Microsoft has confirmed that Azure Linux includes a vulnerable version of the Apache HTTP Server, leaving the door open to HTTP response splitting attacks that can poison caches, hijack sessions, or smuggle malicious requests across an entire web infrastructure. The vulnerability, tracked as CVE-2024-42516, was reported in July 2024 and finally patched by the Apache Foundation on July 10, 2025, in version 2.4.64. But the story doesn’t end with an Azure Linux advisory — any Windows user running Apache locally or any organization using Apache as a reverse proxy needs to check their systems now.
What CVE-2024-42516 Actually Does
CVE-2024-42516 is an HTTP response splitting flaw that lives in the core of Apache HTTP Server. When an attacker can control or influence the Content-Type header of a response from a backend application — maybe through a poorly sanitized input in a proxied service — they can inject carriage-return and line-feed (CRLF) characters. Apache, if not properly guarding against these sequences, will treat the injected content as a full-blown HTTP response, splitting the output and allowing the attacker to smuggle additional response headers and body content into the stream.
The result is a trifecta of possible attacks: cache poisoning, where shared caches or reverse proxies serve corrupted content to many users; cross-site scripting, if the injected payload includes malicious scripts; and request smuggling, where one user’s request gets paired with another’s response, breaking session integrity. In environments where Apache acts as a reverse proxy — think mod_proxy, mod_proxy_fcgi, or mod_proxy_http2 — the damage radius can be especially wide.
Affected versions span from Apache HTTP Server 2.4.0 all the way through 2.4.63. The fix ships in version 2.4.64, which Apache released on July 10, 2025. This CVE is actually a redo of an earlier flaw, CVE-2023-38709, whose patch in 2.4.59 turned out to be incomplete — so the threat has been lurking longer than many defenders realize.
Microsoft’s Advisory: What Azure Linux Users Need to Know
On the same day Apache published its advisory, Microsoft’s Security Response Center updated its own CVE entry for CVE-2024-42516. The message was straightforward: “Azure Linux includes this open‑source library and is therefore potentially affected.” That single sentence has triggered a flood of questions about what else inside Microsoft’s enormous product catalog might be vulnerable.
Let’s be clear: the MSRC statement is a product-specific inventory confirmation, not a blanket exoneration of every other Microsoft offering. It tells you that Azure Linux — Microsoft’s own Linux distribution for edge and cloud workloads — bundles the vulnerable httpd package. It does not say that Windows Server, Windows 11’s WSL environments, Azure Marketplace images, or GitHub Actions runner images are safe. Those remain unverified until Microsoft publishes machine-readable CSAF/VEX attestations, a program the company promised will begin in October 2025.
Practically, this means if you run any Microsoft-supplied artifact that might embed Apache — from container base images in Azure Container Registry to third-party appliances in the Marketplace — you cannot assume you’re in the clear just because Azure Linux is the only product explicitly mentioned. The advisory is a starting point for your own audit, not the finishing line.
Who’s Exposed and What’s at Stake
Windows Users Running Apache
Millions of developers and hobbyists run Apache on Windows through packages like XAMPP, WampServer, or direct installations. If you’re using Apache 2.4.63 or earlier, you’re vulnerable. The fix is the same regardless of OS: upgrade to 2.4.64. Windows binaries are available from the Apache Lounge and other trusted distributors. Don’t overlook WSL instances — if you’ve installed Apache inside a Linux distribution under Windows Subsystem for Linux, that instance needs patching just like a native Linux box.
Azure and Cloud Administrators
If your organization deploys Azure Linux VMs or containers, check every image immediately. Use package managers (apt, yum, zypper) to inspect the installed httpd version. For containers, rebuild images with updated base layers, or replace them with vendor images that already include the patched httpd. Also scan Azure Marketplace images and any CI/CD runner images your pipelines rely on; these can quietly bundle server packages that go unpatched for months.
