| Name | CVE-2026-44453 |
| Description | 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. |
| Source | CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more) |
Vulnerable and fixed packages
The table below lists information on source packages.
| Source Package | Release | Version | Status |
|---|
| h2o (PTS) | bullseye | 2.2.5+dfsg2-6 | vulnerable |
| bookworm | 2.2.5+dfsg2-7 | vulnerable |
The information below is based on the following data on fixed versions.
| Package | Type | Release | Fixed Version | Urgency | Origin | Debian Bugs |
|---|
| h2o | source | (unstable) | (unfixed) | | | |
Notes
[bookworm] - h2o <postponed> (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)
[bullseye] - h2o <postponed> (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)
https://github.com/h2o/h2o/security/advisories/GHSA-rf9v-m59p-mq84
Fixed by: https://github.com/h2o/h2o/commit/6b5370d9d09fcf83aa7620ddf77de1954a192181