The Apache HTTP Server, a widely used web server software, is at risk of being exploited due to a significant security vulnerability found in the ap_escape_quotes() function. This issue affects version 2.4.48 and earlier. The vulnerability, tracked as CVE-2021-39275, can potentially lead to a buffer overflow when given malicious input, which in turn can enable attackers to execute arbitrary code on the affected system or cause a denial of service.

Understanding the Vulnerability

The ap_escape_quotes() function employed in the Apache HTTP Server is meant to provide a convenient way to escape quotes in a string. However, when given malicious input, the function is said to write beyond the end of a buffer. While no included modules pass untrusted data to these functions by default, some third-party and external modules may rely on this function, and thus, could be affected.

Here's a code snippet showcasing a simplified version of the ap_escape_quotes() function

char *ap_escape_quotes(apr_pool_t *p, const char *instring)
    char *outstring;
    char *outpos;
    unsigned char *inpos;
    size_t outlen = ;

    if (!instring) {
        return NULL;

    inpos = (unsigned char *) instring;

    /* Calculate the length of the escaped string */
    while (*inpos) {
        if (*inpos == '"' || *inpos == '\\') {
            outlen += 2;
        else {

    outstring = apr_palloc(p, outlen + 1);
    outpos = outstring;
    inpos = (unsigned char *) instring;

    /* Escape the necessary characters */
    while (*inpos) {
        if (*inpos == '"' || *inpos == '\\') {
            *outpos++ = '\\';
        *outpos++ = *inpos++;

    *outpos = '\';
    return outstring;

Due to insufficient validation and boundary checking in the function, an attacker can craft input in such a way that the function attempts to write beyond the buffer's allocated memory space. This can lead to unintended consequences, potentially providing the attacker with a route to escalate privileges, execute arbitrary code, or disrupt the server's operation.

Exploit Details

While the core development team has not published specific exploit details for CVE-2021-39275, it's crucial to note that the criticality of this vulnerability should not be taken lightly. Server administrators and developers relying on Apache HTTP Server 2.4.48 or earlier should take immediate steps to safeguard their servers and applications.

For instance, you might assess any third-party or external modules that make use of the ap_escape_quotes() function, and if necessary, temporarily disable or modify those modules to prevent exploitation. Additionally, monitor logs and network traffic for unusual patterns that may imply an attempted exploitation of this vulnerability.

Official References and Further Reading

The Apache HTTP Server Project provided limited information regarding CVE-2021-39275 in their official announcement of security fixes made for version 2.4.49 (link). The announcement included a brief mention of the CVE and stated that ap_escape_quotes() could write beyond the end of a buffer when given malicious input.

For more information on this vulnerability, you can also refer to the National Vulnerability Database's (NVD) entry on CVE-2021-39275.

It is highly recommended that you upgrade your Apache HTTP Server to version 2.4.49 or later to secure your server against potential exploitation of CVE-2021-39275. You can download the latest version of the software from the Apache HTTP Server Project's official website.

Once upgraded, thoroughly test your applications for compatibility and proper functioning. Moreover, continue to stay informed about additional patches or announcements related to this vulnerability by subscribing to Apache HTTP Server's security mailing list.


Published on: 09/16/2021 15:15:00 UTC
Last modified on: 10/05/2022 12:28:00 UTC