HTTP/2 server memory exhaustion due to Trailer headers in net/http

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.

Source published

8 Oct 2026 22:53:58 UTC

Source modified

9 Oct 2026 15:34:24 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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