CVE-2026-78659

When Trailer headers are sent by a client, the HTTP server internally uses the header values to populate the Request.Trailer map passed to the server handler. Because Request.Trailer is a map, each entry incurs memory overhead. For HTTP/2 servers, a malicious client can exploit this by sending a Trailer header that declares a large number of fields, causing the server to allocate a disproportionate amount of memory while bypassing Server.MaxHeaderValueCount and Server.MaxHeaderBytes limits. This exploit is not applicable for HTTP/1 servers, which do not support multiplexing a large number of requests over one TCP connection, and whose Server.MaxHeaderBytes are calculated differently.

Weakness

The product does not properly control situations in which an adversary can cause the product to consume or produce excessive resources without requiring the adversary to invest equivalent work or otherwise prove authorization, i.e., the adversary’s influence is “asymmetric.”

Affected Software

Potential Mitigations

References

• https://go.dev/cl/847185
• https://go.dev/cl/847314
• https://go.dev/issue/81857
• https://groups.google.com/g/golang-announce/c/U2fTuyDJznI
• https://groups.google.com/g/golang-announce/c/ZPwCyRUuGBs
• https://pkg.go.dev/vuln/GO-2026-6603