CVE-2026-54340 | h2o has HTTP/2 state amplification

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9265bdd.

Published: 2026-07-16 Last update: 2026-07-17 Assigner: [email protected] Source: [email protected]
NVD Status：Undergoing Analysis，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-54340 is rated Low Risk (36.9/100): CVSS High severity, with low exploitation likelihood (EPSS 0.28%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-54340

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-07-17 — 0.28% —

Full EPSS history (1 record total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-54340

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2026-54340

OS Trackers for CVE-2026-54340

vendor priority summary link
debian not yet assigned CVE-2026-54340 not yet assigned priority: Debian including 1 source packages (h2o), 2 status rows across 2 suites (bookworm, bullseye): open 2. https://security-tracker.debian.org/tracker/CVE-2026-54340
ubuntu medium CVE-2026-54340 medium priority: Ubuntu including 2 source packages (dnsdist, h2o), 12 status rows across 6 suites (bionic, focal, jammy, noble, resolute, upstream): needs-triage 7, not-affected 4, DNE 1. https://ubuntu.com/security/CVE-2026-54340

Affected software / configurations for CVE-2026-54340

Vendor Product Version Raw CPE
No affected products in dataset.

References for CVE-2026-54340

cvelogic Threat Intelligence