CVE-2025-0696

Executive Summary

CVE-2025-0696 is a NULL Pointer Dereference vulnerability in Cesanta Frozen, a JSON parsing library for embedded systems. Versions prior to 1.7 are affected. An attacker can trigger a crash by supplying a specially crafted JSON input, leading to a denial of service (DoS) condition. The vulnerability has a CVSS score of 5.3 (Medium) and an EPSS score of 0.0%, indicating low exploitation likelihood at this time. No public proof-of-concept (PoC) or exploit code is available. The vendor has not released a patch, but upgrading to version 1.7 resolves the issue.

Supply Chain & Package Security

This vulnerability affects the Cesanta Frozen library, which is commonly used in embedded systems and IoT devices. Organizations using this library in their software supply chain should assess their exposure and prioritize upgrading to version 1.7 or later. No known malicious packages or typosquatting attempts have been reported.

CVSS Breakdown & Threat Level

• CVSS Score: 5.3 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: Low (A:L)

The vulnerability is exploitable remotely without authentication, but only results in a crash (denial of service), not data compromise.

Deep Dive & Mechanics

The vulnerability resides in the JSON parsing logic of Cesanta Frozen. When the library processes a maliciously crafted JSON input, it attempts to dereference a NULL pointer, causing a segmentation fault or similar crash. This occurs because the parser does not properly validate certain JSON structures before accessing memory.

The affected function is likely in the core parsing routine, where a pointer is assumed to be valid but can be set to NULL by malformed input. An attacker can exploit this by sending a specially crafted JSON payload to any application using the vulnerable library.

Vulnerable vs. Patched Code

Vulnerable Code (version < 1.7):

// Example vulnerable snippet (conceptual)
void parse_json(const char *input) {
    struct json_token *token = get_next_token(input);
    // No NULL check before dereference
    if (token->type == JSON_TYPE_STRING) {
        // process string
    }
}

Patched Code (version 1.7):

// Example patched snippet (conceptual)
void parse_json(const char *input) {
    struct json_token *token = get_next_token(input);
    if (token == NULL) {
        // Handle error gracefully
        return;
    }
    if (token->type == JSON_TYPE_STRING) {
        // process string
    }
}

The fix adds a NULL pointer check before dereferencing the token, preventing the crash.

Exploitation & Lab Setup

Red Team

No public exploit code is available. To test the vulnerability, craft a JSON payload that triggers a NULL pointer dereference. Example payload:

{"key": null}

Or a deeply nested structure:

{"a":{"b":{"c":null}}}

Use a tool like curl to send the payload to a vulnerable service:

curl -X POST -H "Content-Type: application/json" -d '{"key": null}' http://target:port/api

Blue Team

Monitor for crashes or segmentation faults in applications using Cesanta Frozen. Implement input validation to reject malformed JSON before it reaches the library. Use Web Application Firewalls (WAF) to filter suspicious JSON payloads.

Indicators of Compromise (IOCs)

• Network: Inbound JSON payloads containing null values in unexpected contexts.
• System: Application crash logs showing segmentation faults in the Frozen library.
• File: Presence of Cesanta Frozen library version < 1.7 in the software stack.

Detection Engineering

YARA Rule

rule CVE_2025_0696_Cesanta_Frozen_NULL_Deref {
    meta:
        description = "Detects Cesanta Frozen library versions vulnerable to CVE-2025-0696"
        author = "Valters IT"
        date = "2025-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-0696"
    strings:
        $lib = "Frozen" nocase
        $ver = "version 1." nocase
    condition:
        $lib and $ver and not for any of them ($ver contains "1.7")
}

Sigma Rule

title: Cesanta Frozen NULL Pointer Dereference Exploitation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential exploitation of CVE-2025-0696 via malformed JSON
author: Valters IT
logsource:
    category: application
    product: generic
detection:
    selection:
        EventID: 1000  # Application Error
        Message|contains: "Frozen"
    condition: selection
falsepositives:
    - Legitimate crashes due to other bugs
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-0696 Cesanta Frozen NULL Pointer Dereference Attempt"; content:"null"; depth:100; flow:to_server,established; sid:1000001; rev:1;)

Remediation & Mitigation

1. Upgrade: Update Cesanta Frozen to version 1.7 or later.
2. Input Validation: Implement strict JSON validation before parsing.
3. WAF Rules: Deploy WAF rules to block JSON payloads with unexpected null values.
4. Monitoring: Monitor application logs for crashes and anomalies.
5. Network Segmentation: Isolate vulnerable systems to limit exposure.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert