ERR_HPACK_DECODING_FAILED

Enterno.io Editorial Team

By Anatoly Oshmanovsky · Updated Apr 18, 2026

Key idea:

ERR_HPACK_DECODING_FAILED — HTTP/2 client could not decompress HPACK-encoded headers. HPACK — Huffman-based header compression for HTTP/2 (RFC 7541). Error: server sent malformed HPACK, dynamic table desync, client/server index mismatch. Usually a bug in reverse proxy (old nginx, HAProxy). Fix: update server software.

Below: causes, fixes, FAQ.

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Common Causes

nginx < 1.16 with HPACK bugs
HAProxy < 2.2 HTTP/2 bugs
Custom HTTP/2 implementation (home-grown gateway)
CDN edge node stale cache corrupted HPACK table
Pseudo-headers (:status, :path) wrongly encoded

Step-by-Step Fix

Update nginx to 1.24+ or LTS 1.26
Update HAProxy to 2.8+
Curl test: curl --http2 -v https://host
Chrome DevTools → Network → Protocol column (verify h2)
Fallback to HTTP/1.1: remove Alt-Svc + disable HTTP/2

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

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

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

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

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

Best Practices

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

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

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

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

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

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Research

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Errors

Frequently Asked Questions

HPACK vs QPACK?

HPACK — HTTP/2 (TCP). QPACK — HTTP/3 (QUIC/UDP). QPACK handles out-of-order delivery, allows concurrent headers.

Debug HPACK?

Wireshark + SSLKEYLOGFILE. Or <code>nghttp2</code> CLI tools to analyze frames.

CDN side?

Cloudflare support ticket with request id + date. Akamai — similar support path. Check status page.

HTTP/2 vs HTTP/3?

HTTP/2: TCP, HPACK, head-of-line blocking. HTTP/3: QUIC/UDP, QPACK, stream multiplexing without HoL.