Executive brief
A performance issue was identified in the Go programming language's security library used for validating digital certificates. An attacker could provide a specially crafted certificate or hostname that causes the system to spend an excessive amount of processing power during the validation process. This could lead to a denial-of-service (DoS) condition where the application becomes slow or unresponsive while trying to verify untrusted connections.
Technical details
The (*x509.Certificate).VerifyHostname function in Go's crypto/x509 package previously implemented hostname matching by calling matchHostnames in a loop over all DNS Subject Alternative Name (SAN) entries. This caused the strings.Split(host, ".") operation to execute repeatedly on the same input hostname, leading to quadratic resource consumption (O(n*m) where n is the number of SANs and m is the label count of the hostname). Because hostname validation occurs before certificate chain building in x509.Verify, this overhead is incurred even for untrusted or malicious certificates. Attackers can exploit this to cause high CPU usage on clients or servers performing certificate verification. The issue is fixed in Go 1.25.11 and 1.26.4.
Affected products
- Google Go before 1.25.11, 1.26.0 before 1.26.4
Timeline
- 2026-05-27: disclosed: Issue reported to Go project by Jakub Ciolek
- 2026-06-02: advisory: NVD and Go vulnerability report published