# CVE-2026-27586: Caddy's mTLS client authentication silently fails open when CA certificate file is missing or malformed > Caddy silently bypasses mTLS client authentication if the CA certificate file is missing or malformed, allowing unauthorized access. - **Severity:** High 8.8 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:P`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 24 Feb 2026 - **Updated:** 26 Feb 2026 - **Also known as:** DEBIAN-CVE-2026-27586, GHSA-hffm-g8v7-wrv7, GO-2026-4539, MINI-3rg5-95gh-5rjg, UBUNTU-CVE-2026-27586 ## What is CVE-2026-27586? CVE-2026-27586 is a high-severity authentication bypass vulnerability in caddyserver caddy, published on 24 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, two swallowed errors in `ClientAuthentication.provision()` cause mTLS client certificate authentication to silently fail open when a CA certificate file is missing, unreadable, or malformed. The server starts without error but accepts any client certificate signed by any system-trusted CA, completely bypassing the intended private CA trust boundary. Any deployment using `trusted_ca_cert_file` or `trusted_ca_certs_pem_files` for mTLS will silently degrade to accepting any system-trusted client certificate if the CA file becomes unavailable. This can happen due to a typo in the path, file rotation, corruption, or permission changes. The server gives no indication that mTLS is misconfigured. Version 2.11.1 fixes the vulnerability. Weakness: CWE-755 (Improper Handling of Exceptional Conditions) ## Which versions are affected by CVE-2026-27586? 3 open source packages ship an affected version, including github.com/caddyserver/caddy/v2. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `github.com/caddyserver/caddy/v2` | Go | All versions | 2.11.1 | | `github.com/caddyserver/caddy/v2` | Go | All versions | 2.11.1 | | `github.com/caddyserver/caddy/v2` | Go | All versions | 2.11.1 | Products: Caddyserver Caddy. ## How do I fix CVE-2026-27586? Upgrade github.com/caddyserver/caddy/v2 to 2.11.1 or later. ## Is CVE-2026-27586 being exploited? It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27586 exists. ## Other vulnerabilities in caddyserver caddy - [CVE-2018-21246](https://insights.sbomify.com/CVE-2018-21246/): Caddy vulnerable to Authentication Bypass due to mishandling of TLS client authentication (critical 9.8) - [CVE-2026-27590](https://insights.sbomify.com/CVE-2026-27590/): Caddy: Unicode case-folding length expansion causes incorrect split_path index in FastCGI transport (high 8.9) - [CVE-2026-45135](https://insights.sbomify.com/CVE-2026-45135/): Caddy: Unsafe Unicode Handling in FastCGI splitPos Allows Execution of Non-PHP Files (high 8.1) - [CVE-2026-52845](https://insights.sbomify.com/CVE-2026-52845/): Caddy: FastCGI header normalization bypass in `forward_auth copy_headers` (high 8.1) - [CVE-2026-30851](https://insights.sbomify.com/CVE-2026-30851/): Caddy forward_auth copy_headers Does Not Strip Client-Supplied Headers, Allowing Identity Injection and Privilege Escalation (high 8.1) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-27586 and fix it if it is affected. What it is: CVE-2026-27586 is a high-severity authentication bypass vulnerability in caddyserver caddy, published on 24 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27586 exists. Affected packages and fixed versions: - github.com/caddyserver/caddy/v2 (Go): affected all versions; fixed in 2.11.1 - github.com/caddyserver/caddy/v2 (Go): affected all versions; fixed in 2.11.1 - github.com/caddyserver/caddy/v2 (Go): affected all versions; fixed in 2.11.1 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-27586/ (Markdown: https://insights.sbomify.com/CVE-2026-27586/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-27586 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-27586 - OSV: https://osv.dev/vulnerability/CVE-2026-27586 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-27586/ Summaries are generated and may contain mistakes; the official description above is the record.