Go Emergency Update Blocks Eight CA Keys Due to Undisclosed Security Concerns

Emergency Update Released

Go released emergency updates, versions 1.27.3 and 1.26.10, on October 20. These updates modify Go's certificate verifier to reject any certificate chain that includes one of eight specific intermediate certificate authority keys. The Go team stated that the keys belong to an unnamed certificate authority and were blocked due to "specific security concerns around their operation."

The update was handled on Go's URGENT track, indicating the severity of the issue, and immediate upgrading is recommended. Programs built with these new Go versions will refuse connections to sites and services whose certificates were issued under the blocked keys.

Connection to eMudhra Certificates

While Go did not name the certificate authority, a commenter on the Go issue identified all eight blocked keys as belonging to eMudhra intermediates by matching SHA-256 hashes. Six of these are TLS intermediates issued under the emSign Root TLS CA - G3, and two are code-signing intermediates under emSign Root CS CA - G3.

The identified TLS certificates include EM DV TLS CA - G3B, EM DV TLS CA - G3C, EM OV TLS CA - G3B, EM OV TLS CA - G3C, EM EV TLS CA - G3B, and EM EV TLS CA - G3C. The code-signing certificates are EM OV CS CA - G3B and EM OV CS CA - G3C.

Prior Mozilla Action

Two days prior to Go's update, on October 17, Mozilla added six intermediate certificates from eMudhra's emSign Root TLS CA - G3 to OneCRL, Firefox's list for blocking certificates. The Mozilla bug report related to this action is not publicly accessible, and neither Mozilla nor Go has disclosed the specific reasons for their respective actions. It is not confirmed whether other major browsers or operating systems like Chrome, Apple, or Microsoft have taken similar steps.

Technical Implementation

The filter in Go's update operates by hashing each certificate's public key, ensuring that cross-signed copies of the blocked keys are also caught. This change applies to Go's internal verifier and to macOS and Windows code paths that delegate verification to the operating system. A rejected chain will return the error "x509: no valid chains built."

The update does not include an option to disable this filter. The implementation also means the filter applies even when a program provides its own set of trusted roots.