CVE-2026-28780 is a heap-based buffer overflow vulnerability discovered in the mod_proxy_ajp module of the Apache HTTP Server. This vulnerability is present in all versions up to and including Apache HTTP Server 2.4.66.

In simple words, if your Apache server is configured to use AJP (Apache JServ Protocol) via mod_proxy_ajp, and it connects to a malicious or compromised AJP backend, that backend can send specially crafted data that can make your server overwrite some memory on the heap – with attacker-supplied values.

This vulnerability is fixed in version 2.4.67.

How Does the Vulnerability Work?

This flaw occurs during the handling of certain AJP messages received from the backend server. If the backend AJP server is not trustworthy or is compromised, it might send a maliciously crafted AJP message.

The vulnerable code in mod_proxy_ajp processes the AJP packet and uses a memory buffer for the message. If the message is crafted a certain way, mod_proxy_ajp writes four bytes just after the buffer’s end. This is called a heap-based buffer overflow. Such overflows can be exploited for information leaks, crashes (DoS), or even remote code execution, depending on how the heap is handled and what follows the buffer that is overflowed.

Technical Details and Code Explainer

Let’s go through a simplified explanation of the issue. Here’s a made-up but relevant snippet to illustrate:

/* Pseudo code inspired by vulnerable logic */
char *buffer = malloc(size);
...
if (ajp_read(message, buffer, size)) {
    // Some logic...
    int pos = ...; // an index from the AJP message
    // Write some values to buffer at (pos)
    buffer[pos] = attacker_value1;
    buffer[pos + 1] = attacker_value2;
    buffer[pos + 2] = attacker_value3;
    buffer[pos + 3] = attacker_value4;
    // <-- If pos is (size), these 4 writes go past the buffer!
}

The risk occurs because the index (pos) comes from parsed AJP data – controlled by the backend! If that backend is malicious, it chooses a position at the very end of the buffer, and overwrites 4 bytes of heap memory _after_ the allocated buffer.

What an attacker does:

Potential Exploit and Impact

While this vulnerability can only be exploited if your Apache HTTPD connects to a hostile AJP backend, the consequences can be critical:

- Denial of Service (DoS): Overwriting these four bytes can corrupt memory, crash httpd, or cause random behavior.

Information Disclosure: Carefully crafted exploitation could reveal server memory.

- Remote Code Execution (RCE): In the worst scenario, with ideal heap layout and technique, an attacker could execute code (though this is less likely and much more difficult).

Most common deployment scenarios, like Apache using mod_proxy_ajp to connect only to a trusted Tomcat server, are not at immediate risk. _But_ if an attacker can get between Apache and its backend – or get you to connect to their AJP endpoint – your server is in danger.

How to Fix the Issue

Patch immediately!
The Apache Software Foundation released version 2.4.67 which contains the fix for this vulnerability. The best, and only guaranteed-safe, solution is to update to Apache HTTP Server 2.4.67 or newer.

References & Resources

- Original Apache Security Advisory for CVE-2026-28780
- Apache HTTP Server Downloads
- mod_proxy_ajp Documentation
- About Buffer Overflows (OWASP)

Conclusion

CVE-2026-28780 reminds us why it’s crucial to always patch server software and to only proxy to trusted backends. If you use mod_proxy_ajp on Apache httpd, make sure to update to 2.4.67 or later, and lock down your AJP backend connections. Don’t let a sneaky backend overflow your memory – and possibly compromise your whole web server.

Timeline

Published on: 05/05/2026 21:29:41 UTC
Last modified on: 05/06/2026 20:31:10 UTC