CVE-2021-39275 - Buffer Overflow in Apache HTTP Server’s `ap_escape

Apache HTTP Server is one of the most widely used web servers in the world. That’s why every bug in it can be a big deal. In this article, we break down CVE-2021-39275, a vulnerability in Apache’s ap_escape_quotes() function. We’ll use simple language, show how the issue works, and walk through some example code. If you work with Apache or write modules for it, this is a must-read.

What is CVE-2021-39275?

CVE-2021-39275 is a vulnerability that affects Apache HTTP Server version 2.4.48 and earlier. The root cause is that the built-in function ap_escape_quotes() can write more data than it should into a memory buffer if it receives malicious input. This is called a buffer overflow.

The good news: No official Apache module as shipped will directly expose this bug with unsafe data.
The bad news: Third-party or external modules *can* trigger this issue if they pass untrusted data to ap_escape_quotes().

Understanding ap_escape_quotes()

This function is meant to take a string, search for single or double quote characters, and add a backslash before each one. This process is called escaping. The output gets saved in a buffer, which must be large enough to hold the extra characters.

But what if a programmer doesn’t allocate enough space? Or what if ap_escape_quotes() does not check the buffer size properly?

Here’s a rough example of what the function does

int ap_escape_quotes(char *dest, const char *src) {
    int i = , j = ;
    while (src[i] != '\') {
        if (src[i] == '"' || src[i] == '\'') {
            dest[j++] = '\\';
        }
        dest[j++] = src[i++];
    }
    dest[j] = '\';
    return j;
}

If dest does not point to a buffer big enough to hold every added backslash, the function can write beyond the end of the memory block. This leads to memory corruption.

How Can This Vulnerability Be Exploited?

By itself, Apache does not ship modules that give attackers a way to control what gets passed to ap_escape_quotes(). That makes exploitation unlikely for core Apache installations.

However, if you use external modules (third-party, from GitHub, custom, etc.), those modules might process user-submitted data using this function. If they don’t check the length or size of the destination buffer, an attacker can submit input that causes ap_escape_quotes() to write past the end of the buffer.

Example of risky usage

char buffer[100];
ap_escape_quotes(buffer, user_input); // user_input could be >50 quotes long!

If user_input is, say, a string of 100 quotes (""""..."") then the function would try to write up to 200 bytes into a 100-byte buffer—causing a buffer overflow.

Real-World Impact

If you only use official Apache modules, you’re safe.
If you use third-party modules (especially older or less-maintained ones), or if you develop your own modules, you must be careful.

How Was It Fixed?

The Apache team addressed the issue in later versions by improving memory management around these functions. Upgrading to Apache HTTP Server 2.4.49 or later removes the vulnerable code.

Official fix commit:
https://github.com/apache/httpd/commit/94de739e9f5a7f2dfbbff83db15ea147a57c81

Release notes:
https://httpd.apache.org/security/vulnerabilities_24.html#CVE-2021-39275

Update your server! Upgrade to Apache 2.4.49 or higher.

2. Audit your modules: If you write or use third-party modules, check if any of them use ap_escape_quotes() with user-controlled input.

Buffer sizing: Always make sure the output buffer is large enough.

4. Sandboxing: If you can, run server processes with minimal privileges to limit impact in case of a memory bug.

Final Thoughts

CVE-2021-39275 shows that even small utility functions can be dangerous when a project has millions of users and countless add-ons. Always validate input buffer length, especially when working in C/C++, and keep your software up to date.

If you maintain Apache or module code, this is your nudge to check your buffers and your upgrade status today. Stay safe out there!

*This article was written exclusively for your security learning. Share this with any web server admin or developer who could benefit from it!*

Timeline

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

CVE-2021-39275 - Buffer Overflow in Apache HTTP Server’s `ap_escape_quotes()` Function Explained