CVE-2020-12762: Json-c Buffer Overflow Vulnerability

CVE-2020-12762 Overview

CVE-2020-12762 is an integer overflow vulnerability in the json-c library through version 0.14 that can lead to an out-of-bounds write when processing a large JSON file. The vulnerability is specifically demonstrated in the printbuf_memappend function, which fails to properly validate size calculations before memory operations. This memory corruption vulnerability can potentially allow attackers to execute arbitrary code or cause denial of service conditions when a vulnerable application parses maliciously crafted JSON input.

Critical Impact

Successful exploitation of this integer overflow vulnerability can result in memory corruption, potentially leading to arbitrary code execution with the privileges of the application parsing the malicious JSON data, or causing application crashes resulting in denial of service.

Affected Products

• json-c through version 0.14
• Fedora 30, 31, and 32
• Debian Linux 8.0, 9.0, and 10.0
• Ubuntu Linux 12.04, 14.04 ESM, 16.04 ESM, 18.04 LTS, 19.10, and 20.04 LTS
• Siemens SINEC INS (including version 1.0 and 1.0 SP1)

Discovery Timeline

• 2020-05-09 - CVE-2020-12762 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2020-12762

Vulnerability Analysis

The vulnerability exists in the json-c library's buffer management routines, specifically within the printbuf_memappend function. When processing JSON data, the library needs to dynamically allocate and expand buffers to accommodate varying input sizes. The core issue arises when calculating the new buffer size during reallocation operations.

When a sufficiently large JSON file is processed, the size calculation can wrap around due to integer overflow, resulting in a much smaller buffer being allocated than required. Subsequent write operations then exceed the allocated buffer boundaries, corrupting adjacent memory regions. This out-of-bounds write condition (CWE-787) stems directly from the improper handling of integer overflow conditions (CWE-190).

The attack requires local access with user interaction, meaning a victim must be induced to process a malicious JSON file using an application that depends on the vulnerable json-c library. The vulnerability affects confidentiality, integrity, and availability, as successful exploitation can lead to information disclosure, arbitrary code execution, or application crashes.

Root Cause

The root cause is an integer overflow vulnerability (CWE-190) in the buffer size calculation logic within printbuf_memappend. When the requested buffer size exceeds the maximum representable integer value, the calculation wraps around to a small positive number. The library then allocates this insufficient buffer space and proceeds to write data beyond its boundaries, resulting in an out-of-bounds write (CWE-787). The absence of proper overflow checking before memory allocation operations allows this condition to be exploited.

Attack Vector

The attack vector requires local access to the target system and user interaction. An attacker must craft a malicious JSON file with specific characteristics designed to trigger the integer overflow condition during parsing. The attack scenario involves:

1. Creating a JSON file with extremely large nested structures or string values designed to force buffer reallocation
2. Delivering the malicious JSON file to the target system (via email, download, or other means)
3. Inducing the victim to process the file using an application linked against the vulnerable json-c library
4. The parsing operation triggers the integer overflow, leading to memory corruption

The vulnerability mechanism involves buffer size calculations that fail to account for integer overflow conditions. When the printbuf_memappend function calculates the new buffer size by adding the existing size to the requested additional space, the result can overflow and wrap around to a small value, leading to insufficient memory allocation followed by out-of-bounds writes.

Detection Methods for CVE-2020-12762

Indicators of Compromise

• Unexpected application crashes or segmentation faults when processing JSON files
• Abnormal memory consumption patterns in applications using json-c library
• Core dump files indicating memory corruption in json-c related functions
• Unusual JSON files with extremely large nested structures or string values on the system

Detection Strategies

• Monitor for application crashes involving json-c library functions, particularly printbuf_memappend
• Implement file integrity monitoring for JSON configuration files processed by critical applications
• Deploy memory protection mechanisms such as ASLR and DEP to make exploitation more difficult
• Use static and dynamic code analysis tools to identify applications linked against vulnerable json-c versions

Monitoring Recommendations

• Audit installed packages to identify systems running json-c version 0.14 or earlier
• Monitor system logs for segmentation faults in applications that process JSON data
• Implement input validation for JSON files before processing, rejecting files exceeding reasonable size thresholds
• Track security advisories from vendors using json-c, including Siemens, Debian, Ubuntu, and Fedora

How to Mitigate CVE-2020-12762

Immediate Actions Required

• Update json-c library to a patched version that addresses the integer overflow vulnerability
• Apply vendor-specific security patches from your Linux distribution (Debian, Ubuntu, Fedora, or others)
• Review applications that depend on json-c and prioritize updates for those processing untrusted JSON input
• For Siemens SINEC INS deployments, apply the patches referenced in Siemens Security Advisory SSA-637483

Patch Information

The json-c project has addressed this vulnerability in newer releases. The fix is available in the GitHub Pull Request for JSON-C, which implements proper overflow checking in the buffer size calculations.

Multiple Linux distributions have released security updates:

• Debian: DSA-4741 and multiple LTS announcements
• Ubuntu: USN-4360-1 and USN-4360-4
• Fedora: Package updates for Fedora 30, 31, and 32
• Gentoo: GLSA 202006-13

Organizations using Siemens SINEC INS should consult the Siemens security advisory for specific remediation guidance.

Workarounds

• Implement input validation to reject JSON files exceeding a reasonable size threshold before parsing
• Run applications that process untrusted JSON data in sandboxed environments with restricted privileges
• Use memory protection mechanisms (ASLR, DEP/NX) to make exploitation more difficult
• Consider using alternative JSON parsing libraries for critical applications until patching is complete

# Check installed json-c version on Debian/Ubuntu
dpkg -l | grep libjson-c

# Update json-c on Debian/Ubuntu
sudo apt-get update && sudo apt-get upgrade libjson-c3 libjson-c-dev

# Check installed json-c version on Fedora/RHEL
rpm -qa | grep json-c

# Update json-c on Fedora
sudo dnf update json-c