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.
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Solution

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History

Fri, 09 Oct 2026 09:15:00 +0000

Type Values Removed Values Added
First Time appeared Go Standard Library
Go Standard Library net/http
Go Standard Library net/http2
Vendors & Products Go Standard Library
Go Standard Library net/http
Go Standard Library net/http2

Fri, 09 Oct 2026 01:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-399
CWE-770

Thu, 08 Oct 2026 23:00:00 +0000

Type Values Removed Values Added
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.
Title HTTP/2 server memory exhaustion due to Trailer headers in net/http
References

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cve-icon MITRE

Status: PUBLISHED

Assigner: Go

Published:

Updated: 2026-10-08T22:53:58.192Z

Reserved: 2026-08-24T23:36:03.109Z

Link: CVE-2026-78659

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-10-08T23:17:03.270

Modified: 2026-10-08T23:17:03.270

Link: CVE-2026-78659

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-10-09T08:02:17Z

Weaknesses