Caddy is vulnerable to cross-origin config application via local admin API /load

commit: e0f8d9b2047af417d8faf354b675941f3dac9891 (as-of 2026-02-04) channel: GitHub security advisory (per SECURITY.md)

summary

The local caddy admin API (default listen 127.0.0.1:2019) exposes a state-changing POST /load endpoint that replaces the entire running configuration.

When origin enforcement is not enabled (enforce_origin not configured), the admin endpoint accepts cross-origin requests (e.g., from attacker-controlled web content in a victim browser) and applies an attacker-supplied JSON config. this can change the admin listener settings and alter HTTP server behavior without user intent.

Severity

Medium

Justification:

• The attacker can apply an arbitrary caddy config (integrity impact) by driving a victim’s local admin API.
• Exploitation requires a victim running caddy with the admin API enabled and visiting an attacker-controlled page (or otherwise issuing the request from an untrusted local client).

Affected component

• caddyconfig/load.go: adminLoad.handleLoad (/load admin endpoint)
• Pinned callsite: https://github.com/caddyserver/caddy/blob/e0f8d9b2047af417d8faf354b675941f3dac9891/caddyconfig/load.go#L73

Reproduction

Attachment: poc.zip (integration harness) with canonical and control runs.

unzip -q -o poc.zip -d poc
cd poc/poc-F-CADDY-ADMIN-LOAD-001
make test

Expected output (excerpt):

[CALLSITE_HIT]: adminLoad.handleLoad
[PROOF_MARKER]: http_code=200 admin_moved=true response_pwned=true

Control output (excerpt):

[NC_MARKER]: http_code=403 load_blocked=true admin_moved=false response_pwned=false

Impact

An attacker can replace the running caddy configuration via the local admin API. Depending on the deployed configuration/modules, this can:

• Change admin listener settings (e.g., move the admin listener to a new address)
• Change HTTP server behavior (e.g., alter routes/responses)

Suggested remediation

Ensure cross-origin web content cannot trigger POST /load on the local admin API by default, for example by:

• Enabling origin enforcement by default for unsafe methods, and/or
• Requiring an unguessable token for (and other state-changing admin endpoints).