Plug Multipart Header Parsing Denial-of-Service Vulnerability (CVE-2026-8468)

Plug, a popular web application library for Elixir, is susceptible to a denial-of-service vulnerability (CVE-2026-8468) within its multipart header parsing functionality. The vulnerability resides in the Plug.Conn.read_part_headers/2 function, which fails to enforce limits on the size of the accumulated buffer when parsing multipart/form-data requests. This flaw allows an unauthenticated attacker to send specially crafted HTTP requests containing excessively large multipart headers, leading to uncontrolled memory allocation on the server. By repeatedly sending such requests, an attacker can exhaust available memory resources, ultimately causing the server to crash or become unresponsive, resulting in a denial of service. Specifically, versions >= 1.4.0, < 1.15.4, versions >= 1.16.0, < 1.16.3, versions >= 1.17.0, < 1.17.1, versions >= 1.18.0, < 1.18.2 and versions >= 1.19.0, < 1.19.2 are affected.

Attack Chain

1. The attacker identifies a Plug-based web application that utilizes Plug.Parsers with the :multipart parser or calls Plug.Conn.read_part_headers/2 directly.
2. The attacker crafts a malicious HTTP request with the Content-Type header set to multipart/form-data.
3. Within the multipart data, the attacker constructs a part header with an excessively large size, exceeding expected limits. The attacker omits a closing boundary to continue the uncontrolled header accumulation.
4. The attacker sends the crafted HTTP request to the vulnerable endpoint of the Plug application.
5. The Plug.Conn.read_part_headers/2 function processes the request and begins accumulating the multipart header data without proper length validation.
6. The function continuously allocates memory to store the expanding header buffer, consuming available server resources.
7. The attacker repeats the process by sending multiple malicious requests, accelerating memory exhaustion.
8. Eventually, the server runs out of memory, causing the Plug application to crash or become unresponsive, resulting in a denial of service.

Impact

Successful exploitation of this vulnerability leads to a denial-of-service condition, rendering the affected Plug-based web application unavailable to legitimate users. The impact could range from temporary service disruptions to prolonged outages, depending on the severity of the memory exhaustion and the system’s recovery capabilities. The number of victims depends on the popularity and criticality of the affected applications. There is no evidence of widespread exploitation at this time.

Recommendation

• Upgrade to Plug version 1.15.4, 1.16.3, 1.17.1, 1.18.2, 1.19.2, or later, which includes the patch for CVE-2026-8468 (see References).
• Deploy the Sigma rule Detect Suspicious Multipart Form Request to identify and block requests with abnormally large multipart headers.
• Monitor web server logs for a high volume of multipart/form-data requests with unusually large header sizes.