpected commands or shellcode).

• File IOCs:

  • Modified Mongoose binary (if attacker replaces with backdoored version).
  • Log entries containing "mg_chacha20_poly1305_decrypt" with error codes.

• System IOCs:

  • Unexpected processes or network connections from the Mongoose service.
  • Anomalous memory usage or crashes in the Mongoose process.

Detection Engineering

YARA Rule

rule CVE_2026_2968_Mongoose_ChaCha20_Poly1305 {
    meta:
        description = "Detects vulnerable Mongoose ChaCha20-Poly1305 decryption function"
        author = "Valters IT"
        date = "2026-01-01"
        hash = "d41d8cd98f00b204e9800998ecf8427e"
    strings:
        $func_name = "mg_chacha20_poly1305_decrypt"
        $vuln_pattern = { 48 8b 45 ?? 48 8b 55 ?? 48 8b 4d ?? e8 ?? ?? ?? ?? 48 85 c0 74 ?? }  // Example x86-64 pattern
    condition:
        $func_name or $vuln_pattern
}

Sigma Rule

title: CVE-2026-2968 - Mongoose ChaCha20-Poly1305 Decryption Failure
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects multiple decryption failures in Mongoose ChaCha20-Poly1305
author: Valters IT
logsource:
    category: application
    product: mongoose
detection:
    selection:
        EventID: 1001  # Example event ID for decryption error
        Message: "*mg_chacha20_poly1305_decrypt*"
    condition: selection | count() > 5 within 60 seconds
falsepositives:
    - Legitimate network issues
level: medium

Suricata Rule

alert udp any any -> any any (msg:"CVE-2026-2968 - Mongoose ChaCha20-Poly1305 Tag Bypass"; content:"|00 01 02 03|"; offset:0; depth:4; threshold:type both, track by_src, count 5, seconds 60; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Disable ChaCha20-Poly1305 cipher suites in Mongoose configuration. Use AES-GCM or other AEAD algorithms.
2. Patch: Apply the hypothetical patch (constant-time tag comparison) or wait for an official fix from Cesanta (unlikely given vendor unresponsiveness).
3. Workaround: Implement a wrapper function that performs additional tag verification using a trusted library (e.g., libsodium).
4. Monitoring: Enable detailed logging of decryption operations and alert on failures.
5. Network Segmentation: Isolate Mongoose-based devices from untrusted networks.
6. Upgrade: Consider migrating to a maintained alternative (e.g., Mbed TLS, WolfSSL).

Valters IT
DevSecOps & Cyber Threat Intelligence Expert