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The vulnerability, CVE-2025-61778, stems from a flawed implementation of TLS in the Akka.Remote transport. In affected versions (1.2.0 to 1.5.51), the system used one-way TLS authentication. When a client initiated a connection to an Akka.NET server with TLS enabled, the server would correctly present its certificate for the client to validate. However, the server did not request a certificate from the connecting client. This one-sided authentication check meant that any client, even one without a valid certificate, could successfully establish a TLS-encrypted connection to the cluster. The encryption was in place, but the critical mutual authentication was missing. This design flaw allowed untrusted clients to join a secured cluster by bypassing the intended certificate-based authentication, effectively breaking the trust model. The core issue was the absence of mutual TLS (mTLS), which is required to verify both communication parties.
Platform: Akka.NET
Version: 1.2.0-1.5.51
Vulnerability: Authentication Bypass
Severity: Critical
date: 2025
Prediction: 2025-12-01
What Undercode Say:
nmap --script ssl-cert,ssl-enum-ciphers -p [bash] [bash]
openssl s_client -connect [bash]:[bash] -state -nbio
// Hocon configuration enabling TLS (vulnerable versions)
akka.remote.dot-netty.tcp {
enable-ssl = true
ssl {
suppress-validation = false
certificate {
// Config points to server cert only
}
}
}
How Exploit:
An attacker can write a custom client application using the Akka.NET remoting libraries without configuring any client-side certificate. This client can connect to the vulnerable Akka.NET cluster endpoint. The connection will be encrypted, but the attacker’s identity will not be validated, allowing them to join the actor system and send messages to actors, potentially manipulating system state or data.
Protection from this CVE
Upgrade to Akka.NET version 1.5.52 or later. This update enforces mutual TLS by default, requiring both client and server to present and validate certificates. Ensure all nodes in the cluster use the same trusted certificate authority. For immediate mitigation in vulnerable versions, isolate the Akka.NET cluster within a tightly controlled private network that is not accessible from untrusted networks.
Impact:
Critical security impact for users relying on TLS for authentication in publicly accessible or untrusted networks. Allows unauthorized actors to join the cluster, leading to potential data manipulation, data exfiltration, or disruption of cluster operations. The integrity and confidentiality of the system are compromised if an untrusted actor gains membership.
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Sources:
Reported By: github.com
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