How severe is CVE-2026-40611?

CVE-2026-40611 has a CVSS score of 8.8/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-40611?

CVE-2026-40611 affects the following packages: github.com/go-acme/lego/v4 (Go), github.com/go-acme/lego/v3 (Go), github.com/go-acme/lego (Go). Ecosystems affected: Go.

How do I fix CVE-2026-40611?

Update github.com/go-acme/lego/v4 to 4.34.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40611 is resolved across your whole dependency graph.

How do I detect CVE-2026-40611 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/go-acme/lego/v4. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2026-40611 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2026-40611?

O3 pinpoints whether CVE-2026-40611 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-40611 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-40611 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was CVE-2026-40611 published, and has it been updated?

CVE-2026-40611 was published on April 21, 2026 and was last updated on August 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

HIGH severity

CVE-2026-40611

HIGH

CVE-2026-40611 is a high-severity (CVSS 8.8) vulnerability in github.com/go-acme/lego/v4. O3 Security confirms whether CVE-2026-40611 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

Lego: Arbitrary File Write via Path Traversal in Webroot HTTP-01 Provider

Also known asGHSA-qqx8-2xmm-jrv8GO-2026-5593

Published

Apr 21, 2026

Updated

Aug 7, 2026

Affected

3 pkgs

Patched

1 / 3

Exploits

None indexed

Exploitation data as of Aug 7, 2026 · OSV.dev, FIRST.org (EPSS)

Real-World Exposure

3 pkgs affected

🐹github.com/go-acme/lego/v4🐹github.com/go-acme/lego/v3🐹github.com/go-acme/lego

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Let's Encrypt client and ACME library written in Go (Lego). Prior to 4.34.0, 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. This vulnerability is fixed in 4.34.0.

Affected Packages

3 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/go-acme/lego/v4`	all versions	4.34.0
🐹Go	`github.com/go-acme/lego/v3`	all versions	No fix
🐹Go	`github.com/go-acme/lego`	all versions	No fix

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/go-acme/lego/v4. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/go-acme/lego/v4 to 4.34.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40611 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-40611 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-40611. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2026-40611 in your dependencies?

O3 detects CVE-2026-40611 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works