ERR_SSL_CLIENT_AUTH_SIGNATURE_FAILED — mTLS Cert

Enterno.io Editorial Team

ERR_SSL_CLIENT_AUTH_SIGNATURE_FAILED: Fix

By Anatoly Oshmanovsky · Updated Apr 17, 2026

Key idea:

ERR_SSL_CLIENT_AUTH_SIGNATURE_FAILED appears in mTLS (mutual TLS) when the server requests a client cert but the client cannot sign the challenge. Causes: smart card/eToken locked, private key corrupted, Chrome can not find the cert in the OS store, expired cert. Fix: verify cert in OS store, unlock smart card, reinstall cert.

This error blocks HTTPS access. Below: causes, fixes, working config, FAQ.

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Common Causes

Smart card / eToken locked after a wrong PIN
Private key corrupted or deleted
Chrome does not see the client cert in the OS store
Client cert expired
Server requires a cert from a specific CA; yours is from a different one
Outdated smart-card drivers (CryptoPro, JaCarta, Rutoken in RU)

Step-by-Step Fix

Inspect the cert in the OS: Windows — certmgr.msc, macOS — Keychain, Linux — ~/.pki/nssdb
Smart card: eject and reinsert, enter PIN carefully
Reinstall the certificate from pfx/p12: File → Install Certificate
Update smart-card drivers (CryptoPro, JaCarta, Rutoken)
Test in Chrome: chrome://settings/certificates — see installed certs
Monitor client-cert services with Enterno Monitor

Check SSL Certificate →

Example: Proper nginx TLS config

server {
    listen 443 ssl http2;
    server_name example.com;

    ssl_certificate     /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;

    ssl_protocols       TLSv1.2 TLSv1.3;
    ssl_ciphers         ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256;
    ssl_prefer_server_ciphers off;

    ssl_stapling        on;
    ssl_stapling_verify on;
}

Related SSL Errors

CertificateExpiry, issuer, domains (SAN)

ChainIntermediate and root CA validation

TLS ProtocolTLS version and cipher suite

VulnerabilitiesHeartbleed, POODLE, weak ciphers

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Enter domain

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What Does the SSL Check Cover?

SSL/TLS is the encryption protocol that protects data between the browser and server. Our tool analyzes the certificate, chain of trust, TLS version, and knownvulnerabilities.

Certificate Details

Issuer, validity period, signature algorithm, covered domains (SAN), and validation type (DV/OV/EV).

Chain of Trust

Full chain verification: from leaf certificate through intermediates to root CA.

TLS Analysis

Protocol version (TLS 1.2/1.3), cipher suites, Perfect Forward Secrecy (PFS) support.

Expiry Alerts

Set up a monitor — get Telegram and email alerts 30/14/7 days before expiration.

DV vs OV vs EV Certificates

DV (Domain Validation)

Confirms domain ownership only
Issued in minutes automatically
Free via Let's Encrypt
Suitable for most websites
Most common certificate type

OV / EV

Organization (OV) or Extended Validation (EV)
Issued in 1-5 business days
Costs $50 to $500/year
For finance, e-commerce, government sites
Increases user trust

Who uses this

DevOps

SSL certificate monitoring

Security

TLS config audit

SEO

HTTPS as ranking factor

E-commerce

customer trust

Common Mistakes

❌

Expired certificateBrowsers block sites with expired SSL. Set up auto-renewal or monitoring.

❌

Incomplete certificate chainWithout intermediate CA, some browsers and bots cannot verify the certificate.

❌

Mixed content on HTTPS siteHTTP resources on an HTTPS page — the browser lock icon disappears, reducing trust.

❌

Using TLS 1.0/1.1Legacy TLS versions have known vulnerabilities. Use TLS 1.2+ or 1.3.

❌

Domain mismatch in certificateThe certificate must cover all site domains, including www and subdomains.

Best Practices

✓

Set up auto-renewalLet's Encrypt + certbot with cron — certificate renews automatically every 60-90 days.

✓

Enable HSTSStrict-Transport-Security header forces browsers to always use HTTPS.

✓

Use TLS 1.3TLS 1.3 is faster (1-RTT handshake) and safer — legacy ciphers removed.

✓

Monitor expiration datesCreate a monitor on Enterno.io — get notified well before expiration.

✓

Verify chain after renewalAfter certificate renewal, confirm that intermediate certificates are installed.

Get more with a free account

SSL certificate monitoring, check history and alerts 30 days before expiry.

Learn more

How-to

Glossary

Research

TLS Cipher Suites 2026: Distribution Report

Errors

Alternatives

Frequently Asked Questions

What is mTLS?

Mutual TLS — two-way authentication. The server verifies the client certificate before returning a response. Used in banking, enterprise APIs, gov services.

Why did Chrome not prompt to pick a cert?

If there is a single matching cert — Chrome uses it automatically. If multiple or none — a dialog appears. Tunable in chrome://policy.

Works in Firefox, fails in Chrome — why?

Firefox has its own cert store (NSS), independent of the OS. Chrome uses the system store. Different stores → different access.

How do I import a p12 into Chrome?

chrome://settings/certificates → Your certificates → Import. Enter the p12 password. The cert appears in the list and becomes available for mTLS.