CVE-2025-27788: Ruby JSON Library DOS Vulnerability

CVE-2025-27788 Overview

CVE-2025-27788 is an out-of-bounds read vulnerability affecting the JSON gem for Ruby, a widely-used JSON implementation in the Ruby ecosystem. Starting in version 2.10.0 and prior to version 2.10.2, a specially crafted JSON document could cause an out-of-bounds read operation, most likely resulting in an application crash. This vulnerability exists in the native C extension parser component of the JSON library.

Critical Impact
Applications parsing untrusted JSON input using vulnerable versions of the Ruby JSON gem are susceptible to denial of service attacks through specially crafted documents that trigger out-of-bounds memory reads.

Affected Products

Ruby JSON gem versions 2.10.0 to 2.10.1
Applications using ruby-lang javascript_object_notation library
Ruby applications with native JSON parsing enabled

Discovery Timeline

2025-03-12 - CVE-2025-27788 published to NVD
2025-04-02 - Last updated in NVD database

Technical Details for CVE-2025-27788

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read), which occurs when the software reads data past the end or before the beginning of the intended buffer. In the context of the Ruby JSON parser, the vulnerability exists in the native C extension code responsible for parsing JSON string escape sequences.

The flaw allows a remote attacker to send a specially crafted JSON document that causes the parser to read memory beyond the allocated buffer boundaries. While this primarily results in denial of service through application crashes, out-of-bounds read vulnerabilities can potentially expose sensitive memory contents in certain scenarios.

Root Cause

The root cause of this vulnerability lies in the parser.c file within the JSON gem's native extension. The parser's string unescape routine failed to properly validate buffer boundaries before performing memory read operations. Specifically, the memchr function was called without first verifying that the search pointer (pe) had not already exceeded the string buffer's end boundary (stringEnd).

The vulnerability was introduced in version 2.10.0 when changes to the parsing logic inadvertently removed or bypassed the necessary boundary checks during escape sequence processing.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending a maliciously crafted JSON document to any application that parses untrusted JSON input using the vulnerable library versions. The specially crafted document would contain escape sequences positioned to trigger the out-of-bounds read condition.

The fix adds a boundary check to ensure pe < stringEnd before calling memchr:

     buffer = RSTRING_PTR(result);
     bufferStart = buffer;
 
-    while ((pe = memchr(pe, '\\', stringEnd - pe))) {
+    while (pe < stringEnd && (pe = memchr(pe, '\\', stringEnd - pe))) {
         unescape = (char *) "?";
         unescape_len = 1;
         if (pe > p) {

Source: GitHub Commit c56db31f

Detection Methods for CVE-2025-27788

Indicators of Compromise

Unexpected application crashes during JSON parsing operations
Segmentation fault errors in Ruby applications processing JSON data
Core dumps indicating memory access violations in the JSON gem's native extension
Abnormal termination of web services or API endpoints handling JSON payloads

Detection Strategies

Monitor application logs for segmentation faults or SIGABRT signals during JSON operations
Implement dependency scanning to identify vulnerable JSON gem versions (2.10.0 - 2.10.1)
Use runtime application self-protection (RASP) to detect anomalous memory access patterns
Deploy SentinelOne Singularity to monitor for crash events and memory access violations in Ruby processes

Monitoring Recommendations

Enable verbose logging for JSON parsing operations in production environments
Configure crash reporting and analysis for Ruby application processes
Set up alerts for repeated application restarts that may indicate exploitation attempts
Monitor for unusual patterns in incoming JSON payloads, particularly those with complex escape sequences

How to Mitigate CVE-2025-27788

Immediate Actions Required

Upgrade the Ruby JSON gem to version 2.10.2 or later immediately
Audit applications to identify all instances using vulnerable JSON gem versions
Implement input validation and size limits on JSON documents from untrusted sources
Consider temporarily disabling native JSON parsing if upgrade is not immediately possible

Patch Information

The vulnerability has been fixed in JSON gem version 2.10.2. The fix adds proper boundary validation before performing memory search operations in the parser's escape sequence handling code. Organizations should update their dependencies using:

bash

# Update JSON gem to patched version
gem update json

# Or specify in Gemfile
gem 'json', '>= 2.10.2'

# Then run bundle update
bundle update json

For verification, review the GitHub Security Advisory GHSA-9m3q-rhmv-5q44 and the official release notes for v2.10.2.

Workarounds

No official workarounds are available according to the vendor advisory
As a temporary measure, implement strict input validation on JSON documents before parsing
Consider using alternative JSON parsing libraries temporarily if upgrading is not immediately feasible
Deploy web application firewalls (WAF) rules to filter potentially malicious JSON payloads