CVE-2026-5244: Cesanta Mongoose Buffer Overflow Flaw

CVE-2026-5244 Overview

A heap-based buffer overflow vulnerability has been discovered in Cesanta Mongoose, a popular embedded networking library. This vulnerability affects the mg_tls_recv_cert function within the mongoose.c file, specifically in the TLS 1.3 Handler component. Improper handling of the pubkey argument can lead to memory corruption, potentially allowing remote attackers to compromise affected systems.

Critical Impact

Remote attackers can exploit this heap-based buffer overflow vulnerability in the TLS 1.3 handshake process, potentially leading to arbitrary code execution or denial of service on systems using vulnerable versions of Cesanta Mongoose.

Affected Products

• Cesanta Mongoose versions up to 7.20
• Applications and IoT devices using embedded Mongoose networking library with TLS 1.3 enabled
• Web servers and embedded systems built on vulnerable Mongoose versions

Discovery Timeline

• 2026-04-02 - CVE CVE-2026-5244 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-5244

Vulnerability Analysis

This vulnerability is classified as a heap-based buffer overflow (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw resides in the TLS 1.3 certificate processing routine within the Mongoose library. When the mg_tls_recv_cert function processes incoming TLS certificate data, insufficient bounds checking on the pubkey argument allows an attacker to write beyond the allocated heap buffer boundaries.

The vulnerability is network-accessible, requiring no authentication or user interaction to exploit. An attacker can craft malicious TLS 1.3 handshake packets containing specially crafted public key data that triggers the overflow condition during certificate processing. This can corrupt heap metadata, potentially leading to arbitrary code execution or application crashes.

Root Cause

The root cause stems from insufficient input validation in the AES-GCM decryption function used during TLS 1.3 certificate processing. The original implementation of mg_aes_gcm_decrypt lacked proper handling of authenticated encryption with associated data (AEAD) parameters, specifically missing validation for the authentication tag and associated data length fields. This oversight allowed malformed or malicious input to overflow heap-allocated buffers during cryptographic operations.

Attack Vector

The attack can be launched remotely over a network connection. An attacker initiates a TLS 1.3 handshake with a vulnerable Mongoose server and sends a malicious certificate containing a crafted public key. When the server processes this certificate through the mg_tls_recv_cert function, the overflow occurs in the heap memory region. The exploit has been publicly disclosed and may be actively used in attacks.

The following code shows the security patch applied to address this vulnerability:

 int mg_aes_gcm_decrypt(unsigned char *output, const unsigned char *input,
                        size_t input_length, const unsigned char *key,
                        const size_t key_len, const unsigned char *iv,
-                       const size_t iv_len);
+                       const size_t iv_len, unsigned char *aead,
+                       size_t aead_len, const unsigned char *tag,
+                       const size_t tag_len);

 #endif /* TLS_AES128_H */

Source: GitHub Commit Details

The patch adds proper AEAD parameter handling including aead, aead_len, tag, and tag_len arguments to enable complete authentication tag verification during decryption operations.

Detection Methods for CVE-2026-5244

Indicators of Compromise

• Unusual TLS 1.3 handshake traffic with malformed certificate data targeting Mongoose-based services
• Application crashes or unexpected terminations of services running Mongoose library
• Memory corruption artifacts in core dumps or crash reports from affected applications
• Network traffic containing oversized or malformed public key fields in TLS certificates

Detection Strategies

• Monitor for abnormal TLS handshake patterns, particularly malformed certificate exchanges on services using Mongoose
• Implement network intrusion detection rules to identify oversized certificate payloads in TLS 1.3 traffic
• Deploy memory protection mechanisms (ASLR, stack canaries) to detect and mitigate exploitation attempts
• Use application-level logging to track certificate processing errors in Mongoose-based applications

Monitoring Recommendations

• Enable detailed TLS handshake logging on all Mongoose-based services
• Configure alerting for repeated TLS connection failures or certificate parsing errors
• Monitor heap memory usage patterns in applications using vulnerable Mongoose versions
• Implement network traffic analysis to detect potential exploitation attempts targeting port 443 and other TLS-enabled services

How to Mitigate CVE-2026-5244

Immediate Actions Required

• Upgrade Cesanta Mongoose to version 7.21 or later immediately
• If immediate upgrade is not possible, consider temporarily disabling TLS 1.3 support as a stopgap measure
• Audit all applications and embedded devices using Mongoose library to identify vulnerable deployments
• Implement network segmentation to limit exposure of vulnerable services

Patch Information

Cesanta has released version 7.21 which fully addresses this vulnerability. The fix is available in commit 0d882f1b43ff2308b7486a56a9d60cd6dba8a3f1. The vendor responded professionally and released the patch promptly after responsible disclosure.

For detailed patch information, see the GitHub Release v7.21 and the GitHub Commit Details.

Workarounds

• Disable TLS 1.3 support and use TLS 1.2 until the upgrade can be applied
• Deploy a reverse proxy or Web Application Firewall (WAF) in front of vulnerable Mongoose services to filter malicious TLS traffic
• Restrict network access to Mongoose-based services to trusted IP ranges only
• Enable memory protection features at the operating system level (ASLR, DEP/NX) to increase exploitation difficulty

# Example: Updating Mongoose library to patched version
git clone https://github.com/cesanta/mongoose.git
cd mongoose
git checkout 7.21
# Rebuild your application with the updated library