RUSTSEC-2026-0335 Pre-shared key mode transmits the TGK in cleartext, exposing the SRTP master key

mykey implements MIKEY (RFC 3830) for SRTP key exchange. Pre-shared key mode transmitted the TEK Generation Key (TGK) in the clear.

MikeyMessage::new_psk_init built its KEMAC payload with EncAlg::Null and placed the TGK in enc_data verbatim. Anyone able to read a PSK-Init message therefore recovers the TGK — and so the SRTP master key — without knowing the pre-shared key.

The transmitted value was never even used by the receiver: complete_psk re-derived the TGK from the pre-shared key and ignored enc_data entirely. The field was write-only; it provided nothing and disclosed everything.

A second consequence: the message authentication key was derived from the TGK rather than from the pre-shared key, so an attacker who read the TGK off the wire could also forge a valid message MAC. Integrity was lost along with confidentiality. That aspect is tracked separately as GHSA-xq78-wvcv-hqvj.

Exploitability

A MIKEY message is normally distributed as an SDP a=key-mgmt:mikey attribute, which in AES67 and SMPTE ST 2110 deployments is typically announced over SAP multicast. A passive listener on the media network recovers the session key with no active attack and no credentials.

RFC 3830 §4.2.3 permits NULL encryption only where "the underlying protocols can guarantee security", and warns to use it with caution. An SDP file or a SAP announcement is not such a channel.

The fix

Pre-shared key mode now follows RFC 3830 §3.1 as key transport: the initiator generates a random TGK, frames it as a Key data sub-payload (§6.13), and encrypts it with AES-CM (§4.2.3) under an encryption key and 112-bit salt derived from the pre-shared key (§4.1.4). The message MAC is keyed from the pre-shared key and verified before anything is decrypted.

This changes the wire format, so both ends of an exchange must be upgraded together. The incompatibility is silent: an old peer and a new peer complete the exchange without error and then derive different SRTP keys.

Workarounds

Use ephemeral Diffie-Hellman mode instead of pre-shared key mode, or carry the PSK-Init only over a channel that is already confidential and authenticated end to end.