Can a Stolen TLS Inspection CA Key Decrypt Recorded Sessions?

“An attacker stole the proxy's private key” leaves out the detail that determines the damage: which private key?

An inspecting forward proxy may hold a certificate-authority signing key, generated server certificate keys, live TLS traffic secrets, session-resumption material and plaintext in its own memory or logs. These are different assets.

The CA key grants an impersonation capability

Clients configured to trust an inspection CA can accept certificates issued under it, subject to the certificate chain's constraints and the client's checks. Someone who steals the corresponding signing key may be able to create other acceptable certificates within those constraints.

That is serious, but the key alone does not redirect a user's connection. The attacker also needs a usable interception position or another way to get the client to connect to the impersonated endpoint. Applications with additional certificate checks may reject it.

A signature key is not a transcript decryption key

In a certificate-authenticated TLS 1.3 handshake using ephemeral Diffie–Hellman, the certificate authenticates the exchange. Traffic keys are derived from separate handshake secrets. Obtaining the CA signing key later does not, by itself, reconstruct those past ephemeral secrets from a recording.

Even stealing the server certificate's private key is not equivalent to obtaining the traffic secrets for that type of recorded session. Do not generalize this to every historical TLS mode or every resumption setup; the negotiated key exchange and the compromised material matter.

A compromise of the running inspection appliance can be much broader than theft of its CA key. The appliance handles plaintext while inspecting traffic and may hold session secrets or retain decrypted content. Those exposures require their own investigation and cannot be dismissed by citing forward secrecy.

Describe the incident precisely

Record whether the lost material was the signing key, endpoint keys, live secrets, stored plaintext or control of the appliance itself. Replacing the inspection CA and updating client trust addresses one capability; it does not erase copied logs or settle whether live traffic was read. Accurate key names lead to a much more useful response than calling every secret “the SSL key.”