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Handle empty password from broader set of MySQL clients #1084

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17 changes: 12 additions & 5 deletions server/auth.go
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,15 @@ var (
ErrAccessDeniedNoPassword = fmt.Errorf("%w without password", ErrAccessDenied)
)

// isEmptyPassword returns true if the auth data represents an empty password.
// Some clients send an empty packet (len == 0), while others (e.g. MySQL's libmysql)
// send a single null byte. This matches MySQL server's own handling:
// if (!pkt_len || (pkt_len == 1 && *pkt == 0))
// See: https://github.com/mysql/mysql-server/blob/8.0/sql/auth/sha2_password.cc
func isEmptyPassword(authData []byte) bool {
return len(authData) == 0 || (len(authData) == 1 && authData[0] == 0)
}

func (c *Conn) compareAuthData(authPluginName string, clientAuthData []byte) error {
if authPluginName != c.credential.AuthPluginName {
err := c.writeAuthSwitchRequest(c.credential.AuthPluginName)
Expand Down Expand Up @@ -66,7 +75,7 @@ func scrambleValidation(cached, nonce, scramble []byte) bool {
}

func (c *Conn) compareNativePasswordAuthData(clientAuthData []byte, credential Credential) error {
if len(clientAuthData) == 0 {
if isEmptyPassword(clientAuthData) {
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@lance6716 lance6716 Dec 20, 2025

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I see your MySQL reference is only for caching_sha2_password_authenticate. Will it also apply for compareNativePasswordAuthData and compareSha256PasswordAuthData?

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@ramnes ramnes Dec 20, 2025

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SHA256 authentication does have the exact same check here: https://github.com/mysql/mysql-server/blob/056a391cdc1af9b17b5415aee243483d1bac532d/sql/auth/sql_authentication.cc#L4763

For mysql_native_password, I'm less sure but my understanding is that both cases were supported as well by setting a default \0, and then trimming with get_length_encoded_string: https://github.com/mysql/mysql-server/blob/8.0/sql/auth/sql_authentication.cc#L3003-L3012

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@lance6716 lance6716 Dec 20, 2025

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Please also add the second link to function comment of isEmptyPassword. Rest lgtm

if credential.hasEmptyPassword() {
return nil
}
Expand All @@ -90,8 +99,7 @@ func (c *Conn) compareNativePasswordAuthData(clientAuthData []byte, credential C
}

func (c *Conn) compareSha256PasswordAuthData(clientAuthData []byte, credential Credential) error {
// Empty passwords are not hashed, but sent as empty string
if len(clientAuthData) == 0 {
if isEmptyPassword(clientAuthData) {
if credential.hasEmptyPassword() {
return nil
}
Expand Down Expand Up @@ -135,8 +143,7 @@ func (c *Conn) compareSha256PasswordAuthData(clientAuthData []byte, credential C
}

func (c *Conn) compareCacheSha2PasswordAuthData(clientAuthData []byte) error {
// Empty passwords are not hashed, but sent as empty string
if len(clientAuthData) == 0 {
if isEmptyPassword(clientAuthData) {
if c.credential.hasEmptyPassword() {
return nil
}
Expand Down
2 changes: 1 addition & 1 deletion server/auth_switch_response.go
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Original file line number Diff line number Diff line change
Expand Up @@ -71,7 +71,7 @@ func (c *Conn) handleCachingSha2PasswordFullAuth(authData []byte) error {
}

func (c *Conn) checkSha2CacheCredentials(clientAuthData []byte, credential Credential) error {
if len(clientAuthData) == 0 {
if isEmptyPassword(clientAuthData) {
if credential.hasEmptyPassword() {
return nil
}
Expand Down
12 changes: 12 additions & 0 deletions server/auth_switch_response_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,18 @@ func TestCheckSha2CacheCredentials_EmptyPassword(t *testing.T) {
serverPassword: "secret",
wantErr: ErrAccessDeniedNoPassword,
},
{
name: "null byte client auth, empty server password",
clientAuthData: []byte{0x00},
serverPassword: "",
wantErr: nil,
},
{
name: "null byte client auth, non-empty server password",
clientAuthData: []byte{0x00},
serverPassword: "secret",
wantErr: ErrAccessDeniedNoPassword,
},
}

for _, tt := range tests {
Expand Down
36 changes: 36 additions & 0 deletions server/auth_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,18 @@ func TestCompareNativePasswordAuthData_EmptyPassword(t *testing.T) {
serverPassword: "secret",
wantErr: ErrAccessDeniedNoPassword,
},
{
name: "null byte client auth, empty server password",
clientAuthData: []byte{0x00},
serverPassword: "",
wantErr: nil,
},
{
name: "null byte client auth, non-empty server password",
clientAuthData: []byte{0x00},
serverPassword: "secret",
wantErr: ErrAccessDeniedNoPassword,
},
}

for _, tt := range tests {
Expand Down Expand Up @@ -68,6 +80,18 @@ func TestCompareSha256PasswordAuthData_EmptyPassword(t *testing.T) {
serverPassword: "secret",
wantErr: ErrAccessDeniedNoPassword,
},
{
name: "null byte client auth, empty server password",
clientAuthData: []byte{0x00},
serverPassword: "",
wantErr: nil,
},
{
name: "null byte client auth, non-empty server password",
clientAuthData: []byte{0x00},
serverPassword: "secret",
wantErr: ErrAccessDeniedNoPassword,
},
}

for _, tt := range tests {
Expand Down Expand Up @@ -104,6 +128,18 @@ func TestCompareCacheSha2PasswordAuthData_EmptyPassword(t *testing.T) {
serverPassword: "secret",
wantErr: ErrAccessDeniedNoPassword,
},
{
name: "null byte client auth, empty server password",
clientAuthData: []byte{0x00},
serverPassword: "",
wantErr: nil,
},
{
name: "null byte client auth, non-empty server password",
clientAuthData: []byte{0x00},
serverPassword: "secret",
wantErr: ErrAccessDeniedNoPassword,
},
}

for _, tt := range tests {
Expand Down
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