Response smuggling via malformed framing headers
Published Oct 8, 2026 · Updated Oct 8, 2026
Response smuggling in Go standard library net/http allows remote attackers to inject HTTP/1 responses through HTTP/2 proxying. The HTTP/2 transport accepts malformed framing headers, including multiple Content-Length headers, and preserves them for forwarding by a reverse proxy; golang.org/x/net/http2 and net/http/internal/http2 share the flaw. Exploitation requires attacker-controlled upstream responses and an HTTP/1 client that interprets the forwarded headers permissively, causing response-boundary confusion rather than a documented code-execution primitive.
Summary
What happened
Response smuggling in Go standard library net/http allows remote attackers to inject HTTP/1 responses through HTTP/2 proxying. The HTTP/2 transport accepts malformed framing headers, including multiple Content-Length headers, and preserves them for forwarding by a reverse proxy; golang.org/x/net/http2 and net/http/internal/http2 share the flaw. Exploitation requires attacker-controlled upstream responses and an HTTP/1 client that interprets the forwarded headers permissively, causing response-boundary confusion rather than a documented code-execution primitive.
The record
- CVE
- CVE-2026-78660
- Published
- Oct 8, 2026
- Updated
- Oct 8, 2026
- Vendor
- Go standard library
- Product
- net/http
- Classifications
- CWE-444
- Attack vector
- network
- Privileges
- Unavailable
Timeline
How it unfolded
- Oct 8, 2026CVE publishedPublication date reported by the CVE source.
- Oct 8, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=0 <1.26.9
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Activity data is delayed. The latest report is Sep 28, 2026.
Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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