Lemur: Server-Side Request Forgery via the ACME client following server-controlled URLs

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.4 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:L

EPSS

• CVE-2026-70666: 0.22% estimated exploitation activity in the next 30 days; percentile 12.10 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The ACME client (used to issue certificates from Let's Encrypt / Google Public CA / private ACME CAs) connects to an acmeurl, then issues requests to URLs that the ACME server returns in its directory/order/authorization/finalize responses - this is the classic ACME-client SSRF (RFC 8555 design). Lemur validates acmeurl against an allowlist of public ACME directories, but only at authority creation. The authority UPDATE path (PUT /authorities/<id) accepts a new options blob with an arbitrary acmeurl and never re-validates. An attacker who is a member of an authority's role can repoint an existing ACME authority at a malicious ACME server they control, which returns internal URLs in its responses - coercing Lemur into making JWS-signed POST requests to internal services during the next certificate issuance. Detail Defect A - allowlist only at creation. validateacmeurl (lemur/plugins/lemuracme/plugin.py:35-53) restricts the host to {acme-v02.api.letsencrypt.org,…

Published by the source

18 Aug 2026, 20:51 UTC

Last modified by the source

18 Aug 2026, 20:51 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-xpmj-wjcp-6pww

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# Patch Next advisory brief: CVE-2026-70666

Advisory: Lemur: Server-Side Request Forgery via the ACME client following server-controlled URLs
IDs: CVE-2026-70666, GHSA-xpmj-wjcp-6pww
Record: https://patchnext.com/advisories/cve-2026-70666 (revision 1, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: lemur 1.9.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI lemur: affected <= 1.9.2; fixed in 1.9.3

## Suggested inspection (read-only)
- `python -m pip show lemur`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on lemur.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-xpmj-wjcp-6pww (GitHub-reviewed advisory GHSA-xpmj-wjcp-6pww lists affected ranges: PyPI lemur >=0 <=1.9.2)

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