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CVE Record

CVE-2026-40611: Lego: Arbitrary File Write via Path Traversal in Webroot HTTP-01 Provider

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.

HighCVSS 8.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

Lego versions before 4.34.0 can be tricked by a malicious ACME server into writing or deleting files outside the intended webroot. The business risk is highest where lego runs with broad filesystem permissions or uses an untrusted or compromised ACME endpoint.

Executive priority

Prioritize remediation for certificate automation hosts, especially those running lego with write access beyond the webroot. Treat this as high urgency where privileged service accounts or shared infrastructure are involved.

Technical view

The webroot HTTP-01 challenge provider fails to safely constrain challenge token paths. A crafted token containing path traversal sequences can cause attacker-influenced file writes and deletion to locations writable by the lego process. The issue is classified as CWE-22 and fixed in lego 4.34.0.

Likely exposure

Organizations using go-acme/lego before 4.34.0 with the webroot HTTP-01 provider are the primary exposure group. Risk depends on lego process permissions, configured webroot paths, and whether the ACME server trust boundary could be abused.

Exploitation context

The bundle does not show CISA KEV listing or cited evidence of active exploitation. Exploitation requires lego to process a malicious challenge token from an ACME server, so exposure is more specific than a generic internet-facing bug but can still cause serious host impact.

Researcher notes

Evidence supports path traversal leading to arbitrary file write and deletion in lego before 4.34.0. The source bundle does not provide exploit proof, active exploitation evidence, or complete downstream package applicability details beyond Red Hat references.

Mitigation direction

  • Upgrade go-acme/lego to version 4.34.0 or later.
  • Apply relevant downstream vendor updates, including applicable Red Hat errata.
  • Restrict lego to the minimum filesystem permissions needed.
  • Use only trusted ACME servers and review ACME endpoint configuration.
  • Check vendor guidance before applying product-specific workarounds.

Validation and detection

  • Inventory systems and applications using go-acme/lego.
  • Confirm deployed lego versions are 4.34.0 or later.
  • Identify use of the webroot HTTP-01 challenge provider.
  • Review lego process user permissions and writable paths.
  • Assess whether any downstream Red Hat packages are affected.
Prepared
Confidence
high
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · medium confidence lookup

CWE-22: File access and web shell behavior lookup

File traversal and upload weaknesses can lead teams to review file, web shell, execution, and collection telemetry. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
description · low confidence lookup

Credential and access behavior lookup

The CVE wording references authentication or credential exposure, so valid-account and credential-access review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

Open ATT&CK lookup
description · low confidence lookup

File access behavior lookup

The CVE wording references file access or upload behavior, so file telemetry and web shell review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2026-40611 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
8.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
5Timeline events
2ADP providers
6Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: pocAutomatable: noTechnical Impact: total

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
8.8CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H2.85.9GitHub_M
8.8CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H2.85.9redhat-SADP

Vulnerability scoring details

Base CVSS 3.1 score

8.8High
CVSS 3.1 vector shape for CVE-2026-40611Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPgithub.com/go-acme/lego: Lego: Arbitrary file write and deletion via path traversal from a malicious ACME server
other:Red Hat severity ratingcvssV3_1
  • 2026-04-21T20:01:57.383Z: Reported to Red Hat.
  • 2026-04-21T17:58:35.221Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
go-acmelego< 4.34.0Listed
Weakness

CWE details

CWE-22

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-22 · source CWE mapping

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs