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CVE-2026-52845 high CVSS 8.1

Authentication Bypass in github.com/caddyserver/caddy/v2 (CVE-2026-52845)

Summary

Authentication bypass in github.com/caddyserver/caddy/v2 (CVE-2026-52845). Confidential information can be exposed externally. Exploitable via GET /index.php. Mitigation: upgrade to 2.11.4 or later.

Original description

The description as published by the source.

Summary

forward_auth copy_headers deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through php_fastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with _.

This lets a client send an underscore alias that survives the forward_auth delete step but becomes the same PHP/FastCGI variable:

Remote-Groups  -> HTTP_REMOTE_GROUPS
Remote_Groups  -> HTTP_REMOTE_GROUPS

Remote-User    -> HTTP_REMOTE_USER
Remote_User    -> HTTP_REMOTE_USER

Result: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy.

Details

forward_auth copy_headers intentionally removes client-controlled headers before setting values from the auth response:

  • modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go:212
  • modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go:222

That delete is exact-field deletion through http.Header.Del():

  • modules/caddyhttp/headers/headers.go:255
  • modules/caddyhttp/headers/headers.go:281

So deleting Remote-Groups does not delete Remote_Groups.

Later, FastCGI exports all request headers into CGI variables:

  • modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:410
  • modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:414
  • modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:510

The normalizer replaces hyphens with underscores:

strings.NewReplacer…

Source: GitHub Security Advisories (Global) NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-52845 has been found in github.com/caddyserver/caddy/v2.
Attackers can target a specific entry point like GET /index.php over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 8.1/10.
What to do: upgrade github.com/caddyserver/caddy/v2 to 2.11.4 or later.
If unsure, ask your IT team or search "github.com/caddyserver/caddy/v2 CVE-2026-52845" on the vendor's site.

CVE-2026-52845 (github.com/caddyserver/caddy/v2) — CWE-287 / CVSS v3 8.1
Attack vector: remote (network-reachable) / no user interaction
Attack surface: GET /index.php / Authorization header / php_fastcgi / forward_auth
Patched: 2.11.4 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

Authentication bypass (CWE-287) exists in github.com/caddyserver/caddy/v2. Attackers reach the vulnerable code path via GET /index.php.

📍 Affected scope

Check the vendor advisory for the affected versions of github.com/caddyserver/caddy/v2. Attack surface: GET /index.php / Authorization header / php_fastcgi / forward_auth.

🔥 Severity

Severity: High (CVSS 8.1/10). Confidential information can be exposed externally.

🔧 How to fix

Update to 2.11.4.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual GET /index.php requests with malformed payloads or SQL meta-characters. Run grep -r 'github.com/caddyserver/caddy/v2' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'github.com/caddyserver/caddy/v2' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で github.com/caddyserver/caddy/v2 を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Hunt for indicators of compromise detect 
    grep 'GET /index.php' /var/log/nginx/access.log | grep -E '(unusual_payload|sqli_pattern)'

    アクセスログで GET /index.php への異常なリクエスト (不正な認証ヘッダ・SQLメタ文字)を過去 30〜90日分捜索。WAF/SIEM があれば該当パスのアラート発火履歴を確認。

  3. 3
    Apply patch patch 
    Upgrade github.com/caddyserver/caddy/v2 to 2.11.4

    ステージング環境で 2.11.4 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスと、Step 2 で見つけた異常検知の続報チェックを含めること。

  4. 4
    Post-deployment verification verify 
    Replay attack against GET /index.php on staging to confirm patch closes the vector

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番では Step 2 と同じログクエリでアラート再発が無いか継続監視。

Affected packages

go github.com/caddyserver/caddy/v2
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"2.11.4"}]}]
go github.com/caddyserver/caddy
[{"type":"SEMVER","events":[{"introduced":"0"},{"last_affected":"1.0.5"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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