CVE-2022-1642

A program using swift-corelibs-foundation is vulnerable to a denial of service attack caused by a potentially malicious source producing a JSON document containing a type mismatch. This vulnerability is caused by the interaction between a deserialization mechanism offered by the Swift standard library, the Codable protocol; and the JSONDecoder class offered by swift-corelibs-foundation, which can deserialize types that adopt the Codable protocol based on the content of a provided JSON document. When a type that adopts Codable requests the initialization of a field with an integer value, the JSONDecoder class uses a type-erased container with different accessor methods to attempt and coerce a corresponding JSON value and produce an integer. In the case the JSON value was a numeric literal with a floating-point portion, JSONDecoder used different type-eraser methods during validation than it did during the final casting of the value. The checked casting produces a deterministic crash due to this mismatch. The JSONDecoder class is often wrapped by popular Swift-based web frameworks to parse the body of HTTP requests and perform basic type validation. This makes the attack low-effort: sending a specifically crafted JSON document during a request to these endpoints will cause them to crash. The attack does not have any confidentiality or integrity risks in and of itself; the crash is produced deterministically by an abort function that ensures that execution does not continue in the face of this violation of assumptions. However, unexpected crashes can lead to violations of invariants in services, so its possible that this attack can be used to trigger error conditions that escalate the risk. Producing a denial of service may also be the goal of an attacker in itself. This issue is solved in Swift 5.6.2 for Linux and Windows. This issue was solved by ensuring that the same methods are invoked both when validating and during casting, so that no type mismatch occurs. Swift for Linux and Windows versions are not ABI-interchangeable. To upgrade a service, its owner must update to this version of the Swift toolchain, then recompile and redeploy their software. The new version of Swift includes an updated swift-corelibs-foundation package. Versions of Swift running on Darwin-based operating systems are not affected.

Weakness

The product does not handle or incorrectly handles when a particular element is not the expected type, e.g. it expects a digit (0-9) but is provided with a letter (A-Z).

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/48.html

References

• https://github.com/apple/swift-corelibs-foundation/security/advisories/GHSA-239c-6cv2-wwx8
• https://github.com/apple/swift-corelibs-foundation/security/advisories/GHSA-239c-6cv2-wwx8