php - What does the security implications for default character set in mysqli_real_escape_string() means? -

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in php documentation mysqli_real_escape_string(), written that

caution security: default character set

the character set must set either @ server level, or api function mysqli_set_charset() affect mysqli_real_escape_string().

source mysqli_real_escape_string

in further link character set, mentioned that

the character set should understood , defined, has affect on every action, , includes security implications.

suource character sets

why necessary set character set sake of security , security implications include? can anyone, explain concept behind lines ?

thanks in advance

how sql queries parsed dependent on connection character set. if did query:

$value = chr(0xe0) . chr(0x5c); mysql_query("select '$value'");

then if connection character set latin-1 mysql see invalid:

select 'à\'

whereas if character set shift-jis, byte sequence 0xe0,0x5c interpreted double-byte character:

select '濬'

add string literal escaping security:

$value = mysql_real_escape_string($value); mysql_query("select '$value'");

now if you've correctly set connection character set shift-jis mysql_set_charset, mysql still sees:

select '濬'

but if haven't set connection character set, , mysql's default character set shift-jis php's default character set ascii, php doesn't know trailing 0x5c character part of double-byte sequence, , escapes it, thinking generating valid output:

select 'à\\'

whilst mysql reads using shift-jis as:

select '濬\'

with trailing ' escaped backslash, has left string literal open. next ' character in query end string, leaving whatever follows in raw sql content. if can inject there, query vulnerable.

this problem applies few east asian encodings shift-jis multibyte sequences can contain bytes on own valid ascii characters backslash. if mismatched encodings both treat low bytes always-ascii (strict ascii supersets more-common mismatch of latin-1 vs utf-8), no such confusion possible.

luckily servers default these encodings uncommon, in practice rarely-exploitable issue. if have use mysql_real_escape_string should right. (better avoid using parameterised queries though.)


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