CVE-2025-0696 Overview
CVE-2025-0696 is a NULL pointer dereference vulnerability [CWE-476] in the Cesanta Frozen JSON parsing library. The flaw affects Frozen versions prior to 1.7. An attacker can trigger the vulnerability by supplying a maliciously crafted JSON document to any component that embeds the library. Successful exploitation crashes the host process, producing a denial-of-service condition.
Frozen is a lightweight JSON parser and emitter written in C, commonly embedded in IoT firmware, microcontroller applications, and other resource-constrained systems. The network-reachable nature of many Frozen deployments increases exposure where the parser processes untrusted input from remote sources.
Critical Impact
A remote, unauthenticated attacker can crash any application or device embedding Cesanta Frozen by sending a specially crafted JSON payload, disrupting availability of the affected component.
Affected Products
- Cesanta Frozen library versions prior to 1.7
- Applications and firmware embedding vulnerable Frozen releases
- IoT and embedded devices consuming untrusted JSON via Frozen
Discovery Timeline
- 2025-01-27 - CVE-2025-0696 published to the National Vulnerability Database
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-0696
Vulnerability Analysis
The vulnerability resides in Frozen's JSON parsing routines. When the library processes a malformed or unexpected token sequence, it dereferences a pointer that has not been validated against NULL. The resulting segmentation fault terminates the host process. Because JSON parsing is typically performed inline on incoming data, the crash occurs before any application-level input validation can intervene.
The issue is exploitable over the network without authentication or user interaction. Impact is limited to availability, with no confidentiality or integrity effects. Devices and services that automatically restart may enter repeated crash loops if the attacker continues sending the payload.
Root Cause
The root cause is missing pointer validation inside Frozen's parser state machine. Certain input sequences cause an internal helper to return a NULL reference that downstream code then dereferences without a preceding check. This is a classic CWE-476 pattern where error paths and boundary conditions bypass the assumptions made by hot-path parsing code.
Attack Vector
Exploitation requires only the ability to submit JSON input to the vulnerable component. Typical attack channels include HTTP request bodies, WebSocket messages, MQTT payloads, and configuration files fetched from remote endpoints. The attacker crafts a JSON document containing the malformed structure that triggers the unchecked dereference. Refer to the Nozomi Networks Vulnerability Advisory for parser-level detail.
No verified public exploit code is available. The vulnerability mechanism is described in prose because verified proof-of-concept payloads have not been released.
Detection Methods for CVE-2025-0696
Indicators of Compromise
- Unexpected termination or segmentation faults in processes that parse JSON using Frozen
- Repeated service restarts on embedded devices immediately following inbound network requests
- Core dumps referencing Frozen parser functions such as json_walk or parse_value
- Sudden spikes in malformed JSON traffic directed at API endpoints or device management interfaces
Detection Strategies
- Inventory embedded software for the Frozen library and record versions below 1.7
- Deploy schema validation at ingress layers to reject JSON that fails structural checks before it reaches Frozen
- Instrument crash reporting to alert on repeated faults in the same JSON parsing thread
- Use network intrusion detection signatures for anomalous or truncated JSON payloads targeting known Frozen consumers
Monitoring Recommendations
- Correlate device or service reboots with preceding inbound requests to identify targeted crash attempts
- Track error rates on JSON-processing endpoints and alert on statistical deviations
- Log source IP addresses submitting malformed JSON for downstream blocking and threat intelligence enrichment
- Enable stack traces or minidumps on embedded systems where feasible to confirm the fault occurs in Frozen code paths
How to Mitigate CVE-2025-0696
Immediate Actions Required
- Upgrade Cesanta Frozen to version 1.7 or later in all applications and firmware images
- Identify third-party components and vendor firmware that statically link Frozen and request updated builds
- Restrict network access to services exposing JSON parsing to trusted sources until patches are deployed
- Enable automatic process restart with backoff to limit sustained denial-of-service impact
Patch Information
Upgrade to Frozen 1.7 or newer. Because Frozen is typically embedded rather than dynamically linked, patching requires rebuilding and redistributing any application or firmware that ships the library. Coordinate with device vendors and software suppliers for updated releases. Consult the Nozomi Networks Vulnerability Advisory for advisory details.
Workarounds
- Place a validating reverse proxy or API gateway in front of exposed services to enforce strict JSON schema conformance
- Rate-limit inbound requests to reduce the effectiveness of repeated crash attempts
- Isolate affected devices on segmented networks with allowlisted management sources
- Disable optional network-facing features that consume untrusted JSON where operationally acceptable
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
