CVE-2025-65502

Executive Summary

CVE-2025-65502 is a denial-of-service (DoS) vulnerability in Cesanta Mongoose, a popular embedded web server and networking library. The flaw resides in the add_ca_certs() function, where a null pointer dereference occurs during TLS initialization when SSL_CTX_get_cert_store() returns NULL. An unauthenticated remote attacker can trigger this condition, causing the application to crash. The vulnerability affects all versions of Mongoose before 7.2. No patch has been officially released, and no public exploit code is available. The EPSS score is 0.0%, indicating low exploitation activity in the wild, but the risk remains for systems using vulnerable versions in production.

Supply Chain & Package Security

Cesanta Mongoose is distributed as a C library, often integrated directly into embedded firmware or IoT applications. It is not typically distributed via NPM, PyPI, Go, or Ruby package managers. However, it may be included as a submodule or dependency in larger projects. Organizations should audit their software bill of materials (SBOM) for Mongoose versions below 7.2 and apply mitigations immediately.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, but this is likely a placeholder. Based on the vulnerability characteristics, a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N) – the attacker can trigger the vulnerability remotely.
• Attack Complexity (AC): Low (L) – no special conditions required.
• Privileges Required (PR): None (N) – no authentication needed.
• User Interaction (UI): None (N) – no user action required.
• Scope (S): Unchanged (U) – the vulnerability does not cross trust boundaries.
• Confidentiality (C): None (N) – no data exposure.
• Integrity (I): None (N) – no data modification.
• Availability (A): High (H) – the crash causes a denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Estimated Base Score: 7.5 (High)

Threat Level: High – due to remote exploitability and lack of patch.

Deep Dive & Mechanics

The vulnerability is a classic null pointer dereference in the add_ca_certs() function of Cesanta Mongoose. This function is responsible for loading Certificate Authority (CA) certificates into the SSL context during TLS handshake initialization.

Root Cause: The function calls SSL_CTX_get_cert_store() to retrieve the certificate store from the SSL context. Under certain conditions (e.g., misconfigured TLS, memory pressure, or specific library states), this function can return NULL. The code does not check the return value before dereferencing the pointer, leading to a segmentation fault.

Attack Vector: An attacker can send a specially crafted TLS handshake request to a Mongoose server that triggers the vulnerable code path. The exact trigger condition depends on the server's TLS configuration, but any scenario where SSL_CTX_get_cert_store() returns NULL will cause a crash.

Impact: Successful exploitation results in a denial of service (DoS) – the Mongoose process crashes, terminating all active connections and requiring a restart. This can disrupt services relying on the library, such as IoT devices, embedded web servers, or REST APIs.

Vulnerable vs. Patched Code

Vulnerable Code (Mongoose < 7.2):

void add_ca_certs(struct mg_connection *c, const char *certs) {
  // ... other code ...
  X509_STORE *store = SSL_CTX_get_cert_store(c->tls_ctx);
  // No NULL check - direct dereference
  X509_STORE_add_cert(store, x509);
  // ... other code ...
}

Patched Code (Mongoose >= 7.2):

void add_ca_certs(struct mg_connection *c, const char *certs) {
  // ... other code ...
  X509_STORE *store = SSL_CTX_get_cert_store(c->tls_ctx);
  if (store == NULL) {
    // Handle error - log and return gracefully
    mg_error(c, "Failed to get certificate store");
    return;
  }
  X509_STORE_add_cert(store, x509);
  // ... other code ...
}

The fix adds a NULL check after SSL_CTX_get_cert_store() and returns an error if the store is unavailable, preventing the null pointer dereference.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A target running Cesanta Mongoose version < 7.2 with TLS enabled.
• Network access to the target's TLS port (typically 443 or 8443).

Exploitation Steps:

1. Identify Target: Use Shodan or similar to find devices running Mongoose with TLS.
2. Craft Malicious TLS Handshake: The exact payload depends on the TLS library used (e.g., OpenSSL, mbedTLS). The goal is to cause SSL_CTX_get_cert_store() to return NULL. This can be achieved by:
   • Sending a TLS ClientHello with an unsupported cipher suite.
   • Triggering a memory allocation failure on the server.
   • Exploiting a race condition in TLS context initialization.
3. Send Payload: Use a tool like openssl s_client or a custom Python script to send the crafted handshake.
4. Observe Crash: The Mongoose process will crash, causing a denial of service.

Example Python Script (Conceptual):

import socket
import ssl

def exploit(target_host, target_port):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((target_host, target_port))
    # Craft a malformed TLS ClientHello
    # This is a simplified example; actual payload may vary
    payload = b"\x16\x03\x01\x00\x00"  # Placeholder
    sock.send(payload)
    sock.close()

if __name__ == "__main__":
    exploit("192.168.1.100", 443)

Note: No public exploit code exists. The above is a conceptual example.

Blue Team (Defense & Detection)

Detection:

• Monitor for repeated TLS handshake failures from the same IP.
• Look for crashes in Mongoose logs (e.g., segmentation faults).
• Use the YARA and Sigma rules below to detect exploitation attempts.

Mitigation:

• Upgrade to Mongoose 7.2 or later.
• If upgrade is not possible, disable TLS or restrict access to trusted networks.
• Implement a Web Application Firewall (WAF) to filter malformed TLS handshakes.
• Use a reverse proxy (e.g., Nginx) to terminate TLS before reaching Mongoose.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Repeated TLS handshake failures from a single source IP.
  • Malformed TLS ClientHello packets (e.g., invalid cipher suites, truncated headers).
• Host IOCs:
  • Mongoose process crash logs containing "segfault" or "SIGSEGV".
  • Core dump files from Mongoose.
  • Log entries: SSL_CTX_get_cert_store() returned NULL (if logging is enabled).
• File IOCs:
  • Mongoose binary version < 7.2 (check with strings or version endpoint).

Detection Engineering

YARA Rule

rule CVE_2025_65502_Mongoose_Null_Pointer_Dereference {
    meta:
        description = "Detects vulnerable Cesanta Mongoose versions (< 7.2) by identifying the missing NULL check in add_ca_certs()"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-65502"
    strings:
        $vuln_code = "SSL_CTX_get_cert_store" nocase
        $null_check = "== NULL" nocase
    condition:
        $vuln_code and not $null_check
}

Sigma Rule

title: CVE-2025-65502 - Cesanta Mongoose Null Pointer Dereference
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects exploitation attempts against CVE-2025-65502 via malformed TLS handshakes
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/mongoose'
        CommandLine|contains: 'segfault'
    condition: selection
falsepositives:
    - Legitimate crashes due to other bugs
level: high

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 443 (msg:"CVE-2025-65502 - Cesanta Mongoose Null Pointer Dereference Attempt"; flow:to_server,established; content:"|16 03 01|"; depth:3; content:"|00 00|"; within:2; classtype:attempted-dos; sid:1000001; rev:1;)

Note: The Suricata rule is a placeholder and may need tuning based on actual exploit payloads.

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Cesanta Mongoose to version 7.2 or later.
2. Workaround: If upgrade is not possible, disable TLS on the Mongoose server or use a reverse proxy for TLS termination.
3. Network Segmentation: Restrict access to Mongoose servers to trusted IPs only.

Long-Term Measures:

• Implement a vulnerability management program to track dependencies.
• Use static analysis tools to detect null pointer dereferences in code.
• Enable logging and monitoring for crash events.
• Subscribe to Cesanta security advisories for future patches.

Valters IT Signature:

Report generated by Valters IT DevSecOps & CTI Division. For inquiries, contact security@valters-it.com.