GHSA-9m44-rr2w-ppp7

Summary

The X-Wing decapsulation path accepts attacker-controlled encapsulated ciphertext bytes without enforcing the required fixed ciphertext length. The decapsulation call is forwarded into a C API, which expects a compile-time fixed-size ciphertext buffer of 1120 bytes. This creates an FFI memory-safety boundary issue when a shorter Data value is passed in, because the C code may read beyond the Swift buffer.

The issue is reachable through initialization of an HPKE.Recipient, which decapsulates the provided encapsulatedKey during construction. A malformed encapsulatedKey can therefore trigger undefined behavior instead of a safe length-validation error.

Details

The decapsulate function of OpenSSLXWingPrivateKeyImpl does not perform a length check before passing the encapsulated data to the C API.

func decapsulate(_ encapsulated: Data) throws -> SymmetricKey {
    try SymmetricKey(unsafeUninitializedCapacity: Int(XWING_SHARED_SECRET_BYTES)) { sharedSecretBytes, count in
        try encapsulated.withUnsafeBytes { encapsulatedSecretBytes in
            let rc = CCryptoBoringSSL_XWING_decap(
                sharedSecretBytes.baseAddress,
                encapsulatedSecretBytes.baseAddress,
                &self.privateKey
            )
            guard rc == 1 else {
                throw CryptoKitError.internalBoringSSLError()
            }
            count = Int(XWING_SHARED_SECRET_BYTES)
        }
    }
}

The C API does not have a runtime length parameter and instead expects a fixed-size buffer of 1120 bytes.

#define XWING_CIPHERTEXT_BYTES 1120

OPENSSL_EXPORT int XWING_decap(
    uint8_t out_shared_secret[XWING_SHARED_SECRET_BYTES],
    const uint8_t ciphertext[XWING_CIPHERTEXT_BYTES],
    const struct XWING_private_key *private_key);

Since decapsulate accepts arguments of any length, an attacker controlled input can trigger an out-of-bounds read. The vulnerable code path can be reached through by...