Swiftly Broken: Redirecting AWS SDK Traffic via Region Injection

In the cloud, location is everything. When you initialize an AWS SDK, the first thing it asks for is a Region. Usually, you dutifully provide us-east-1 (because who likes latency?) or eu-central-1. The SDK takes this string, nods politely, and uses it to construct the endpoints it will talk to. It’s the compass by which your application navigates the Amazon nebula.

But what happens when that compass is broken? What if, instead of pointing North to Virginia, you could force the compass to point... well, anywhere you want?

This vulnerability in the AWS SDK for Swift is a classic case of "Implicit Trust." The developers assumed that a region would always be a well-behaved string like us-west-2. They didn't anticipate that a chaotic neutral developer (or a chaotic evil hacker) might feed it evil.com or other special characters. It turns out, when you blindly concatenate strings to build URLs, you aren't just building endpoints; you're building bridges to places you never intended to go.

At its core, this is an Input Validation failure (CWE-20), but let's get specific. The AWS SDK for Swift constructs service URLs dynamically. The logic generally resembles this pattern:

https://<service>.<region>.amazonaws.com

The vulnerability lies in the fact that the SDK treated the <region> component as a trusted variable. It accepted whatever string was passed to it—whether from a hardcoded string, a configuration file, or the notorious AWS_REGION environment variable—and injected it directly into the URL structure without verifying if it was actually a valid AWS region or even a valid host label.

Why does this matter? DNS host labels are strict. They should be alphanumeric and utilize hyphens (RFC 1123). They shouldn't contain dots (.), slashes (/), or colons (:). By failing to sanitize this input, the SDK allowed the region variable to break the semantic structure of the URL. While the exact endpoint resolution logic in the Swift SDK is complex (involving partitions and endpoint rulesets), the lack of validation meant the "region" could potentially alter the domain hierarchy itself.

The fix provided in version 1.5.79 is a textbook example of "Defense in Depth"—which is corporate speak for "We realized we should probably check this." The patch introduces a validator that ensures the region string strictly adheres to valid host label syntax.

The Vulnerable Logic (Conceptual)

Before the patch, the code essentially did this:

// Dangerous: No validation
let region = config.region // e.g., "us-east-1"
let endpoint = "https://s3.\(region).amazonaws.com"

If region was set to us-east-1.attacker-controller, the endpoint could become funky, potentially confusing downstream parsers or specific resolver logic.

The Fix

The remediation involves a regex check or character set validation before the region is ever used. The logic now enforces that the region contains only alphanumeric characters and hyphens.

// The Fix: Strict Allow-listing
let validHostLabelRegex = /^[a-zA-Z0-9-]+$/
 
guard let region = config.region, 
      validHostLabelRegex.matches(region) else {
    throw ClientError.invalidConfiguration("Region contains invalid characters")
}
// Proceed to build endpoint

> [!NOTE] > By explicitly banning dots (.), the SDK prevents the region parameter from creating new subdomains or traversing the URL hierarchy. It forces the region to stay inside its designated box in the URL structure.

Let's put on our black hats. This vulnerability has a CVSS of 3.7 (Low) because it requires the attacker to control the configuration (e.g., AWS_REGION). You might say, "If I can change env vars, I already own the box!"

Not necessarily.

Consider a scenario in a CI/CD pipeline or a containerized environment where developers can inject environment variables but cannot access the filesystem or the actual code. Or consider an application that reads config from a remote, less-secure source.

The Attack Chain

1. Preparation: The attacker identifies a Swift application running inside a container. They have the ability to modify the deployment manifest or .env file.
2. Injection: They set the AWS_REGION variable to a malicious value.
   • Attempt: custom-region.evil.com
3. Execution: The application restarts. It attempts to connect to AWS STS (Security Token Service) to assume a role.
4. The Hook: The SDK constructs the URL. Depending on the exact endpoint ruleset triggering, the traffic is routed to a host the attacker controls, or a non-existent host that the attacker can register.
5. The Catch: The application sends a standard AWS HTTP request to the attacker's server. This request includes the Authorization header, signed with the application's actual AWS credentials (using SigV4).

Even though the attacker cannot decrypt the secret key from the signature, they have captured a valid, signed request. They can now replay this request to the actual AWS API (within the 15-minute timestamp window) to execute the action the app intended, or analyze the headers to fingerprint the infrastructure.

While this isn't a remote code execution (RCE) bug that will set the internet on fire, it represents a fundamental breakdown in trust boundaries.

1. Data Exfiltration: If the application sends data (e.g., uploading a file to S3) and the endpoint is hijacked, that data goes straight to the attacker.
2. Denial of Service: By setting an invalid region, an attacker can simply brick the application, preventing it from reaching critical cloud services.
3. SSRF Foundation: This is a stepping stone. In complex microservices, forcing an app to talk to 127.0.0.1 (if the region allows IP inputs, though the fix bans dots) could trigger internal admin panels. The fix banning dots (.) specifically highlights that preventing IP address usage or domain traversing was a primary goal.

It's a reminder that configuration data is untrusted input and should be treated with the same paranoia as user input.