CVE-2026-58055

nghttp2's nghttpx proxy through 1.69.0 forwards an HTTP/1.1 Upgrade request that also carries a Content-Length header and body onto reusable keep-alive backend connections, re-adding the Upgrade and Connection headers while passing Content-Length verbatim. A backend that resolves the resulting ambiguous message in the attacker's favor enables HTTP request/response smuggling and cross-client response-queue poisoning.

CVSS v4.0

6.3 MEDIUMCVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS v3.1

5.4 MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N

EPSS

0.25% chance of exploitation in the next 30 days, 17th percentile

Published

2026-06-28

Updated

2026-06-29

Proof-of-concept exploits (1)

• bikini/exploitarium/tree/main/nghttp2-nghttpx-upgrade-queue-poison-poc

References

• CVE Record
• NVD entry

Related

• CVE-2026-58049
• CVE-2026-58050
• CVE-2026-58051
• CVE-2026-58052
• CVE-2026-58053
• CVE-2026-58056