Apache HTTP Server 2.4.18, while an older version, contains several critical vulnerabilities that allow for local root privilege escalation, denial of service (DoS), and certificate bypass. Critical Exploits & Vulnerabilities
CARPE (CVE-2019-0211): Local Root Privilege EscalationOne of the most significant exploits affecting 2.4.18 is the "CARPE" vulnerability found in versions 2.4.17 through 2.4.38.
The Mechanism: This is a Use-After-Free (UAF) flaw in the scoreboard. A less-privileged child process (like a PHP script) can manipulate the shared memory to gain root privileges when the server performs a graceful restart.
Exploitation: Attackers typically overwrite function pointers in the shared memory to execute arbitrary code with root authority.
HTTP/2 Certificate Authentication Bypass (CVE-2016-4979)When both mod_http2 and mod_ssl are enabled, version 2.4.18 fails to properly enforce the SSLVerifyClient require directive for HTTP/2 requests.
The Flaw: Attackers can leverage the ability to send multiple requests over a single connection to bypass access restrictions. Fix: This is addressed in version 2.4.23 or later.
HTTPoxy (CVE-2016-5387)This vulnerability allows remote attackers to redirect outbound HTTP traffic from applications to an arbitrary proxy server.
How it works: Apache 2.4.18 incorrectly trusts a user-supplied Proxy header and uses it to set the HTTP_PROXY environment variable for CGI-like scripts.
Impact: This can lead to sensitive data interception or man-in-the-middle attacks.
Memory Disclosures (CVE-2019-10082 & CVE-2019-0196)Versions ranging from 2.4.18 to 2.4.39 are susceptible to memory-related attacks via fuzzed network input.
Session Handling (CVE-2019-10082): Can trigger a read of freed memory during connection shutdown, potentially exposing sensitive information.
Request Handling (CVE-2019-0196): Leads to access of freed memory during string comparisons when determining the request method. Denial of Service (DoS) Vectors Apache HTTPD: CVE-2019-0211: Use After Free - Rapid7
Apache HTTP Server version 2.4.18, released in December 2015, is a legacy version of the software that contains several significant security vulnerabilities discovered in the years following its release. While 2.4.18 itself was intended to be a stable release, its lack of modern patches makes it a primary target for specific exploit techniques. Major Vulnerabilities in Apache 2.4.18
Because this version falls within the 2.4.17 to 2.4.38 range, it is susceptible to several high-impact exploits, most notably in local privilege escalation and memory handling. 1. Local Privilege Escalation (CVE-2019-0211)
Commonly referred to as CARPE (DIEM), this is one of the most critical exploits affecting version 2.4.18.
The Flaw: It involves an out-of-bounds array access during a "graceful restart" (apache2ctl graceful). apache httpd 2.4.18 exploit
The Exploit: An attacker with low-level permissions on the server (such as through a compromised PHP script) can write to the shared memory used by Apache's parent process. When the server performs its daily log rotation and restarts, the parent process—which runs with root privileges—executes the attacker's code.
Impact: This allows a local user to gain full root access to the entire server. 2. Optionsbleed (CVE-2017-9798)
This vulnerability is an information disclosure bug that earned its name due to similarities with the infamous Heartbleed flaw.
The Flaw: It is a use-after-free bug that occurs when the server processes an OPTIONS request.
The Exploit: If a webmaster uses the Limit directive with an invalid or custom HTTP method in a .htaccess file, the server can leak small chunks of its process memory in the "Allow" header of its response.
Impact: Remote attackers can repeatedly send OPTIONS requests to scrape sensitive data, such as passwords or secret keys, from the server's memory. 3. HTTP/2 and DoS Vulnerabilities
Version 2.4.18 was among the early versions to support the mod_http2 module, which introduced several stability issues.
Thread Blocking (CVE-2019-9517): A remote attacker can exploit a denial-of-service (DoS) vulnerability by flooding the connection with requests while never reading the responses. This exhausts the server's worker threads, causing the application to stop responding.
