When OpenTofu is acting as a TLS client authenticating a certificate chain provided by a TLS server, an excluded subdomain constraint in a certificate chain does not restrict the usage of wildcard SANs in the leaf certificate.
For example a constraint that excludes the subdomain test.example.com does not prevent a leaf certificate from claiming the SAN *.example.com.
Details
When acting as a TLS client, OpenTofu relies on the implementation of TLS certificate verification from the standard library of the Go programming language.
The Go project has recently published the following advisory for that which indirectly affects OpenTofu's behavior:
- CVE-2025-61727: Improper application of excluded DNS name constraints when verifying wildcard names in crypto/x509
OpenTofu acts as a TLS client when calling a module or provider registry to request metadata, when downloading module or provider packages from "https" URLs, and when interacting with remote services for state storage and encryption. In these situations, OpenTofu could potentially accept as valid a certificate chain containing conflicting information about whether it is valid for the target hostname.
All certificates in the chain are still checked separately for validity, and so a successful attack requires an attacker-controlled server to produce a chain of valid-but-contradictory certificates and have access to the private keys associated with each one, and for the attacker to then arrange for OpenTofu to attempt to connect to the affected hostname.
Patches
OpenTofu v1.10.8 addresses these vulnerabilities by being built against Go 1.24.11, which contains an improved version of the upstream implementation.
The OpenTofu v1.9 and v1.8 series are also affected by these vulnerabilities. However, those series are built with a version of Go for which no upstream fix is available. Adopting Go 1.24.11 for those series would effectively end support for certain versions of macOS and Linux, and the OpenTofu Project has determined that the impact of these vulnerabilities is not high enough to justify that disruption in a patch release. For those using the OpenTofu v1.9 or v1.8 releases we recommend planning to upgrade to OpenTofu v1.10.8 in the near future.
References
When OpenTofu is acting as a TLS client authenticating a certificate chain provided by a TLS server, an excluded subdomain constraint in a certificate chain does not restrict the usage of wildcard SANs in the leaf certificate.
For example a constraint that excludes the subdomain test.example.com does not prevent a leaf certificate from claiming the SAN *.example.com.
Details
When acting as a TLS client, OpenTofu relies on the implementation of TLS certificate verification from the standard library of the Go programming language.
The Go project has recently published the following advisory for that which indirectly affects OpenTofu's behavior:
OpenTofu acts as a TLS client when calling a module or provider registry to request metadata, when downloading module or provider packages from "https" URLs, and when interacting with remote services for state storage and encryption. In these situations, OpenTofu could potentially accept as valid a certificate chain containing conflicting information about whether it is valid for the target hostname.
All certificates in the chain are still checked separately for validity, and so a successful attack requires an attacker-controlled server to produce a chain of valid-but-contradictory certificates and have access to the private keys associated with each one, and for the attacker to then arrange for OpenTofu to attempt to connect to the affected hostname.
Patches
OpenTofu v1.10.8 addresses these vulnerabilities by being built against Go 1.24.11, which contains an improved version of the upstream implementation.
The OpenTofu v1.9 and v1.8 series are also affected by these vulnerabilities. However, those series are built with a version of Go for which no upstream fix is available. Adopting Go 1.24.11 for those series would effectively end support for certain versions of macOS and Linux, and the OpenTofu Project has determined that the impact of these vulnerabilities is not high enough to justify that disruption in a patch release. For those using the OpenTofu v1.9 or v1.8 releases we recommend planning to upgrade to OpenTofu v1.10.8 in the near future.
References