CVE-2015-3183

What it actually does

The chunked transfer coding implementation fails to properly parse chunk headers, allowing remote attackers to conduct HTTP request smuggling attacks via crafted requests that exploit mishandling of large chunk-size values and invalid chunk-extension characters.

Why it matters to the business

Attackers can bypass access controls or security filters on web applications, leading to unauthorized data access or actions that undermine application integrity and increase operational risk over time.

How an attack unfolds

An attacker crafts an HTTP request with malformed chunk headers containing large size values or invalid extensions; the server misparses the request, allowing smuggled follow-on requests to be processed as if they originated from the attacker, bypassing intended security boundaries.

How to fix it

Upgrade Apache HTTP Server to version 2.4.14 or later, which corrects the chunk header parsing logic in modules/http/http_filters.c.

How to confirm the fix worked

Verify the installed Apache version is 2.4.14 or newer; test with known chunked-encoding smuggling payloads to confirm the server no longer processes malformed requests incorrectly.

Typical attack path

1. Identify a target running vulnerable Apache HTTP Server
2. Craft HTTP request with oversized chunk-size or invalid chunk-extension characters
3. Transmit the request to trigger parsing failure
4. Smuggle additional HTTP requests that evade filters or access controls
5. Perform unauthorized actions such as bypassing authentication or injecting content

How to detect it

Examine web server access logs for requests using Transfer-Encoding: chunked with unusually large chunk sizes or non-standard extension characters; correlate sequences of requests that appear to have been smuggled together.

• Transfer-Encoding: chunked with chunk-size values exceeding normal limits
• invalid characters in chunk-extension fields
• malformed chunked requests followed by unexpected subsequent requests

What your security team should do

• Prioritize upgrade of all Apache HTTP Server instances to 2.4.14 or later given high EPSS score
• Implement web application firewall rules to block suspicious chunked requests
• Review downstream web applications for additional request smuggling defenses
• Monitor logs for anomalous Transfer-Encoding patterns