CVE-2026-70666 Lemur: Server-Side Request Forgery via the ACME client following server-controlled URLs
An attacker with authority-role access can redirect ACME client requests to internal services or cloud metadata endpoints via server-controlled URLs.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-70666?
CVE-2026-70666 is a high-severity server-side request forgery vulnerability in Netflix lemur, published on 18 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Lemur manages TLS certificate creation. Prior to 1.9.3, an authority-role member could update acme_url through PUT /api/1/authorities/ without revalidation and direct setup_acme_client_no_retry to an attacker-controlled ACME server. ACME directory and order responses contain newNonce, newOrder, authorizations, and finalize URLs chosen by that server. The Lemur ClientV2 followed those URLs without requiring their host to match the configured directory host, allowing JWS-signed requests to internal services or cloud metadata endpoints. The issue required an ACME authority and a user authorized for that authority, but did not require global administrator privileges. The fix revalidates updates and introduces _PinnedClientNetwork to enforce a single allowed host for the complete ACME flow. This issue is fixed in version 1.9.3.
Weakness: CWE-918
Which versions are affected by CVE-2026-70666?
3 open source packages ship an affected version, including lemur.
| Package | Affected | Fixed in |
|---|---|---|
lemur PyPI | All versions | 1.9.3 |
lemur PyPI | All versions | 1.9.3 |
lemur PyPI | All versions | 1.9.3 |
How do I fix CVE-2026-70666?
Upgrade lemur to 1.9.3 or later.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-70666 being exploited?
There is no known exploitation of CVE-2026-70666 in the wild and no public exploit code.
Sources
Also known as GHSA-xpmj-wjcp-6pww, PYSEC-2026-3680.