Skip to content

mint: Unbounded streams map growth via PUSH_PROMISE without follow-up HEADERS

High severity GitHub Reviewed Published Jun 2, 2026 in elixir-mint/mint

Package

erlang mint (Erlang)

Affected versions

>= 0.2.0, < 1.9.0

Patched versions

1.9.0

Description

Summary

Mint's HTTP/2 client accepts PUSH_PROMISE frames from any server it connects to and inserts every promised stream into a per-connection map without consulting max_concurrent_streams. A malicious or compromised HTTP/2 server can flood the client with PUSH_PROMISE frames and withhold the matching response HEADERS, pinning one map entry per frame indefinitely until the client process runs out of memory.

Details

'Elixir.Mint.HTTP2':handle_push_promise/3 in lib/mint/http2.ex dispatches every inbound PUSH_PROMISE frame to 'Elixir.Mint.HTTP2':decode_push_promise_headers_and_add_response/5, which inserts a :reserved_remote entry into conn.streams for the promised ID. The only validation applied is 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. A server that emits PUSH_PROMISE frames and never sends the matching HEADERS never trips that check, and the existing tally counts only streams in open states, not :reserved_remote entries.

HTTP/2 server push is accepted by default (client_settings.enable_push defaults to true), so no application opt-in is required. A single long-lived HTTP/2 connection to a hostile server lets it pin one conn.streams entry per PUSH_PROMISE frame, with no upper bound.

PoC

  1. Stand up a raw TCP HTTP/2 server that completes the handshake and ACKs the client's SETTINGS.
  2. Wait for the client's request HEADERS and capture its odd stream ID.
  3. Send a flood of PUSH_PROMISE frames (flags = END_HEADERS) associated with the captured stream, each promising a fresh even stream ID and carrying a minimal HPACK-encoded header block.
  4. Never send the matching response HEADERS for any of the promised IDs.
  5. The client's conn.streams map grows by one entry per PUSH_PROMISE frame (~148 bytes/entry); memory grows linearly and the BEAM process eventually crashes with OOM.

Impact

Remote, unauthenticated denial-of-service against any process using Mint as an HTTP/2 client against an untrusted or attacker-influenced server. Server push is on by default, so no application code change can prevent it short of disabling push or upgrading. Affected populations include outbound HTTP/2 clients in web backends, webhook delivery systems, scrapers, federated and proxy components, and any service that follows redirects to third-party HTTP/2 origins.

Workarounds

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

Resources

References

@ericmj ericmj published to elixir-mint/mint Jun 2, 2026
Published by the National Vulnerability Database Jun 2, 2026
Published to the GitHub Advisory Database Jul 9, 2026
Reviewed Jul 9, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(31st percentile)

Weaknesses

Allocation of Resources Without Limits or Throttling

The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. Learn more on MITRE.

CVE ID

CVE-2026-48862

GHSA ID

GHSA-g586-ccqf-7x4r

Source code

Credits

Dependabot alerts are not supported on some or all of the ecosystems on this advisory.

Learn more about GitHub language support

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.