CVE-2026-78659

Description

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.

CVSS breakdown

Affected products

• golang.org/x/net / golang.org/x/net/http20 – 0.60.0
• Go standard library / net/http0 – 1.26.9
• Go standard library / net/http/internal/http21.27.0-0 – 1.27.2

References

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