HTTP/2 State Amplification Vulnerability in H2O Server by H2O
CVE-2026-54340

7.5HIGH

Key Information:

Vendor

H2o

Status
H2o
Vendor
CVE Published:
16 July 2026

What is CVE-2026-54340?

An HTTP/2 state amplification vulnerability exists in the H2O web server, where HPACK decompression can be exploited in conjunction with Slowloris-style stream stalling. This issue allows for potential retention of amplified decoded header state by stalled streams, requiring additional configuration limits to mitigate risks. The vulnerability has been addressed in the latest updates. It is crucial for users of H2O Server to ensure they are running the patched version to safeguard against these types of amplification attacks.

Affected Version(s)

h2o < 9265bdd

References

https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-...
x_refsource_CONFIRM
https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055...
x_refsource_MISC

CVSS V3.1

Score:
7.5
Severity:
HIGH
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.