And that's a story about setting up AWS configuration files and learning about best practices along the way!
This specific string, fetch-url-file-3A-2F-2F-2Froot-2F.aws-2Fconfig, is a high-risk security payload typically used to test for Server-Side Request Forgery (SSRF) vulnerabilities. If a web application is vulnerable, an attacker can use this string to trick the server into reading its own internal configuration files—in this case, the AWS root user's CLI configuration.
Below is a draft for a technical blog post exploring how this payload works, what it targets, and how to defend against it.
The Anatomy of a Cloud Attack: Deconstructing the "fetch-url-file" SSRF Payload
In modern cloud security, small strings can carry massive risks. One such string that frequently appears in bug bounty reports and security logs is:fetch-url-file-3A-2F-2F-2Froot-2F.aws-2Fconfig
At first glance, it looks like a garbled URL. In reality, it is a surgical tool designed to extract the "crown jewels" of an AWS environment: the root user's configuration. What is this payload doing?
This payload is a URL-encoded instruction used in Server-Side Request Forgery (SSRF) attacks. Let's break it down: fetch-url-file-3A-2F-2F-2Froot-2F.aws-2Fconfig
fetch-url: This often refers to a vulnerable parameter in a web application (e.g., a "preview" feature or an "image fetcher") that accepts a URL and makes a request on the user's behalf.
file:///: This is a URI scheme used to access files on the local machine rather than resources on the internet. The 3A-2F-2F-2F is the URL-encoded version of :///.
/root/.aws/config: This is the specific target. It points to the configuration file for the AWS Command Line Interface (CLI) for the root user. Why is /root/.aws/config a target?
On an AWS EC2 instance, the .aws directory typically contains two critical files:
config: Stores configuration settings like default regions and output formats.
credentials: Stores the actual Access Keys and Secret Access Keys. And that's a story about setting up AWS
Attackers target the config file first to confirm they can read files from the system. If they can read config, they can likely read credentials. If those keys belong to a highly privileged user or the root account, the attacker can gain full control over the entire AWS environment. How the Attack Works
Discovery: An attacker finds a feature that fetches content (e.g., https://example.com...).
Payload Injection: The attacker replaces the legitimate URL with the malicious payload:https://example.com
Execution: If the application doesn't validate the "url" input, the server's backend will follow the instruction, read the local file from its own disk, and return the contents to the attacker. How to Protect Your Infrastructure
To prevent this kind of data leakage, developers and DevOps teams should implement these layers of defense:
The string is URL-encoded and partially obfuscated. Let's break it down: Moreover, even if the config file only references
| Encoded Segment | Decoded Value |
| --- | --- |
| file-3A | file: (The colon : is encoded as %3A) |
| 2F | / |
| 2F | / |
| 2F | / |
| root | root |
| 2F | / |
| .aws | .aws |
| 2F | / |
| config | config |
Final decoded result:
file:///root/.aws/config
When you use the AWS CLI, your configuration settings are stored in a file located at ~/.aws/config on Linux, macOS, or Unix, and at %USERPROFILE%\.aws\config on Windows. This configuration file is crucial for specifying your AWS credentials, default region, and other settings that the AWS CLI needs to interact with AWS services.
The /root/.aws/config file itself might not always contain secrets—but in many real-world misconfigurations, administrators store credentials directly in the config file using the following syntax:
[default]
aws_access_key_id = AKIAIOSFODNN7EXAMPLE
aws_secret_access_key = wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
region = us-east-1
Moreover, even if the config file only references a profile, it almost always coexists with /root/.aws/credentials. An attacker who can read /root/.aws/config can often guess or traverse to /root/.aws/credentials.
In 2020, multiple misconfigured Jupyter Notebooks exposed file:///root/.aws/credentials via public endpoints, leading to account takeovers within hours.
The AWS CLI allows you to create multiple profiles for different AWS accounts or roles. You can specify profiles in the config file like this:
[profile dev]
aws_access_key_id = YOUR_DEV_ACCESS_KEY
aws_secret_access_key = YOUR_DEV_SECRET_KEY
region = us-east-1
[profile prod]
aws_access_key_id = YOUR_PROD_ACCESS_KEY
aws_secret_access_key = YOUR_PROD_SECRET_KEY
region = us-west-2
To use a profile, you can specify it in your AWS CLI commands with the --profile option:
aws --profile dev s3 ls