Unbounded conn.streams growth in Mint HTTP/2 client via unenforced PUSH_PROMISE concurrency

Weakness Type (CWE)

CWE-770 — CWE-770 Allocation of Resources Without Limits or Throttling

CAPEC

CAPEC-130 — CAPEC-130 Excessive Allocation

CVSS 4.0 Score

8.2

HIGH

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Vulnerability description

Allocation of Resources Without Limits or Throttling vulnerability in elixir-mint Mint allows attacker-controlled HTTP/2 servers to exhaust memory in a Mint client via PUSH_PROMISE flooding.

In lib/mint/http2.ex, 'Elixir.Mint.HTTP2':decode_push_promise_headers_and_add_response/5 inserts a :reserved_remote entry into conn.streams for every promised stream ID. The neighbouring 'Elixir.Mint.HTTP2':assert_valid_promised_stream_id/2 only verifies that the promised ID is even and not already present; client_settings.max_concurrent_streams is not consulted at promise time. The concurrency cap is only checked when the response HEADERS for the promised stream arrive, so a server that emits PUSH_PROMISE frames and withholds the matching HEADERS never trips that check.

HTTP/2 server push is accepted by default (client_settings.enable_push defaults to true). A single long-lived HTTP/2 connection to a hostile server lets that server pin one conn.streams entry per PUSH_PROMISE frame it sends, with no upper bound, until the client process runs out of memory.

This issue affects mint: from 0.2.0 before 1.9.0.

Affected

`pkg:hex/mint`

Module	Source File	Routine
`Mint.HTTP2`	`lib/mint/http2.ex`	`Mint.HTTP2.handle_push_promise/3`
		`Mint.HTTP2.decode_push_promise_headers_and_add_response/5`

Status	Type	Version	Changes / Fixed in
affected	semver ^ⓘ	`0.2.0`	< `1.9.0`

`pkg:github/elixir-mint/mint`

Module	Source File	Routine
`Mint.HTTP2`	`lib/mint/http2.ex`	`Mint.HTTP2.handle_push_promise/3`
		`Mint.HTTP2.decode_push_promise_headers_and_add_response/5`

Status	Type	Version	Changes / Fixed in
affected	git ^ⓘ	`65c6394d05`	< `70b97b6a52`

Workarounds

Disable HTTP/2 server push on connections to untrusted servers by passing client_settings: [enable_push: false] to 'Elixir.Mint.HTTP':connect/4. This makes Mint reject any inbound PUSH_PROMISE frame with a PROTOCOL_ERROR before the vulnerable code path is reached.

References

https://github.com/elixir-mint/mint/security/advisories/GHSA-g586-ccqf-7x4r vendor-advisory related
https://osv.dev/vulnerability/EEF-CVE-2026-48862 related
https://github.com/elixir-mint/mint/commit/70b97b6a5209fb288b0e04d8e657dda26c59de67 patch

Credits

Finder: Peter Ullrich
Remediation developer: Eric Meadows-Jönsson
Analyst: Jonatan Männchen / EEF

CVE record as JSON: GET /cves/CVE-2026-48862.json
OSV record as JSON: GET /osv/EEF-CVE-2026-48862.json