CVE-2026-91043 in mint
Summary
by MITRE • 09/28/2026
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 max_header_list_size 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 join_cookie_headers/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.
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Analysis
by VulDB Data Team • 09/28/2026
The vulnerability identified as an allocation of resources without limits or throttling represents a critical flaw in how the Elixir-based HTTP client library, Mint, processes incoming HTTP/2 header frames. This issue specifically impacts versions from 1.1.0 up to but not including 1.11.0. The core technical deficiency lies in the discrepancy between the enforcement of size limits on compressed data versus decoded data within the HPACK encoding scheme used by HTTP/2. While RFC 9113 section 6.5.2 mandates that servers and clients must limit the total size of header fields after decoding, Mint.HTTP2 incorrectly applies its max_header_list_size configuration only to the compressed wire format. This misalignment allows a malicious server to exploit the compression ratio inherent in HPACK to bypass client-side memory protections effectively.
The operational mechanism of this denial-of-service attack relies on the specific behavior of HPACK indexed fields and binary handling in Elixir. An HPACK indexed field can be as small as one byte when transmitted over the network, yet it may reference a dynamic table entry that expands into up to 4 KB of actual data upon decoding. Furthermore, the function join_cookie_headers/1 within lib/mint/http2.ex aggregates all cookie values from a response into a single new binary structure. Consequently, a header block that appears compliant with the default 256 KB wire limit can result in the client allocating approximately one gigabyte of memory for a single HTTP response. This massive expansion occurs because the library does not account for the potential size increase during decompression before enforcing limits.
The impact of this vulnerability is severe, leading to significant resource exhaustion on the host system running the Mint client. When multiple such oversized responses are received in quick succession within a single connection delivery, they rapidly consume available memory resources. This can exhaust the memory allocated to the specific process that owns the HTTP/2 connection or, depending on the application architecture and garbage collection settings, deplete the memory of the entire Erlang Virtual Machine. Such exhaustion results in a complete denial of service for the affected client applications, rendering them unresponsive or causing them to crash entirely due out-of-memory conditions.
From a classification perspective, this vulnerability aligns with CWE-770: Allocation of Resources Without Limits or Throttling and falls under MITRE ATT&CK technique T1499: Endpoint Denial of Service. The attack vector involves an adversary controlling the server-side response to manipulate resource consumption on the client side. To mitigate this risk, organizations using affected versions of Mint must upgrade immediately to version 1.11.0 or later where the header size limits are correctly enforced against the decoded data rather than just the compressed wire format. Until upgrading is possible, implementing network-level rate limiting or filtering for unusually large HTTP/2 frames may provide partial protection, though application-layer patching remains the definitive remediation strategy to ensure robust defense against this specific memory exhaustion vector.