CVE catalog

CVE-2026-44453

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 6b5370d, h2o is vulnerable to a Denial of Service attack when calling alloca under certain conditions. When serving

[CVE catalog]

Severity: High · CVSS 7.5
Published: Jul 16, 2026, 11:16:00 PM UTC
Source: [NVD] · [CVE record]
EPSS: 0.49% · Oct 25, 2026

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 6b5370d, h2o is vulnerable to a Denial of Service attack when calling alloca under certain conditions. When serving static files, h2o builds the file path on stack, by calling alloca. The maximum size of the memory allocated using alloca can be as huge as ~600KB, which exceeds the default pthread stack size used by musl libc (128KB). If the amount of memory allocated by alloca exceeds the stack size, the h2o server crashes with a segmentation fault, while it tries to touch the guard page. This issue has been fixed by commit 6b5370d.

Vulnerable and fixed packages

Distribution-reported package versions and fixes. These do not depend on matching against the OpenFactory package catalog.

Distribution	Package	Release	Version	Status
Debian	h2o	bookworm	`2.2.5+dfsg2-7`	vulnerable
Cpe	h2o	Not reported	Not reported	vulnerable
Ubuntu	h2o	pro	`2.2.5+dfsg2-6`	vulnerable
Ubuntu	h2o	pro	`2.2.5+dfsg2-6.1`	vulnerable
Ubuntu	h2o	pro	`2.2.5+dfsg2-6.1ubuntu1`	vulnerable
Ubuntu	h2o	pro	`2.2.5+dfsg2-6.1ubuntu2`	vulnerable
Ubuntu	h2o	pro	`2.2.5+dfsg2-6.1ubuntu2+esm1`	vulnerable
Ubuntu	h2o	24.04	`2.2.5+dfsg2-7`	vulnerable
Ubuntu	h2o	24.04	`2.2.5+dfsg2-8`	vulnerable
Ubuntu	h2o	24.04	`2.2.5+dfsg2-8.1ubuntu1`	vulnerable
Ubuntu	h2o	24.04	`2.2.5+dfsg2-8.1ubuntu2`	vulnerable
Ubuntu	h2o	24.04	`2.2.5+dfsg2-8.1ubuntu3`	vulnerable
Ubuntu	dnsdist	pro	`1.8.0-1`	vulnerable
Ubuntu	dnsdist	pro	`1.8.2-1`	vulnerable
Ubuntu	dnsdist	pro	`1.8.2-2`	vulnerable
Ubuntu	dnsdist	pro	`1.8.2-3`	vulnerable
Ubuntu	dnsdist	pro	`1.8.3-1`	vulnerable
Ubuntu	dnsdist	pro	`1.8.3-2`	vulnerable
Ubuntu	dnsdist	pro	`1.8.3-2build1`	vulnerable
Ubuntu	dnsdist	pro	`1.8.3-2build2`	vulnerable
Ubuntu	dnsdist	pro	`1.8.3-2ubuntu0.1~esm1`	vulnerable

Fixed-version evidence

Distribution	Package	Type	Release	Fixed version	Urgency	Origin	Bugs
Debian	h2o	source	bookworm	`(unfixed)`	not yet assigned	Not reported	Not reported

Notes

[bookworm] h2o: Minor issue, DoS; the alloca() in lib/handler/file.c on_req() is present, but the crash premise is musl's 128KB default pthread stack, and h2o passes no pthread attr so glibc worker threads get the 8MB default

OpenFactory catalog packages

[Debian 12 (Bookworm)]

[h2o]Affected
Details (2)
- 2.2.5+dfsg2-7 · amd64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
- 2.2.5+dfsg2-7 · arm64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
[h2o-doc]Affected
Source package: [h2o]
Details (2)
- 2.2.5+dfsg2-7 · all · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
- 2.2.5+dfsg2-7 · all · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
[libh2o0.13]Affected
Source package: [h2o]
Details (2)
- 2.2.5+dfsg2-7 · amd64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
- 2.2.5+dfsg2-7 · arm64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
[libh2o-dev]Affected
Source package: [h2o]
Details (2)
- 2.2.5+dfsg2-7 · amd64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
- 2.2.5+dfsg2-7 · arm64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
[libh2o-dev-common]Affected
Source package: [h2o]
Details (2)
- 2.2.5+dfsg2-7 · all · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
- 2.2.5+dfsg2-7 · all · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
[libh2o-evloop0.13]Affected
Source package: [h2o]
Details (2)
- 2.2.5+dfsg2-7 · amd64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
- 2.2.5+dfsg2-7 · arm64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
[libh2o-evloop-dev]Affected
Source package: [h2o]
Details (2)
- 2.2.5+dfsg2-7 · amd64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12
- 2.2.5+dfsg2-7 · arm64 · bookworm
  Affected
  nvd · h2o · debian · bookworm; nvd · h2o · debian · 12

Observed: Oct 27, 2026, 7:12:53 AM UTC

References

[Debian Security Tracker] · [FIRST EPSS] · [National Vulnerability Database (NVD)] · [OSV Package Vulnerability Database]