CVE-2024-25622

What it actually does

The headers directive in inner configuration scopes overrides and ignores all header configurations from outer scopes, preventing expected header modifications from being applied.

Why it matters to the business

Unexpected response headers may be sent or omitted, potentially causing client applications to behave incorrectly or security headers to be missing, which can affect multiple downstream services using this server.

How an attack unfolds

An attacker does not actively trigger the flaw; instead, an administrator's use of a header directive inside a path-level scope silently drops outer-scope header rules, resulting in missing or incorrect headers that may enable client-side attacks or bypass intended controls.

How to fix it

Update h2o to commit 123f5e2b65dcdba8f7ef659a00d24bd1249141be or a later version that restores proper scope inheritance for the headers directive.

How to confirm the fix worked

After upgrade, confirm that header directives defined in outer scopes continue to apply when an inner scope also contains a header directive; test by sending requests to nested paths and inspecting response headers.

Typical attack path

1. Review public server configuration for nested scopes
2. Identify use of the header directive inside an inner scope
3. Request resources that should receive outer-scope headers
4. Observe absence of expected headers such as security-related ones in responses

How to detect it

Inspect server configuration files for any header directive appearing inside inner scopes such as path blocks, then manually request affected paths and compare received headers against those expected from outer scopes.

What your security team should do

• Upgrade all h2o instances to the fixed commit immediately
• Audit existing configuration files for header directives placed inside path or host scopes
• Add regression tests that verify header inheritance across scopes during configuration changes