CVE-2026-28815: Swift Crypto Information Disclosure Flaw

CVE-2026-28815 Overview

CVE-2026-28815 is an out-of-bounds read vulnerability affecting Apple's swift-crypto library. 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. This issue is fixed in swift-crypto version 4.3.1.

Critical Impact

Remote attackers can exploit this vulnerability without authentication to cause denial of service through application crashes or potentially disclose sensitive memory contents from the affected system.

Affected Products

• Apple swift-crypto versions prior to 4.3.1
• Applications using the X-Wing HPKE decapsulation functionality
• Systems processing untrusted encapsulated keys through the C decapsulation path

Discovery Timeline

• 2026-04-03 - CVE-2026-28815 published to NVD
• 2026-04-03 - Last updated in NVD database

Technical Details for CVE-2026-28815

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read) and affects the X-Wing HPKE (Hybrid Public Key Encryption) implementation within swift-crypto. The flaw resides in the C decapsulation path, which fails to properly validate the length of incoming encapsulated keys before processing them. When a malformed, shortened key is provided, the decapsulation routine attempts to read beyond the bounds of the allocated buffer, leading to undefined behavior.

The impact of this vulnerability is primarily availability-focused, with a network-based attack vector requiring no privileges or user interaction. Successful exploitation can cause application crashes, resulting in denial of service. In certain runtime configurations without adequate memory protections, there is also potential for information disclosure through memory leakage.

Root Cause

The root cause lies in insufficient input validation within the C decapsulation code path of the X-Wing HPKE implementation. The code does not adequately verify that the provided encapsulated key meets the expected length requirements before attempting to process it. This missing boundary check allows an attacker to supply a truncated key that causes the decapsulation routine to read memory beyond the actual buffer boundaries.

Attack Vector

The attack can be executed remotely over a network without requiring authentication, user interaction, or special privileges. An attacker crafts a malicious request containing a shortened X-Wing HPKE encapsulated key and sends it to an application using the vulnerable swift-crypto library. When the application attempts to decapsulate this malformed key, the C code path reads beyond the allocated buffer, causing either:

1. An application crash if memory protections detect the invalid access
2. Disclosure of adjacent memory contents if the read succeeds and the data is returned to the attacker

The vulnerability mechanism involves the decapsulation function receiving an undersized encapsulated key input. Without proper length validation, the function proceeds to read the expected number of bytes from the buffer, extending past its actual boundaries. For technical details regarding the specific code paths affected, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-28815

Indicators of Compromise

• Unexpected application crashes or segmentation faults in services utilizing swift-crypto HPKE functionality
• Anomalous network requests containing malformed or truncated encapsulated key data
• Memory access violations logged by runtime protection mechanisms
• Unusual patterns of HPKE decapsulation failures in application logs

Detection Strategies

• Monitor application logs for crashes or errors related to HPKE decapsulation operations
• Implement input validation rules to flag requests with abnormally short encapsulated key payloads
• Deploy memory safety monitoring tools to detect out-of-bounds read attempts
• Use network intrusion detection to identify malformed cryptographic protocol messages

Monitoring Recommendations

• Enable verbose logging for cryptographic operations within applications using swift-crypto
• Configure crash reporting and analysis tools to capture and alert on swift-crypto related failures
• Establish baseline metrics for HPKE operations to detect anomalous patterns indicating exploitation attempts
• Review application error rates for services processing external cryptographic inputs

How to Mitigate CVE-2026-28815

Immediate Actions Required

• Upgrade swift-crypto to version 4.3.1 or later immediately
• Audit all applications and services that depend on swift-crypto for X-Wing HPKE functionality
• Implement additional input validation to reject encapsulated keys that do not meet minimum length requirements
• Consider temporarily disabling X-Wing HPKE features if upgrade is not immediately possible

Patch Information

The vulnerability has been addressed in swift-crypto version 4.3.1. Organizations should update their dependencies to this version or later. The fix implements proper length validation for encapsulated keys before the decapsulation process begins, preventing the out-of-bounds read condition.

For detailed patch information, see the GitHub Security Advisory.

Workarounds

• Implement application-level input validation to verify encapsulated key lengths before passing to swift-crypto
• Deploy network-level filtering to block requests with malformed cryptographic payloads
• Enable runtime memory protections such as ASLR and stack canaries to limit exploitation impact
• Consider using request rate limiting for endpoints that process HPKE operations to reduce attack surface

# Update swift-crypto dependency to patched version
# In Package.swift, update the dependency:
# .package(url: "https://github.com/apple/swift-crypto.git", from: "4.3.1")

# Verify installed version after update
swift package show-dependencies | grep swift-crypto