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Hackney: Per-chunk timeout with unbounded body accumulation enables slow-drip OOM

High severity GitHub Reviewed Published May 25, 2026 in benoitc/hackney • Updated Jun 30, 2026

Package

erlang hackney (Erlang)

Affected versions

>= 2.0.0, < 4.0.1

Patched versions

4.0.1

Description

Summary

hackney_h3:await_response_loop/6 in src/hackney_h3.erl accumulates the HTTP/3 response body in memory without any size cap. The after Timeout clause is a per-message inactivity timer, not a wall-clock deadline: every received stream_data chunk, housekeeping select message, or settings frame resets it. A malicious HTTP/3 server that drips one small chunk every Timeout - 1 ms with Fin = false and never terminates the stream keeps the loop alive indefinitely while the accumulation buffer grows without bound, eventually exhausting the BEAM process heap.

Details

In src/hackney_h3.erl, await_response_loop/6 (line 430) builds the body with:

NewBody = <<AccBody/binary, Data/binary>>

There is no max_body check and no monotonic deadline. The after Timeout clause at line 463 is restarted on each loop iteration. A server that ensures at least one message arrives within Timeout ms indefinitely (one small chunk per interval is sufficient) prevents the timeout from firing while AccBody grows linearly. The same module's wait_connected/3 (lines 388-389) shows the correct pattern: track an absolute start time and pass a shrinking Remaining budget into each receive. This loop does not.

Configurations

Only the HTTP/3 transport is affected. Applications using the default TCP/TLS hackney transport are not vulnerable. The vulnerability requires using hackney_h3 directly or passing {transport, h3} to hackney:request/5.

PoC

  1. Stand up an HTTP/3 server that responds with 200 OK headers (Fin = false), then emits a small stream_data chunk every Timeout - margin ms with Fin = false indefinitely.
  2. Issue hackney:request(get, Url, [], <<>>, [{transport, h3}]) against it.
  3. Watch the client process heap grow monotonically. The configured timeout never fires; the process is eventually killed by max_heap_size or the OS OOM killer.

Impact

Remote denial of service via unbounded memory consumption. Affects hackney 2.0.0 through 4.0.0 when using the HTTP/3 transport against an attacker-controlled or attacker-influenced server. Each affected request consumes unbounded memory until the BEAM is killed. CVSS v4.0: 8.2 (HIGH).

Resources

References

@benoitc benoitc published to benoitc/hackney May 25, 2026
Published by the National Vulnerability Database May 28, 2026
Published to the GitHub Advisory Database Jun 26, 2026
Reviewed Jun 26, 2026
Last updated Jun 30, 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.
(49th percentile)

Weaknesses

Improper Certificate Validation

The product does not validate, or incorrectly validates, a certificate. Learn more on MITRE.

Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource. Learn more on MITRE.

CVE ID

CVE-2026-47077

GHSA ID

GHSA-jq4m-q6p2-8gwc

Source code

Credits

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