Callback-url-file-3a-2f-2f-2fhome-2f-2a-2f.aws-2fcredentials [updated] Jun 2026

I’ve been looking into how common "callback URL" parameters can be weaponized to exfiltrate sensitive cloud metadata. A common payload I'm seeing in logs looks like this: ?callbackUrl=file:///home/*/.aws/credentials 🔍 What is happening? Attackers use the

This is essentially a pointing to a wildcard path inside a user’s home directory, targeting the standard AWS credentials file. The * (asterisk) is particularly interesting — it may be used as a wildcard in poorly implemented callback handlers to match multiple user directories or as a path traversal technique.

If a web application accepts a callback URL from a user and uses its own backend permissions to fetch that URL, an attacker can manipulate the request. By swapping a valid web URL (e.g., https://example.com ) with a file:// URI scheme, the attacker tricks the hosting server into reading its own local operating system files. 2. The Cloud Metadata and Credential Harvest

from urllib.request import build_opener, HTTPHandler, HTTPSHandler opener = build_opener(HTTPHandler, HTTPSHandler) # Now opener will not handle file:// callback-url-file-3A-2F-2F-2Fhome-2F-2A-2F.aws-2Fcredentials

Remember:

: Access to S3 buckets, databases, and other services often follows credential theft. Persistence

While many security tools block access to the Instance Metadata Service (IMDS) at 169.254.169.254 , they often forget to block the file:// scheme, making this a common "plan B" for attackers. How to Protect Your Application I’ve been looking into how common "callback URL"

Rachel was both impressed and concerned. "Impressive, but also a bit reckless, don't you think? I mean, we're talking about sensitive credentials here."

: Never allow arbitrary callback URLs. Maintain a strict whitelist of approved domains in your application settings. code snippet

This specific pattern typically emerges in attacks or Open Redirect exploitation attempts. It occurs when an application improperly accepts local file system URIs within its OAuth, webhook, or callback validation mechanisms. The * (asterisk) is particularly interesting — it

When the application attempts to process or echo the callback address, it treats the file:// protocol as an active system directive. Instead of redirecting a user's browser, the back-end application server reads the contents of the target system file into memory. 3. Plaintext Credential Exfiltration

: A parameter often used in OAuth, webhooks, or image-fetching services.

: The file:// URI scheme is a local handling mechanism. Instead of pointing to a remote web server over http:// or https:// , it tells the application engine or system parser to fetch a resource stored directly on the host machine's local file system.