Slow Loris on H2 (CVE-2018-17189): In versions 2.4.37 and prior, sending request bodies in a "slow loris" fashion (extremely slowly) unnecessarily occupies server threads, leading to a DoS. Summary of Risks Requirement CVE-2019-0211 Privilege Escalation Local access + Graceful restart CVE-2017-9798 Information Disclosure Specific .htaccess config CVE-2019-9517 Denial of Service mod_http2 enabled Remediation
Security researchers from organizations like Tenable and the Apache Software Foundation recommend upgrading to the latest stable version of Apache 2.4.x (currently 2.4.62 or higher) to mitigate these risks. Version 2.4.18 is no longer considered secure for production environments exposed to the internet. CVE-2017-9798 Detail - NVD
Apache HTTP Server version 2.4.18 is susceptible to critical vulnerabilities, including CVE-2019-0211, which allows local privilege escalation to root, and multiple Denial of Service (DoS) flaws targeting HTTP/2 and module handling. Security advisories urge immediate upgrading to the latest stable release (2.4.60 or later) to mitigate these risks and associated "httpoxy" vulnerabilities. For comprehensive vulnerability details, consult Apache HTTPD: CVE-2019-0211: Use After Free - Rapid7
Apache HTTP Server version 2.4.18 is affected by several vulnerabilities, with CVE-2016-0736 CVE-2019-0211
being among the most notable. Below is a guide on how these vulnerabilities function and how to secure your server. 1. Cryptographic Padding Oracle (CVE-2016-0736) This vulnerability exists in the mod_session_crypto
module. It allows a remote attacker to decrypt and modify session data stored in a user's browser. Exploit-DB
: The module failed to verify the integrity of encrypted session data before decryption. Because it used CBC (Cipher Block Chaining) mode without authenticated encryption, it was susceptible to a Padding Oracle Attack Apache HTTP Server 2
: An attacker can gain unauthorized access by decrypting session cookies or forging new session data to impersonate users. Exploit Availability : Verified exploit scripts are available on platforms like Exploit-DB (EDB-ID: 40961) 2. Local Privilege Escalation (CVE-2019-0211) Often referred to as CARPE (DIEM)
, this flaw affects Apache 2.4.17 through 2.4.38 on Unix-based systems. Exploit-DB
: A vulnerability in how the "scoreboard" (shared memory used for worker communication) is handled. A low-privileged user (e.g.,
) who can execute code (via PHP or CGI) can manipulate the scoreboard. When the parent process performs a graceful restart, it can be tricked into executing arbitrary code with root privileges
: Full system compromise by escalating from a web user to the root user. Exploit Availability : A public proof-of-concept is available on Exploit-DB (EDB-ID: 46676) 3. HTTP Request Smuggling (CVE-2016-8743)
Apache 2.4.18 was overly "liberal" in how it handled whitespace in HTTP request headers. CVE Details Apache mod_session_crypto - Padding Oracle - Exploit-DB
Understanding the Apache HTTPD 2.4.18 Vulnerability Landscape
If you are running Apache HTTP Server version 2.4.18, you are operating on a version released in early 2016. In the world of web security, that is an eternity. While 2.4.18 was a stable release for its time, several high-risk vulnerabilities and functional exploits have been discovered in the years since. 1. Key Vulnerabilities (CVEs) affecting 2.4.18
While there isn't one single "silver bullet" exploit for 2.4.18, it is susceptible to several critical flaws that allow for Request Smuggling, Denial of Service (DoS), and Information Disclosure. CVE-2016-8743: Enforcing HTTP Response Correctness
This is one of the most significant issues discovered shortly after the 2.4.18 release. Apache was found to be too lenient in how it parsed HTTP response headers.
The Exploit: An attacker can inject malicious characters into headers.
