CVE-2026-91043: HPACK-indexed cookie fields in Mint HTTP/2 responses bypass max_header_list_size and exhaust client memory
Allocation of Resources Without Limits or Throttling vulnerability in elixir-mint mint allows a malicious HTTP/2 server to exhaust memory on the client host and cause a denial of service.
Mint.HTTP2 enforces the client's maxheaderlistsize setting only on the compressed size of an inbound header block, while RFC 9113 section 6.5.2 defines the limit on the decoded header list. An HPACK indexed field costs one byte on the wire and decodes to a dynamic table entry of up to 4 KB, and joincookieheaders/1 in lib/mint/http2.ex copies every cookie value of a response into one new binary. A header block under the default 256 KB wire limit therefore makes the client allocate about 1 GB for a single response, and several such responses in one delivery exhaust the memory of the process that owns the connection or of the whole VM.
This issue affects mint: from 1.1.0 before 1.11.0.
Affected Software
Event History
Frequently Asked Questions
Which deployments are exposed?
Clients using Mint versions 1.1.0 through before 1.11.0 for HTTP/2 connections are affected. Exposure requires the client to process responses from a malicious HTTP/2 server.
What does an attacker need to do to trigger the denial of service?
The attacker needs to control, or be able to serve responses through, an HTTP/2 endpoint contacted by the Mint client. They can send HPACK-indexed cookie fields that remain below the configured compressed header-block limit but expand substantially after decoding.
Are default settings affected?
Yes. Under the default 256 KB wire-level max_header_list_size limit, a single response can cause roughly 1 GB of allocation. Multiple such responses in one delivery can exhaust the owning process's memory or the entire VM.
What is the fixed version?
Upgrade Mint to version 1.11.0 or later. The affected range is from 1.1.0 before 1.11.0.