ERR_SSL_DECOMPRESSION_FAILURE_ALERT — CRIME Mitigation

Enterno.io Editorial Team

ERR_SSL_DECOMPRESSION_FAILURE_ALERT: What It Is

By Anatoly Oshmanovsky · Updated Apr 17, 2026

Key idea:

ERR_SSL_DECOMPRESSION_FAILURE_ALERT — TLS alert 30. Historically appeared when client and server failed to agree on compression. Rarely seen in 2026: TLS compression is off on every modern server since the CRIME attack (CVE-2012-4929). If you see it — you have a legacy server attempting compression, or a misconfigured TLS library.

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

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Common Causes

Legacy server still offers TLS compression (rare in 2026)
Custom TLS library with a deflate-handling bug
Proxy/DPI actively modifying compressed content
CPAN/OpenSSL < 1.0.0 with compression enabled

Step-by-Step Fix

Disable TLS compression on the server (off by default in OpenSSL 1.0.0+)
Update OpenSSL/LibreSSL to the latest version
Check openssl s_client -connect example.com:443 -comp — should show no compression method
nginx: application-layer compression (gzip) is safe and different from TLS compression
Remove any custom SSL_COMP settings

Check SSL Certificate →

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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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.

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DV vs OV vs EV Certificates

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Common Mistakes

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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.

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How-to

Glossary

Research

TLS Cipher Suites 2026: Distribution Report

Errors

Alternatives

Frequently Asked Questions

What is the CRIME attack?

CRIME (CVE-2012-4929) — attacker measures compressed response size containing secrets (cookies, CSRF tokens). TLS compression leaked secrets via side-channel.

Is TLS compression the same as gzip content-encoding?

No. TLS compression is transport-layer. gzip is application-layer. gzip is safe.

In which TLS version is compression?

TLS 1.0-1.2 — optional. TLS 1.3 — removed entirely. Never in TLS 1.3.

How to check compression is off?

<a href="/en/ssl">Enterno SSL</a> — negotiated TLS 1.3 guarantees no compression.