CVE-2024-45403 [Low] | Security Vulnerability in H2o

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. When h2o is configured as a reverse proxy and HTTP/3 requests are cancelled by the client, h2o might crash due to an assertion failure. The crash can be exploited by an attacker to mount a Denial-of-Service attack. By default, the h2o standalone server automatically restarts, minimizing the impact. However, HTTP requests that were served concurrently will still be disrupted. The vulnerability has been addressed in commit 1ed32b2. Users may disable the use of HTTP/3 to mitigate the issue.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2025-11-21	0.82%	0.33%	-0.49%
2	2025-11-18	0.53%	0.82%	+0.30%
3	2025-08-07	—	0.53%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2025-11-21

0.82%

0.33%

-0.49%

2025-11-18

0.53%

0.82%

+0.30%

2025-08-07

—

0.53%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
3.7	3.1	LOW	`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L` 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:H) Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work. 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:L) Might cause slowdowns, glitches, or partial disruption—not a full brick.	2.2	1.4	[email protected]
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]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

3.7

3.1

LOW

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L 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:H): Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
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:L): Might cause slowdowns, glitches, or partial disruption—not a full brick.

2.2

1.4

[email protected]

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]

OS Trackers for CVE-2024-45403

vendor	priority	summary	link
`debian`	unimportant	CVE-2024-45403 unimportant priority: Debian including 1 source packages (h2o), 2 status rows across 2 suites (bookworm, bullseye): resolved 2.	https://security-tracker.debian.org/tracker/CVE-2024-45403
`ubuntu`	medium	CVE-2024-45403 medium priority: Ubuntu including 1 source packages (h2o), 8 status rows across 8 suites (bionic, focal, jammy, noble, oracular, plucky, questing, upstream): needs-triage 5, ignored 2, DNE 1.	https://ubuntu.com/security/CVE-2024-45403

Affected software / configurations for CVE-2024-45403

Vendor	Product	Version	Raw CPE
dena	h2o	>= 2024-06-18, < 2024-09-04	cpe:2.3:a:dena:h2o::::::::

References for CVE-2024-45403

URL	Tags
https://github.com/h2o/h2o/commit/16b13eee8ad7895b4fe3fcbcabee53bd52782562	Patch
https://github.com/h2o/h2o/commit/1ed32b23f999acf0c5029f09c8525f93eb1d354c	Patch
https://github.com/h2o/h2o/security/advisories/GHSA-4xp5-3jhc-3m92	Vendor Advisory
https://h2o.examp1e.net/configure/http3_directives.html	Product

URL

CVE-2024-45403 | H2O assertion failure when HTTP/3 requests are cancelled

Exploit prediction scoring system (EPSS) score for CVE-2024-45403

Common vulnerability scoring system (CVSS) metrics for CVE-2024-45403

Weakness enumeration for CVE-2024-45403

OS Trackers for CVE-2024-45403

Affected software / configurations for CVE-2024-45403

References for CVE-2024-45403