The Impact: This leads to HTTP Request Smuggling or Cache Poisoning. If your Apache server sits behind a proxy or load balancer, an attacker can "smuggle" a second request inside a legitimate one, potentially bypassing security controls. CVE-2017-9798: "Optionsbleed"
This vulnerability affects the way Apache handles the LIMIT directive in .htaccess files.
The Exploit: By sending a specially crafted OPTIONS request to a server with a corrupted configuration, the server may leak small chunks of its memory.
The Impact: While it only leaks a few bytes at a time, repeated attempts can reveal sensitive process information or environment variables. CVE-2016-1546: mod_http2 Denial of Service Version 2.4.18 was early in Apache's support for HTTP/2. 3.1 CVE-2016-5387 – "HTTPOXY"
The Exploit: A flaw in the mod_http2 engine allowed an attacker to consume excessive CPU and memory by sending specific H2 stream patterns.
The Impact: A simple remote attacker could crash the web server or make it unresponsive to legitimate users (DoS). 2. Is there a "Remote Code Execution" (RCE) exploit?
Users often search for an RCE exploit for 2.4.18. While there is no widely known, direct "unauthenticated RCE" that works on a default configuration, version 2.4.18 is frequently targeted in Local Privilege Escalation (LPE) chains.
For example, if an attacker gains low-level access to your server (perhaps through a vulnerable PHP script), they can use vulnerabilities in older Apache binaries to gain Root access. A famous example is CVE-2019-0211, which allows a low-privilege child process to execute code as the parent (root) during a graceful restart. 3. How to Identify if You Are Vulnerable You can check your version quickly via the command line: httpd -v # or apache2 -v Use code with caution.
If the output shows Server version: Apache/2.4.18, you are missing nearly a decade of security patches. 4. Mitigation and Best Practices
The only responsible way to "fix" an exploit for version 2.4.18 is to move away from it.
Upgrade Immediately: Most modern Linux distributions (Ubuntu 20.04+, Debian 10+) provide much newer versions. Update your package manager: sudo apt-get update && sudo apt-get upgrade apache2 Use code with caution.
Disable Unused Modules: If you cannot upgrade immediately, reduce your attack surface by disabling mod_http2 and mod_proxy if they aren't strictly necessary.
Strict Header Parsing: Ensure your configuration includes HttpProtocolOptions Strict to mitigate request smuggling (though this was introduced in later patches).
Apache 2.4.18 is outdated and contains known flaws that allow for Request Smuggling and Denial of Service. Because exploits for these vulnerabilities are publicly available in frameworks like Metasploit, running this version on a public-facing server is a high risk.
Note on intent: This report is written for educational and defensive purposes. It analyzes the historical vulnerabilities associated with this specific version to help system administrators understand risks, patch management, and forensic indicators.
HTTP_PROXY environment variable injection via Proxy: headerhttpoxy scanner tools, Metasploit auxiliary module.mod_cgi or mod_cgid is disabled.For modern penetration testers, manual exploitation of 2.4.18 is inefficient. Tooling support includes:
auxiliary/scanner/http/apache_http_request_smuggling has detection logic for 2.4.18 patterns.http-request-smuggling.nse can fingerprint vulnerable proxy configurations.A typical Nmap scan to confirm presence:
nmap -sV --script=http-request-smuggling.nse -p 80,443 target.com
A typical low-skill attacker workflow against 2.4.18:
nmap -sV -p80 --script http-apache-negotiation <target>Server: Apache/2.4.18python3 httpoxy_scanner.py --url http://target/cgi-bin/test.cgicurl -X OPTIONS http://target/ -H "Limit: 0" → leak memory.Result: Information disclosure → privilege escalation on hosted application (e.g., WordPress plugins).
Report ID: INFOSEC-APR-2026-01 Date: April 23, 2026 Subject: Vulnerability assessment of Apache HTTP Server version 2.4.18