GHSA-239c-6cv2-wwx8
HIGHSwift-corelibs-foundation denial of service in JSON decoding with JSONDecoder
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
github.com/apple/swift-corelibs-foundationReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects SwiftURL packages — download data is not available via public APIs for these ecosystems.
Description
Impact
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 it's 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.
Resolution
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.
The resolution is also included in recent development snapshots of Swift available on swift.org for those platforms.
Versions of Swift running on Darwin-based operating systems are not affected.
Workarounds
Users that can control which payload is parsed with JSONDecoder can ensure that fields that are intended to initialize Swift integer types use a JSON numeric constant without a fractional part. This will avoid the crash, but it is rare that a user has full control on the JSON payload they will parse.
As a workaround, users that can alter their current software but cannot perform an upgrade can perform JSON parsing directly (e.g., through the JSONSerialization class) rather than using the JSONDecoder class.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦SwiftURL | github.com/apple/swift-corelibs-foundation | all versions | 5.6.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/apple/swift-corelibs-foundation. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/apple/swift-corelibs-foundation to 5.6.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-239c-6cv2-wwx8 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-239c-6cv2-wwx8 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-239c-6cv2-wwx8. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-239c-6cv2-wwx8 in your dependencies?
O3 detects GHSA-239c-6cv2-wwx8 across SwiftURL dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.