CVE-2026-2968: Cesanta Mongoose RCE Vulnerability

CVE-2026-2968 Overview

A cryptographic signature verification vulnerability has been identified in Cesanta Mongoose up to version 7.20. This security flaw affects the mg_chacha20_poly1305_decrypt function located in /src/tls_chacha20.c, specifically within the Poly1305 Authentication Tag Handler component. The vulnerability allows improper verification of cryptographic signatures, potentially enabling attackers to bypass authentication mechanisms in TLS communications.

Critical Impact

Attackers exploiting this vulnerability could potentially forge or tamper with encrypted communications by circumventing Poly1305 authentication tag verification, compromising the integrity guarantees of ChaCha20-Poly1305 encrypted data.

Affected Products

• Cesanta Mongoose versions up to 7.20
• Applications and embedded systems utilizing Mongoose's TLS implementation with ChaCha20-Poly1305 cipher suite
• IoT devices and web servers built on the Mongoose networking library

Discovery Timeline

• 2026-02-23 - CVE-2026-2968 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-2968

Vulnerability Analysis

This vulnerability stems from improper verification of cryptographic signatures (CWE-345) within the ChaCha20-Poly1305 AEAD (Authenticated Encryption with Associated Data) implementation in Cesanta Mongoose. The mg_chacha20_poly1305_decrypt function fails to properly validate the Poly1305 authentication tag during decryption operations.

ChaCha20-Poly1305 is a widely-used AEAD cipher suite that combines the ChaCha20 stream cipher with the Poly1305 message authentication code. The Poly1305 tag provides integrity verification, ensuring that encrypted data has not been modified. When this verification is improperly implemented, attackers may be able to modify ciphertext without detection.

The attack is characterized as having high complexity and is noted to be difficult to exploit. It can be launched remotely over the network, though the exploitation requires sophisticated cryptographic knowledge and precise timing. The vendor was contacted about this disclosure but did not respond.

Root Cause

The root cause lies in the implementation of the Poly1305 authentication tag verification within the mg_chacha20_poly1305_decrypt function. The code fails to perform constant-time comparison or adequate validation of the computed authentication tag against the received tag. This implementation flaw allows potential authentication bypass scenarios where malformed or tampered ciphertext may pass verification checks.

Attack Vector

The attack vector is network-based, targeting TLS connections that utilize the ChaCha20-Poly1305 cipher suite. An attacker positioned as a man-in-the-middle or with the ability to intercept and modify network traffic could potentially exploit this vulnerability.

The exploitation involves intercepting ChaCha20-Poly1305 encrypted TLS packets, modifying the ciphertext, and crafting authentication tags that exploit the improper verification logic. Due to the high complexity of the attack, successful exploitation requires detailed knowledge of the target's cryptographic state and precise manipulation of encrypted data.

For detailed technical information and proof-of-concept details, refer to the GitHub CVE Resource.

Detection Methods for CVE-2026-2968

Indicators of Compromise

• Unusual TLS handshake failures or renegotiations in applications using Mongoose
• Unexpected decryption errors followed by successful data processing
• Network traffic anomalies showing modified TLS packets with ChaCha20-Poly1305 cipher suite
• Log entries indicating authentication tag verification inconsistencies

Detection Strategies

• Implement network intrusion detection rules to identify malformed TLS packets targeting ChaCha20-Poly1305 connections
• Monitor application logs for cryptographic verification failures and exceptions in Mongoose-based applications
• Deploy TLS inspection capabilities to analyze cipher suite usage and detect anomalous patterns
• Use SentinelOne's behavioral analysis to detect exploitation attempts targeting cryptographic libraries

Monitoring Recommendations

• Enable verbose logging for TLS operations in Mongoose-based applications
• Monitor for applications using affected Mongoose versions (7.20 and earlier)
• Implement alerts for sudden increases in TLS-related errors or connection resets
• Track connections using ChaCha20-Poly1305 cipher suite for anomalous behavior patterns

How to Mitigate CVE-2026-2968

Immediate Actions Required

• Identify all systems and applications using Cesanta Mongoose versions up to 7.20
• Consider temporarily disabling ChaCha20-Poly1305 cipher suite support and using alternative AEAD ciphers such as AES-GCM
• Implement additional network-layer protections for systems that cannot be immediately updated
• Review TLS configurations and restrict cipher suite options to exclude vulnerable implementations

Patch Information

At the time of publication, the vendor (Cesanta) has not responded to disclosure attempts. Organizations should monitor the official Cesanta Mongoose repository for security updates. Additional vulnerability details are available at VulDB #347335 and the GitHub PoC Repository.

Workarounds

• Configure TLS to prefer alternative cipher suites such as AES-128-GCM or AES-256-GCM that do not rely on the affected Poly1305 implementation
• Implement network segmentation to limit exposure of vulnerable Mongoose-based services
• Deploy Web Application Firewalls (WAF) or TLS proxies that perform additional authentication tag verification
• Consider implementing application-layer encryption as an additional security layer for sensitive communications

# Configuration example - Disable ChaCha20-Poly1305 in Mongoose (if supported)
# Add to Mongoose TLS configuration
export MG_TLS_CIPHERS="TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256"
# Alternatively, configure cipher preference in application code
# mg_tls_ctx_set_ciphers(ctx, "AES256-GCM-SHA384:AES128-GCM-SHA256");