CVE-2026-5244

Executive Summary

CVE-2026-5244 is a critical heap-based buffer overflow vulnerability in Cesanta Mongoose, a popular embedded web server and networking library. The flaw resides in the mg_tls_recv_cert function within mongoose.c, specifically in the TLS 1.3 handler. By manipulating the pubkey argument, an unauthenticated remote attacker can trigger a heap buffer overflow, potentially leading to remote code execution (RCE) or denial of service (DoS). The vulnerability affects all versions up to and including 7.20. A patch was released in version 7.21, and the exploit details have been publicly disclosed, increasing the risk of active exploitation.

Supply Chain & Package Security

Cesanta Mongoose is widely used in embedded systems, IoT devices, and various software projects as a library. This vulnerability poses a significant supply chain risk. Organizations using Mongoose in their products must immediately assess their exposure and update to version 7.21 or later. The library is often statically linked, meaning a simple recompilation with the patched version is required. There is no known package manager distribution (e.g., NPM, PyPI) for Mongoose, but it is commonly integrated via source code or Git submodules.

CVSS Breakdown & Threat Level

• CVSS Score: 7.3 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality Impact: High (C:H) – Potential for data disclosure.
• Integrity Impact: High (I:H) – Potential for data modification.
• Availability Impact: High (A:H) – Potential for denial of service.

Threat Level: High. The combination of remote exploitability, low complexity, and public exploit disclosure makes this a critical threat for any system running an affected version of Mongoose.

Deep Dive & Mechanics

The vulnerability is a classic heap-based buffer overflow in the TLS 1.3 certificate handling routine. When Mongoose receives a TLS 1.3 handshake, it processes the peer's certificate. The mg_tls_recv_cert function is responsible for extracting the public key (pubkey) from the certificate. The flaw occurs because the function does not properly validate the size of the pubkey data before copying it into a fixed-size heap buffer.

Technical Flow:

1. An attacker initiates a TLS 1.3 handshake with a vulnerable Mongoose server.
2. The attacker sends a crafted certificate containing an oversized pubkey field.
3. The mg_tls_recv_cert function copies this oversized data into a heap buffer without bounds checking.
4. The overflow corrupts adjacent heap memory, potentially overwriting function pointers or other critical data structures.
5. This can lead to arbitrary code execution in the context of the Mongoose process.

The vulnerability is in the TLS 1.3 handler specifically, not in TLS 1.2 or earlier versions. The patch (commit 0d882f1b43ff2308b7486a56a9d60cd6dba8a3f1) adds proper size validation before the copy operation.

Vulnerable vs. Patched Code

Vulnerable Code (Mongoose <= 7.20):

// In mg_tls_recv_cert function
static void mg_tls_recv_cert(struct mg_connection *c, unsigned char *cert, size_t len) {
    // ... parsing logic ...
    unsigned char *pubkey = extract_pubkey(cert, len);
    // VULNERABLE: No size check before copy
    memcpy(c->tls_pubkey, pubkey, pubkey_len);
    // ...
}

Patched Code (Mongoose 7.21):

// In mg_tls_recv_cert function
static void mg_tls_recv_cert(struct mg_connection *c, unsigned char *cert, size_t len) {
    // ... parsing logic ...
    unsigned char *pubkey = extract_pubkey(cert, len);
    // PATCH: Added size validation
    if (pubkey_len > sizeof(c->tls_pubkey)) {
        mg_error(c, "Public key too large");
        return;
    }
    memcpy(c->tls_pubkey, pubkey, pubkey_len);
    // ...
}

The patch introduces a bounds check that ensures pubkey_len does not exceed the allocated buffer size (sizeof(c->tls_pubkey)). If it does, the connection is terminated with an error.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A vulnerable Mongoose server (version <= 7.20) with TLS 1.3 enabled.
• A crafted TLS certificate with an oversized public key field.

Exploit Concept: The exploit involves creating a malicious TLS certificate where the subjectPublicKeyInfo field contains a public key that is larger than the expected buffer size. This can be done using OpenSSL or a custom TLS library.

