CVE-2026-40280: Gotenberg SSRF via case-insensitive URL scheme bypass in webhook and downloadFrom deny-lists

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.9% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-05-05 · updated 2026-07-24
Weakness: CWE-918

Description

Gotenberg is an API-based document conversion tool. In versions 8.30.1 and earlier, the default private-IP deny-lists for the --webhook-deny-list and --api-download-from-deny-list flags use a case-sensitive regular expression (^https?://) to match URL schemes. Because Go's net/url.Parse() normalizes the scheme to lowercase before establishing the outbound TCP connection, an attacker can bypass the deny-list by simply capitalizing part of the URL scheme (e.g., HTTP://, HTTPS://, or Http://). This allows unauthenticated requests to reach internal network services, including private IP ranges, loopback addresses, and cloud instance metadata endpoints such as HTTP://169.254.169.254/latest/meta-data/. This bypasses the same security control that was patched in CVE-2026-27018. This issue has been fixed in version 8.31.0.

Affected products

gotenberg: before 8.31.0 — fixed in 8.31.0

How to fix

Upgrade to a fixed version: gotenberg 8.31.0. Check the vendor advisory for mitigations.

References

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