CVE-2026-78659
HIGHDescription
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.
How to fix
No published remediation has been found for this vulnerability's affected products yet.
Mitigation guidance may be in the linked vendor advisories in the References section below.
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CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Exploit Intelligence
Low risk: more likely to be exploited than 12% of all known CVEs.
References
Related Vulnerabilities
Other CWE-405 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2019-11479 | High | 7.5 | 92% | - | Fix |
| CVE-2024-11187 | High | 7.5 | 17% | - | Fix |
| CVE-2025-8677 | High | 7.5 | 11% | - | Fix |
| CVE-2021-38447 | High | 8.6 | 2.1% | - | Fix |
| CVE-2021-21359 | Medium | 5.9 | 1.7% | - | Fix |
| CVE-2018-15492 | High | 7.5 | 1.2% | - | - |
Common questions
How do I fix CVE-2026-78659?
No published fix for CVE-2026-78659 has been found yet. Check the linked vendor advisories for guidance as it becomes available.
Is CVE-2026-78659 being actively exploited?
Not that we know of. CVE-2026-78659 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.23% is the estimated probability that it will be exploited in the next 30 days. That is higher than 12% of all scored CVEs.
How severe is CVE-2026-78659?
CVE-2026-78659 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-10-09.