Swiftly Broken: Hijacking AWS SDK Traffic via Region Injection

When we import an SDK like aws-sdk-swift, we treat it as immutable infrastructure. We assume that when we ask it to talk to S3 or DynamoDB, it knows how to find the door. We provide credentials, maybe a region, and we let the black box handle the HTTP requests. But what happens when that black box is a little too trusting of the instructions we give it?

This specific vulnerability, tracked as GHSA-PC9J-5V36-2MWW, highlights a classic boundary condition failure. The AWS SDK for Swift blindly trusted the region configuration parameter. It didn't care if you said the region was us-east-1 or us-east-1.attacker.com/oops. It just took your string and glued it into a URL.

While AWS classifies this as a "defense-in-depth" enhancement—implying that if an attacker can set your environment variables, you're already in trouble—it represents a fascinating mechanism for persistence and data redirection. It’s the digital equivalent of swapping the street signs in a neighborhood so the mailman delivers your letters to the thieves next door.

The root cause here is rudimentary: unchecked string interpolation. In the world of cloud SDKs, endpoints are often constructed dynamically. A typical AWS endpoint looks something like https://<service>.<region>.amazonaws.com. To build this, the code takes the service name (e.g., s3) and the user-supplied region.

Prior to version 1.5.79, the SDK's logic effectively boiled down to: url = "https://" + service + "." + region + ".amazonaws.com". (This is a simplification, but accurately represents the logical flow).

There was no regex check to ensure the region string looked like a valid region (e.g., [a-z]{2}-[a-z]+-\d). If an attacker, or a compromised configuration script, set the region to a value containing dots or slashes, the SDK would happily construct a Frankenstein URL. This isn't a memory corruption bug or a buffer overflow; it's a logic error where the code assumed the configuration provider was a benevolent actor.

Let's look at the logic conceptually. The vulnerability exists in the endpoint resolution pipeline. In the vulnerable versions, the code handled the input essentially as a raw string pass-through.

Vulnerable Logic (Conceptual Swift):

// The naive implementation
let region = config.region // e.g., "us-east-1.evil.com"
let endpoint = "https://service.\(region).amazonaws.com"
// Result: https://service.us-east-1.evil.com.amazonaws.com

If the attacker controls a wildcard DNS for evil.com, that request resolves to their server, not AWS. The SDK doesn't know the difference; it's just following orders.

The Fix (v1.5.79):

The remediation introduced strict validation. The SDK now asserts that the region string conforms to RFC-compliant host label standards before using it.

// The hardened implementation
func validateRegion(_ region: String) throws {
    // Regex: Alphanumeric characters and hyphens only
    let regionRegex = /^[a-zA-Z0-9-]+$/
    guard region.wholeMatch(of: regionRegex) != nil else {
         throw ClientError.invalidConfiguration("Invalid region format")
    }
}

By enforcing an allowlist of characters (alphanumeric and hyphens), the patch neutralizes the ability to inject periods (.) or slashes (/), which are necessary to traverse out of the intended subdomain structure.

To exploit this, we don't need to send a malicious packet to the server. We need to be on the server (or the developer's laptop). This is a post-compromise or insider threat scenario. Let's say we have gained a foothold on a CI/CD runner and we want to steal request headers or just disrupt operations subtly.

The Attack Chain:

1. Access: Gain shell access or ability to modify .env files / ~/.aws/config.
2. Configuration: Set the environment variable: export AWS_REGION='us-east-1.attacker.net'
3. Execution: The Swift application initializes the S3 client using the environment configuration.
4. Traffic Gen: The app calls s3.listBuckets().
5. Resolution: The SDK constructs the URL: https://s3.us-east-1.attacker.net.amazonaws.com.

Wait, that URL ends in amazonaws.com. Does that help us? It depends on how the SDK handles the suffix. If the SDK logic is strictly {region}.amazonaws.com, we are limited.

However, if we can inject a path or if the suffix logic is conditional, we can break out entirely. Even with the suffix forced, if we register attacker.net.amazonaws.com (unlikely) or if we simply confuse the DNS resolver or internal proxy (SSRF), we win. More dangerously, if the SDK logic allows for custom endpoints via region mapping, we might reroute traffic entirely to an IP address.

You might look at the CVSS score of 3.7 and laugh. "Low severity? Why are we reading this?" The severity is low because of the prerequisites (local access), not the consequence.

If successful, the impact is Information Disclosure and Integrity Loss.

1. Credential Leaking: While AWS SigV4 signatures are time-bound and scoped, sending them to a third party reveals the Access Key ID and the structure of the request. In some poorly configured setups, other sensitive headers might be included.
2. Denial of Service: By pointing the application to a black hole, an attacker can silently disable cloud capabilities without modifying the application binary.
3. Bypassing Egress Controls: If an organization allows traffic to *.amazonaws.com but blocks everything else, this injection keeps the traffic matching the allowlist (domain suffix) while routing it to a host the attacker might control via complex DNS poisoning or internal routing tricks.

The fix is simple: Update your dependencies. The AWS SDK for Swift version 1.5.79 introduces the necessary validation logic.

If you are stuck on an older version, your mitigation strategy is rigorous configuration management.

1. Lock Down Environment Variables: Ensure that AWS_REGION and AWS_DEFAULT_REGION cannot be modified by unprivileged processes.
2. Audit Config Files: Regularly scan ~/.aws/config and other configuration sources for non-standard region strings.
3. Network Policies: Use rigid egress filtering that restricts traffic to specific AWS IP ranges, not just DNS names, although this is administratively heavy.

Ultimately, this is a lesson in input validation. Even configuration files provided by the "admin" are input. And all input is evil until proven otherwise.