Meshtastic is an open-source mesh networking firmware designed to facilitate decentralized communication. Starting with version 2.5, the firmware introduced asymmetric encryption using Public Key Infrastructure (PKI) for direct messages to enhance security. However, to maintain backward compatibility, if the pki_encrypted flag is absent in a message, the firmware silently reverts to legacy AES-256-CTR symmetric channel encryption. This fallback mechanism was intentional but created a critical security weakness. End-user applications, including the Web app, iOS/Android apps, and SDK-based applications, lack the capability to differentiate between messages encrypted with PKI and those encrypted with the legacy symmetric method. Consequently, an attacker who knows the shared channel key can craft and inject spoofed direct messages that appear to be PKI-encrypted to the user, effectively performing a downgrade attack. This undermines the integrity and authenticity guarantees that the PKI encryption was supposed to provide. The vulnerability is tracked as CVE-2025-53627 and classified under CWE-1287 (Improper Validation of Specified Type of Input). It has a CVSS v3.1 score of 5.3 (medium severity), reflecting that it can be exploited remotely without authentication or user interaction, but it does not impact confidentiality directly. The flaw was addressed and fixed in Meshtastic firmware version 2.7.15. No known exploits are reported in the wild as of the publication date. This vulnerability highlights the risks of silent fallback mechanisms in cryptographic implementations and the importance of clear feedback to end users regarding encryption status.

For European organizations, especially those deploying Meshtastic in critical communication infrastructures, IoT networks, or emergency response systems, this vulnerability poses a risk to message integrity and authenticity. Attackers with access to the shared channel key can inject spoofed messages that appear securely encrypted, potentially misleading users or triggering incorrect automated responses. This can undermine trust in the communication system and may lead to misinformation, operational disruption, or exploitation in coordinated attacks. Although confidentiality is not directly compromised, the integrity loss can have cascading effects in sensitive environments. Given the growing interest in decentralized and resilient communication networks in Europe, particularly in rural or emergency contexts, the impact could be significant if unpatched devices are widely used. The vulnerability also raises compliance concerns under European data protection and cybersecurity regulations, which mandate the integrity and authenticity of communications. Organizations relying on Meshtastic firmware should assess their exposure and prioritize patching to maintain secure operations.

The primary mitigation is to upgrade all Meshtastic firmware instances to version 2.7.15 or later, where the vulnerability is fixed. Organizations should audit their device inventories to identify affected versions and enforce firmware updates promptly. Additionally, developers of Meshtastic-based applications should implement explicit indicators to inform users whether a direct message was decrypted using PKI or legacy encryption, enhancing user awareness and trust. Network administrators should restrict access to shared channel keys and monitor for anomalous message injection attempts. Employing network segmentation and access controls can limit the exposure of the shared keys. For deployments where upgrading is temporarily not feasible, organizations should consider disabling legacy encryption fallback if configurable or isolate vulnerable devices from critical networks. Finally, integrating cryptographic validation and message authentication at the application layer can provide an additional security layer against spoofing.