Summary
When a Single Text File Breaks a Trust Boundary (Bug Bounty writeup) During a recent security review, I encountered an interesting vulnerability in an open-source tool that, at first glance, appeared to be implementing a completely legitimate feature. The feature was simple: allow users to place the application’s working directory inside a subdirectory of a larger repository. Nothing unusual there. However, after tracing the code path responsible for resolving that directory, I discovered that a single text file could completely redefine where the application believed its source data lived. The result was a path traversal condition that allowed the application to operate on arbitrary locations outside the intended repository boundary. The Feature The application supports a special file that allows users to relocate the effective source directory. The implementation looked conceptually similar to this: switch data, err := readFile(“.special-root-file”); { case fileNotFound: sourceDir = originalSourceDir default: sourceDir = originalSourceDir.Join( trimSpace(data), ) } At first glance, this seems harmless. The problem is that the value read from the file is joined directly to the original directory without any validation that the resulting path remains inside the repository. Since path joining functions typically normalize sequences such as: ../../../ the final path can end up pointing somewhere entirely different from what the developer originally intended. A Missing Validation Step Consider a file containing: ../../../../../../some/other/location After path normalization, the application no longer operates inside its repository. Instead, it treats the attacker-controlled destination as the new source directory. The crucial issue is not the traversal itself. The issue is that no validation occurs after the path is resolved. The application trusts the final path without verifying whether it remains inside the expected boundary. Building a Proof of Concept To validate the behavior, I created an isolated test environment. First, I prepared a directory outside the application’s normal working tree and placed a file containing sensitive-looking data: mkdir -p /tmp/source mkdir -p /tmp/outside echo “TOP-SECRET-CONTENT” > /tmp/outside/dot_leaked-secret Next, I configured the special root file: printf ‘../outside’ > /tmp/source/.special-root-file Once the application processed that file, every subsequent operation began interacting with the external directory rather than the original source directory. Source Directory Takeover When querying the effective source path, the application reported: /tmp/outside instead of: /tmp/source At that point, control of the source directory had effectively been transferred. File Enumeration Files located exclusively in the external directory became visible through the application’s normal management commands. The repository itself did not contain those files. Get julichaan’s stories in your inbox Join Medium for free to get updates from this writer. Nevertheless, the application considered them legitimate managed content. Arbitrary File Read Using standard functionality, it was possible to read the contents of files located in the external directory. The retrieved data matched the original content exactly. This demonstrated a local file disclosure primitive. Arbitrary File Write The application’s synchronization features could then copy that content into locations under the user’s profile. The vulnerability therefore extended beyond information disclosure and enabled unintended file creation based on attacker-controlled input. Why This Matters Many tools rely on an implicit trust model: “If content comes from this repository, it is safe to process.” That assumption breaks down when repository-controlled data can redefine where the repository itself lives. Once that trust boundary disappears, legitimate functionality begins operating on attacker-selected locations. Depending on the environment, this can expose:
- SSH keys
- API tokens
- Cloud credentials
- Browser data
- Development secrets
- Configuration files The exact impact depends on the victim’s system and the application’s available features, but the underlying issue remains the same: repository content should never be able to escape its intended scope without explicit validation. Root Cause Analysis The vulnerability exists because the implementation performs path resolution but not path containment verification. Conceptually, the workflow becomes: Repository │ ▼ Read relocation file │ ▼ Normalize path │ ▼ Trust result A secure implementation requires one additional step: Repository │ ▼ Read relocation file │ ▼ Normalize path │ ▼ Verify containment │ ▼ Accept or reject Without that containment check, any directory reachable through traversal sequences can become the application’s new source of truth. Recommended Fix After computing the final path, the application should verify that it remains within the original repository hierarchy. A common approach is to calculate the relative path between the original directory and the candidate directory and reject any result that escapes the boundary through .. components. This preserves the legitimate use case while preventing traversal outside the intended scope. Final Thoughts Security issues do not always emerge from complex exploitation chains. Sometimes they originate from small assumptions hidden inside convenience features. In this case, a single text file was enough to redefine an application’s trust boundary and redirect its operations toward arbitrary locations on the local filesystem. The code correctly handled path resolution. The code correctly supported nested directories. What it did not do was verify that the resolved destination still belonged to the place it was expected to trust. As is often the case, the vulnerability was not caused by what the code did, but by what it failed to verify. Certain technical details, repository information, identifiers, timelines, and program-specific context have been intentionally omitted or generalized in accordance with the disclosure requirements of the vulnerability disclosure program under which this research was performed.