tls: server selected unsupported compression format

Error message

tls: server selected unsupported compression format

What it means

TLS 1.2 and earlier carry a compression_method field in the ServerHello. Modern TLS permits only compressionNone (0); any other selection enables CRIME/BREACH-style compression oracle attacks and Go implements no compression, so the handshake is aborted with alertIllegalParameter.

Source

Thrown at src/crypto/tls/handshake_client.go:890

}

func (hs *clientHandshakeState) serverResumedSession() bool {
	// If the server responded with the same sessionId then it means the
	// sessionTicket is being used to resume a TLS session.
	return hs.session != nil && hs.hello.sessionId != nil &&
		bytes.Equal(hs.serverHello.sessionId, hs.hello.sessionId)
}

func (hs *clientHandshakeState) processServerHello() (bool, error) {
	c := hs.c

	if err := hs.pickCipherSuite(); err != nil {
		return false, err
	}

	if hs.serverHello.compressionMethod != compressionNone {
		c.sendAlert(alertIllegalParameter)
		return false, errors.New("tls: server selected unsupported compression format")
	}

	supportsPointFormat := false
	offeredNonCompressedFormat := false
	for _, format := range hs.serverHello.supportedPoints {
		if format == pointFormatUncompressed {
			supportsPointFormat = true
		} else {
			offeredNonCompressedFormat = true
		}
	}
	if !supportsPointFormat && offeredNonCompressedFormat {
		return false, errors.New("tls: server offered only incompatible point formats")
	}

	if c.handshakes == 0 && hs.serverHello.secureRenegotiationSupported {
		c.secureRenegotiation = true
		if len(hs.serverHello.secureRenegotiation) != 0 {

View on GitHub (pinned to b6b368adc5)

Solutions

Treat this as a server-side defect; the server must select null compression.
Capture the ServerHello to confirm the field value and report to the server operator.
Investigate the network path for a tampering device.

Defensive patterns

Strategy: try-catch

Type guard

func isUnsupportedCompression(err error) bool {
    return err != nil && strings.Contains(err.Error(), "server selected unsupported compression format")
}

Try / catch

if _, err := tls.Dial("tcp", addr, cfg); err != nil {
    if isUnsupportedCompression(err) {
        // Server is non-conformant or path is being tampered with; do not retry.
        reportServerDefect(addr, err)
    }
}

Prevention

Report server defects that select non-null compression.
Investigate the path for tampering.

When it happens

Trigger: Server selects DEFLATE (compression method 1) or another non-null method; corrupted ServerHello decoding; malicious server probing for vulnerable clients.

Common situations: Very old or non-conformant TLS stacks; tampered traffic; essentially never seen from modern servers.

Understand the failure class

SSL/TLS and certificate errors — how TLS handshakes and certificate validation fail.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/8a8d2f9f1f2240eb. Report an issue: GitHub.