Enterprise IT and Reverse-Proxy Deployments
Organizations using Apache as a reverse proxy — including those that front internal apps, load balancers, or API gateways — face the most severe risk. A single poisoned response can ripple through downstream caches and clients. If you cannot patch immediately, deploy aggressive WAF rules to strip CR/LF characters from incoming headers, and configure your proxy to set the Content-Type header explicitly rather than trusting whatever the backend sends. But treat these as temporary shields; the only true fix is the code patch.
How We Got Here: A Flaw That Outlasted Its First Patch
The timeline of CVE-2024-42516 is a lesson in how stubborn response-splitting bugs can be. The original issue was reported on July 18, 2024, and assigned CVE-2023-38709. Apache shipped a fix in version 2.4.59, but after further review, the security team found that the fix didn’t fully close the door — crafted headers could still trigger the split. Rather than rev the old CVE, Apache assigned a new one, CVE-2024-42516, and worked on a more thorough patch.
That second fix landed on July 7, 2025, in the development branch, and was publicly announced three days later with the release of 2.4.64. This means the vulnerability has been known to attackers for nearly a year, and while exploitation in the wild hasn’t been confirmed publicly, the window of exposure for unpatched systems is unusually long. The delay between the incomplete fix and the final one is a stark reminder that even high-profile open-source projects can ship patches that need a second look.
What to Do Right Now
1. Patch, patch, patch:
- If you control the Apache installation, upgrade to version 2.4.64 directly from Apache or your Linux distribution. Debian, Ubuntu, Red Hat, and Amazon Linux have all begun shipping updates that include this fix.
- On Windows, download the latest 64-bit binaries from Apache Lounge or your preferred provider, and run the upgrade wizard. Verify the version string after installation with httpd -v.
- For environments where you don’t control the binary but use a managed service, contact your provider and ask for confirmation that they’ve applied the patch.
2. Scan everything:
- Use vulnerability scanners like Tenable Nessus, Qualys, or open-source tools like Trivy to detect httpd packages across your VMs, containers, and cloud images. Focus on any image or host that might run a web server, even if you think it’s only used internally.
- For Azure users, enable Azure Defender for Containers to scan images in Azure Container Registry. If you use GitHub Actions or Azure DevOps, check the runner images’ software inventories — hosted agents often include httpd.
3. Mitigate where you can’t patch immediately:
- Deploy mod_security with rules that block HTTP requests and responses containing CRLF sequences in headers. Commercial WAFs can do the same.
- At the reverse-proxy layer, explicitly set the Content-Type header based on a whitelist, rather than passing through whatever the backend emits. For example, in mod_headers: Header always set Content-Type “application/octet-stream” can act as a temporary safeguard.
- Consider disabling HTTP/1.1 keep-alive or enabling strict request smuggling detection if your architecture supports it, though this is a deeper change that requires testing.
4. Don’t trust the absence of an advisory as proof of safety.
- For any Microsoft-supplied artifact not explicitly listed in the MSRC advisory, assume it is unverified. Until Microsoft’s VEX program delivers machine-readable attestations for its full portfolio — expected to ramp up in October 2025 — your best defense is your own inspection.
- If you run third-party appliances from the Azure Marketplace, contact the publisher and ask for their response to CVE-2024-42516. Many will have their own update schedules.
Outlook: Supply-Chain Transparency Is Coming, but Slowly
CVE-2024-42516 is not the most technically exotic bug, but it exposes a soft underbelly of modern cloud operations: we rely on layers of software we didn’t compile ourselves, and we trust vendors to tell us when those layers break. Microsoft’s move to publish VEX documents for Azure Linux is a step forward, but the gap between a single-product attestation and a complete product mapping leaves defenders in an uncomfortable middle ground.
Until that mapping arrives, the burden of verification falls on you. The good news is that the fix is simple and available, and the attack surface is well-defined — if you know where to look. The bad news is that many organizations don’t know all the places Apache httpd might be hiding in their estate, from long-forgotten dev VMs to container base images that haven’t been rebuilt in months.
Treat this as a chance to tighten image hygiene, audit your software inventory, and build automated checks into your CI/CD pipelines. The next vulnerability might be harder to patch, but the discipline you build today will make it much less painful when it arrives.