- Reported
-
- Issued
-
- Package
-
mykey
(crates.io)
- Type
-
Vulnerability
- Categories
-
- Keywords
-
#mikey
#srtp
#rfc3830
#authentication-bypass
- Aliases
-
- References
-
- CVSS Score
- 7.5
HIGH
- CVSS Details
-
- Attack Vector
- Network
- Attack Complexity
- Low
- Privileges Required
- None
- User Interaction
- None
- Scope
- Unchanged
- Confidentiality Impact
- None
- Integrity Impact
- High
- Availability Impact
- None
- CVSS Vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- Patched
-
Description
mykey implements MIKEY (RFC 3830) for SRTP key exchange. No receive path
verified the MIKEY message authentication code.
crypto::verify_mac existed and was correct, but it was called only from unit
tests — neither MikeyMessage::from_bytes nor MikeyMessage::complete_psk
invoked it.
new_psk_init computed an HMAC-SHA-1-160 over the serialized message and
appended it, so the tag was present on the wire and looked like integrity
protection. It was decorative: a receiver accepted any message regardless of
whether the MAC was correct, wrong, or absent.
Consequences
- An attacker able to inject or modify MIKEY messages in transit faced no
integrity check. Modified RAND, SSRC or security-policy payloads were accepted
silently. Note that this does not require a man-in-the-middle position: since
nothing was checked, a wholly forged PSK-Init was accepted.
- Pre-shared key mode therefore provided no mutual authentication, despite the
documentation claiming that it did. Possession of the pre-shared key was never
actually proven by either party.
- Replayed init messages were not rejected.
The practical severity was bounded by GHSA-fwjg-cm92-j3hj (cleartext TGK), which
already let a passive observer recover the key material outright without needing
to forge anything.
The fix
complete_psk now verifies the message MAC before any decryption, so a
message that fails authentication yields no key material at all. A MacAlg::Null
payload, an absent MAC and a truncated MAC are each refused rather than treated
as "no check required".
The authentication key is now derived from the pre-shared key per RFC 3830 §4.1.4
rather than from the TGK — a prerequisite, since verifying against a TGK-derived
key would have been circular while the TGK travelled in the message itself.
Optional §6.9 verification messages were also added, giving the exchange genuine
mutual authentication.
Replay protection is still absent: a captured PSK-Init can be replayed and will
be accepted, because the optional COUNTER timestamp is emitted rather than a
mandatory NTP type and no record of seen timestamps or RAND values is kept. That
gap is tracked at https://github.com/waxspin/mykey/issues/24 and is not covered
by this advisory.
Workarounds
Treat parsed MIKEY messages as unauthenticated input: do not rely on pre-shared
key mode for peer authentication or message integrity, and carry it over a
transport that provides both.
Advisory available under CC0-1.0
license.