full-disclosure - [FD] PHP-FPM fpm_log.c memory leak and buffer overflow

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Message-ID: <56A6448D.5010803@search-lab.hu>
Date: Mon, 25 Jan 2016 16:51:41 +0100
From: Imre RAD <imre.rad@...rch-lab.hu>
To: fulldisclosure@...lists.org
Subject: [FD] PHP-FPM fpm_log.c memory leak and buffer overflow

The FastCGI Process Manager (FPM) SAPI of PHP was vulnerable to memory
leak and buffer overflow in the access logging feature.

PHP-FPM offers customization of the access log lines based on format
string variables which can be specified with the access.format option of
the FPM configuration file.
The log lines were compiled in php-fpm.c. The %{something}e fields were
processed at line 237:

len2 = snprintf(b, FPM_LOG_BUFFER - len, "%s", env ? env : "-");
...
len += len2;
...
    if (!test && strlen(buffer) > 0) {
         buffer[len] = '\n';
        write(fpm_log_fd, buffer, len + 1);
    }

In case the string being appended to the access log line buffer was
longer than the remaining space, the len variable became longer than the
buffer (FPM_LOG_BUFFER) size, because snprintf returns the number of
characters (excluding the terminating null byte) which would have been
written to the final string if enough space had been available. Then the
PHP engine performed an out-of-boundaries read and also wrote a \n
character outside of the allocated memory.

The fix is available with the commit
http://git.php.net/?p=php-src.git;a=commit;h=2721a0148649e07ed74468f097a28899741eb58f
The fixed versions of PHP are: 5.5.31, 5.6.17 and 7.0.2.

More information:
http://www.search-lab.hu/about-us/news/111-some-unusual-vulnerabilities-in-the-php-engine

Imre Rad
Search-Lab Ltd.
http://www.search-lab.hu/
http://www.scademy.com/

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