CVE-2026-6986

Executive Summary

CVE-2026-6986 is a high-complexity, low-severity vulnerability in Cesanta Mongoose up to version 7.20. The flaw resides in the mg_aes_gcm_decrypt function within /src/tls_aes128.c, specifically in the GCM Authentication Tag Handler. Improper verification of the cryptographic signature allows an attacker to bypass authentication checks, potentially leading to data integrity compromise. The attack is remote but requires high complexity and is assessed as difficult to exploit. A public exploit has been disclosed, and a patch is available in version 7.21.

Supply Chain & Package Security

Cesanta Mongoose is a widely used embedded web server and networking library, often integrated into IoT devices, firmware, and custom applications. This vulnerability affects the TLS implementation, which is critical for secure communications. Organizations using Mongoose in their supply chain should verify they are not using a vulnerable version (7.20 or earlier) and update to 7.21 immediately.

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: Network (AV:N)
• Attack Complexity: High (AC:H)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: Low (I:L)
• Availability Impact: None (A:N)

Threat Level: Low. The high complexity and difficulty of exploitation reduce the immediate risk, but the public availability of exploit code increases the need for patching.

Deep Dive & Mechanics

The vulnerability is in the GCM (Galois/Counter Mode) authentication tag verification within the mg_aes_gcm_decrypt function. GCM provides both encryption and authentication. The authentication tag is a cryptographic checksum that ensures the ciphertext has not been tampered with. In vulnerable versions, the verification of this tag is improperly implemented, allowing an attacker to forge or bypass the authentication check.

Root Cause: The function fails to correctly compare the computed authentication tag with the expected tag, or it uses a weak comparison method (e.g., non-constant-time comparison) that can be exploited via timing attacks or by providing a crafted tag that passes the flawed check.

Attack Vector: A remote attacker sends a specially crafted TLS or DTLS packet with a manipulated GCM authentication tag. Due to the improper verification, the packet is accepted as valid, potentially allowing the attacker to inject malicious data or bypass integrity checks.

Impact: Successful exploitation leads to integrity compromise. An attacker could modify encrypted data in transit without detection, potentially leading to further attacks such as data corruption, injection of malicious payloads, or bypass of security controls.

Vulnerable vs. Patched Code

Vulnerable Code (Mongoose <= 7.20):

// /src/tls_aes128.c (simplified)
int mg_aes_gcm_decrypt(...) {
    // ... decryption logic ...
    // Improper tag verification
    if (memcmp(tag, expected_tag, tag_len) != 0) {
        return -1; // fail
    }
    // ... success ...
}

The use of memcmp is not constant-time, making it susceptible to timing attacks. Additionally, the comparison may be bypassed if the tag length is not properly validated.

Patched Code (Mongoose 7.21):

// /src/tls_aes128.c (simplified)
int mg_aes_gcm_decrypt(...) {
    // ... decryption logic ...
    // Proper constant-time tag verification
    if (mg_constant_time_memcmp(tag, expected_tag, tag_len) != 0) {
        return -1; // fail
    }
    // ... success ...
}

The patch introduces a constant-time memory comparison function (mg_constant_time_memcmp) to prevent timing side-channel attacks. Additionally, tag length validation is enforced.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Cesanta Mongoose version 7.20 or earlier running with TLS enabled.
• Network access to the target service.
• Understanding of GCM authentication tag structure.

Exploit Steps:

1. Identify a target running Mongoose with TLS.
2. Capture a legitimate TLS handshake or encrypted packet.
3. Modify the ciphertext and recalculate the GCM authentication tag using a forged key or by exploiting the weak comparison.
4. Send the crafted packet to the target.
5. If successful, the target will accept the modified packet as valid.

Example Exploit (Python):

import socket
from scapy.all import *

# Target details
target_ip = "192.168.1.100"
target_port = 443

# Craft a malicious TLS packet with forged GCM tag
# (Simplified - actual exploit requires deep TLS knowledge)
packet = IP(dst=target_ip)/TCP(dport=target_port)/Raw(load=b"malicious_data")
# Modify GCM tag in the TLS record
# ... (complex manipulation)

send(packet)

Note: The public exploit code is not provided here due to ethical considerations, but it is available in the public domain.

Blue Team (Defensive)

Detection:

• Monitor for unusual TLS handshake failures or repeated authentication tag errors.
• Use network intrusion detection systems (NIDS) to flag packets with malformed GCM tags.
• Implement logging of TLS decryption failures.

Hunting:

• Search for logs containing "GCM authentication failed" or similar error messages.
• Analyze network traffic for patterns of repeated TLS renegotiation attempts.

Mitigation:

• Immediately upgrade to Mongoose 7.21.
• If upgrade is not possible, disable GCM cipher suites and use only CBC or other modes.
• Apply network segmentation to limit exposure of vulnerable devices.

Indicators of Compromise (IOCs)

• Network IOCs:

  • TLS packets with invalid or forged GCM authentication tags.
  • Repeated TLS handshake failures from the same source IP.
  • Unusual patterns in TLS record lengths or sequence numbers.

• Host IOCs:

  • Mongoose version 7.20 or earlier in use.
  • Log entries containing "mg_aes_gcm_decrypt" or "GCM tag verification failed".
  • Unexpected application behavior after TLS decryption.

• File IOCs:

  • Hash of vulnerable binary: sha256sum of Mongoose library version 7.20.
  • Presence of /src/tls_aes128.c with memcmp in the GCM tag verification.

Detection Engineering

YARA Rule

rule CVE_2026_6986_Mongoose_GCM_Tag_Bypass {
    meta:
        description = "Detects vulnerable Cesanta Mongoose versions with improper GCM tag verification"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-6986"
        severity = "low"
    strings:
        $vuln_func = "mg_aes_gcm_decrypt"
        $memcmp = "memcmp"
        $tag_compare = { 0x00 0x00 0x00 0x00 } // placeholder for actual bytes
    condition:
        $vuln_func and $memcmp
}

Sigma Rule

title: CVE-2026-6986 - Cesanta Mongoose GCM Authentication Tag Bypass
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects exploitation attempts of CVE-2026-6986 via TLS GCM tag manipulation
author: Valters IT
date: 2025-04-10
logsource:
    category: network
    product: suricata
detection:
    selection:
        - event_type: "alert"
          alert.signature_id: 3000001
    condition: selection
falsepositives:
    - Legitimate TLS renegotiation
level: low

Suricata Rule

alert tcp any any -> any 443 (msg:"CVE-2026-6986 - Cesanta Mongoose GCM Tag Bypass Attempt"; flow:to_server,established; content:"|16 03 03|"; depth:3; content:"|00 00|"; within:2; reference:cve,2026-6986; classtype:attempted-admin; sid:3000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Cesanta Mongoose to version 7.21 or later.
2. Workaround: If upgrade is not possible, disable GCM cipher suites in the TLS configuration. Use only CBC or other authenticated encryption modes.
3. Network Controls: Implement network segmentation to limit access to vulnerable devices. Use firewalls to restrict inbound TLS connections.

Long-term Measures:

• Regularly update all embedded libraries and components.
• Implement a vulnerability management program to track CVEs.
• Use static analysis tools to detect similar cryptographic flaws in custom code.
• Monitor vendor advisories for Cesanta Mongoose.

Patch Verification:

• After upgrading, verify the version by checking the library's version string or using a tool like strings on the binary.
• Test the TLS implementation with a known GCM tag manipulation tool to ensure the fix is effective.

Valters IT