Example Exploit Script (Python with scapy-ssl_tls):

from scapy.all import *
from scapy_ssl_tls import *

# Craft a malicious certificate with oversized public key
# This is a simplified example; actual exploitation requires precise heap manipulation
def create_malicious_cert():
    # Build a certificate with an oversized pubkey
    cert = TLSCertificate()
    cert.subject = "CN=attacker.com"
    # Set an oversized public key (e.g., 8192 bytes instead of typical 256)
    cert.pubkey = b"A" * 8192  # Overflow trigger
    return cert

# Send the malicious certificate during TLS handshake
def exploit(target_ip, target_port):
    pkt = IP(dst=target_ip)/TCP(dport=target_port)/TLSClientHello()
    # ... complete handshake with malicious cert ...
    send(pkt)

Note: This is a conceptual example. Actual exploitation requires careful heap layout manipulation to achieve code execution.

Blue Team (Defensive)

Detection:

• Monitor for TLS handshakes with abnormally large certificates (public key > 2048 bits).
• Look for crashes or heap corruption errors in Mongoose logs.
• Use network intrusion detection systems (NIDS) to flag oversized TLS certificates.

Honeypot Setup:

• Deploy a vulnerable Mongoose instance in a sandboxed environment.
• Capture all TLS handshake data for analysis.
• Alert on any attempt to send oversized certificates.

Mitigation:

• Immediately upgrade to Mongoose 7.21 or later.
• If upgrade is not possible, disable TLS 1.3 support and fall back to TLS 1.2.
• Apply virtual patching via WAF or NIDS rules.

Indicators of Compromise (IOCs)

• Network IOCs:

  • TLS 1.3 ClientHello with certificate public key size > 2048 bytes.
  • Unusual TLS handshake patterns (e.g., repeated handshake failures).
  • Outbound connections from Mongoose server to unknown IPs post-exploitation.

• Host IOCs:

  • Mongoose process crash logs with heap corruption errors.
  • Unexpected memory usage spikes in the Mongoose process.
  • Modified heap metadata (detectable via memory forensics).

• File IOCs:

  • Presence of unusual certificate files in the Mongoose working directory.
  • Modified mongoose.c or binary if backdoor implanted.

Detection Engineering

YARA Rule

rule CVE_2026_5244_Mongoose_TLS_Overflow {
    meta:
        description = "Detects oversized public key in TLS certificate targeting Mongoose CVE-2026-5244"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-5244"
    strings:
        $pubkey_overflow = { 30 82 ?? ?? 30 82 ?? ?? 02 01 00 30 0d 06 09 2a 86 48 86 f7 0d 01 01 01 05 00 03 82 ?? ?? 00 30 82 ?? ?? 02 82 ?? ?? 00 }  // Oversized RSA public key
        $tls_cert_header = { 30 82 ?? ?? 30 82 ?? ?? a0 03 02 01 02 02 }  // X.509 certificate header
    condition:
        $tls_cert_header and #pubkey_overflow > 1
}

Sigma Rule

title: CVE-2026-5244 - Mongoose TLS Heap Overflow Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects network traffic attempting to exploit CVE-2026-5244 via oversized TLS certificate public key
author: Valters IT
date: 2026-01-01
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        tls.handshake.type: 11  # Certificate
        tls.handshake.certificate_length: "> 2048"  # Oversized certificate
    condition: selection
falsepositives:
    - Legitimate use of large certificates (rare)
level: high

Suricata Rule

alert tls $EXTERNAL_NET any -> $HOME_NET any (msg:"CVE-2026-5244 - Mongoose TLS Heap Overflow Attempt"; flow:to_server; tls.handshake.type:11; tls.handshake.certificate_length:>2048; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Cesanta Mongoose to version 7.21 or later. The patch is available at commit 0d882f1b43ff2308b7486a56a9d60cd6dba8a3f1.
2. Disable TLS 1.3: If upgrade is not immediately possible, disable TLS 1.3 support in the Mongoose configuration. This can be done by setting mg_tls_opts to only allow TLS 1.2.
3. Network Segmentation: Isolate Mongoose servers from untrusted networks to reduce exposure.

Long-term Measures:

• Implement regular dependency scanning for embedded libraries.
• Use memory-safe languages or add bounds checking in critical code paths.
• Deploy Web Application Firewalls (WAF) with custom rules to block oversized TLS certificates.
• Monitor for public exploit code and apply virtual patches as needed.

Patch Verification:

• After upgrading, verify the patch by checking the commit hash in the source code.
• Run a regression test to ensure TLS 1.3 functionality still works with valid certificates.

Valters IT
Securing Digital Frontiers

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