CVE-2026-56866 Go http.Transport - HTTP Request Smuggling

Go http.Transport contains a cross-user response poisoning vulnerability caused by improper handling of HTTP/1 CONNECT requests with non-empty bodies, letting attackers cause connection desynchronization and response injection, exploit requires use of shared Transport in reverse proxies.

CVSS base score 7.5 /10
High
CISA KEV
Not listed
Public exploit
None known
Authentication
Not required
EPSS
0.31% chance of exploitation in 30 days
View the NVD record ↗

Details

Type
HTTP Request Smuggling
Impact
Data Tampering, Information Disclosure
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Published
8 October 2026
Updated
15 October 2026
Weakness
CWE-444

Vendor fix

Update to the latest Go version with the fix for CONNECT request body handling.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

References

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