CVE-2026-28815: swift-crypto X-Wing HPKE Out-of-Bounds Read

Overview

CVE-2026-28815 is a high-severity vulnerability in Apple's swift-crypto library affecting the X-Wing Hybrid Public Key Encryption (HPKE) implementation. An attacker supplying a specially crafted short encapsulated key can trigger an out-of-bounds read in the C decapsulation code path.

Vulnerability Details

X-Wing is a hybrid KEM (Key Encapsulation Mechanism) that combines X25519 Diffie-Hellman with ML-KEM-768 (Kyber) for post-quantum resistance. The defect exists in the native C implementation of the decapsulation routine: when the encapsulated key material is shorter than the expected format, a length validation is bypassed, allowing the library to read past the end of an allocated buffer.

Depending on memory layout and platform-level protections (ASLR, guard pages), the result is either:

• Process crash — denial of service for applications performing X-Wing decapsulation
• Memory disclosure — leaking bytes adjacent to the key buffer, potentially exposing sensitive heap data

The flaw is limited to the X-Wing decapsulation path; standard X25519 and ML-KEM-768 operations are unaffected.

Affected Components

• Library: swift-crypto (Apple)
• Component: CryptoBoringWrapper / X-Wing HPKE C bindings
• Versions: All releases prior to 4.3.1
• Platforms: Any Swift project linking swift-crypto, including server-side Swift, iOS/macOS apps using the library directly

Mitigation

Update swift-crypto to version 4.3.1 or later. In your Package.swift:

.package(url: "https://github.com/apple/swift-crypto.git", from: "4.3.1")

Run swift package update to pull the patched version.

If an immediate upgrade is not possible, consider disabling X-Wing HPKE operations at the application layer until the patch can be applied.

References

• NVD Entry — CVE-2026-28815
• Apple swift-crypto GitHub
• swift-crypto 4.3.1 Release Notes