CVE-2021-36369 is a high-severity vulnerability affecting Dropbear SSH client versions through 2020.81. The core issue arises from a non-RFC-compliant implementation in the client-side SSH code that improperly checks the available authentication methods presented by the SSH server during the login process. This flaw enables a malicious SSH server to manipulate the authentication flow to its advantage. Specifically, the server can alter the login sequence to bypass additional security mechanisms such as FIDO2 hardware tokens or SSH-Askpass, which are typically used to strengthen authentication. By exploiting this vulnerability, an attacker controlling or impersonating an SSH server can abuse a forwarded SSH agent to authenticate to other servers without detection. This means that once a user connects to a compromised or malicious SSH server, the attacker can leverage the forwarded agent credentials to move laterally or escalate privileges on other systems that trust the same credentials. The vulnerability is classified under CWE-287 (Improper Authentication), indicating a failure to properly verify authentication credentials or methods. The CVSS v3.1 base score is 7.5, reflecting a high impact on integrity with no impact on confidentiality or availability, no user interaction required, and no privileges needed to exploit. There are no known exploits in the wild as of the published date, and no specific patches or vendor advisories are listed in the provided data. The vulnerability affects the Dropbear SSH client, a lightweight SSH implementation commonly used in embedded systems, network devices, and some Linux distributions, especially where resource constraints exist. The lack of RFC compliance in authentication method checks is a subtle but critical flaw that undermines the security guarantees of SSH agent forwarding and multi-factor authentication mechanisms.

For European organizations, the impact of CVE-2021-36369 can be significant, particularly for those relying on Dropbear SSH clients in their infrastructure. Many embedded devices, IoT gateways, and network appliances deployed across industries such as telecommunications, manufacturing, and critical infrastructure use Dropbear due to its lightweight nature. If these devices are configured to use SSH agent forwarding and connect to untrusted or compromised SSH servers, attackers could exploit this vulnerability to gain unauthorized access to internal systems by abusing forwarded credentials. This lateral movement risk threatens the integrity of internal networks and can lead to unauthorized command execution or privilege escalation without triggering typical multi-factor authentication alerts. Organizations using FIDO2 tokens or SSH-Askpass for enhanced security may have a false sense of protection, as this vulnerability bypasses those controls. The stealthy nature of the attack—leveraging legitimate authentication forwarding—makes detection difficult. Additionally, sectors with stringent security requirements, such as finance, healthcare, and government agencies in Europe, could face compliance and operational risks if this vulnerability is exploited. The absence of known exploits in the wild reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits targeting embedded devices with Dropbear. The impact is compounded in environments where patching embedded systems is challenging, leading to prolonged exposure.

To mitigate CVE-2021-36369, European organizations should take the following specific actions: 1) Inventory and identify all devices and systems using Dropbear SSH clients, especially embedded and IoT devices. 2) Where possible, upgrade Dropbear to versions released after 2020.81 that address this vulnerability; if no official patch exists, monitor vendor advisories closely and apply updates promptly once available. 3) Disable SSH agent forwarding on clients and devices that do not require it, as this feature is central to the attack vector. 4) Implement strict network segmentation to limit SSH connections from untrusted servers and reduce the risk of lateral movement. 5) Employ SSH connection monitoring and anomaly detection tools that can flag unusual authentication flows or unexpected agent forwarding usage. 6) For critical systems, consider replacing Dropbear with more robust SSH clients that fully comply with RFC standards and support secure multi-factor authentication. 7) Educate system administrators and security teams about the risks of agent forwarding and the importance of verifying SSH server authenticity. 8) Use hardware security modules or secure enclave technologies to protect SSH keys and agent forwarding credentials where feasible. These measures go beyond generic advice by focusing on the specific exploitation mechanism and the operational context of Dropbear in embedded environments.