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Hackney has an infinite loop on non-token byte at start of an Alt-Svc entry

High severity GitHub Reviewed Published May 25, 2026 in benoitc/hackney • Updated Jun 26, 2026

Package

erlang hackney (Erlang)

Affected versions

>= 2.0.0, < 4.0.1

Patched versions

4.0.1

Description

Summary

CVE-2026-47066 is an infinite loop (CWE-835) in hackney's Alt-Svc response header parser (src/hackney_altsvc.erl). When an HTTP server returns an Alt-Svc header whose value begins with a non-token byte (e.g. !, @, =, ;), the parser enters a tight tail-recursive loop that pins an Erlang scheduler at 100% CPU and permanently hangs the calling connection process. Because the parser is invoked synchronously on every HTTP response, any attacker-controlled origin can trigger the hang with a single-byte header value.

Details

1. Parser dispatch

parse_and_cache/3 is called inside the hackney connection process on each HTTP response. It collects all Alt-Svc header values via collect_altsvc_headers/1, concatenates them, and passes the result to parse/1, which calls parse_entries(Header, []).

2. Failed token consumption

parse_entries/2 → parse_entry/1 → parse_protocol/1 → parse_token(Data, <<>>). The function parse_token/2 pattern-matches leading bytes: alphanumeric, -, _, whitespace, and comma all have explicit clauses. Any other byte (e.g. !) falls through to the catch-all:

parse_token(Rest, <<>>) -> {undefined, Rest}.

This returns the input unchanged — no byte is consumed.

3. No-progress loop

parse_entry propagates {undefined, Rest} back to parse_entries/2, which calls skip_comma(Rest). Because the first byte is not ,, skip_comma also returns Rest unchanged. parse_entries then recurses with the identical buffer:

parse_entries(Data, Acc)  % Data identical to previous iteration

Erlang tail recursion never preempts on a pure CPU loop, so the scheduler is pinned and the process never yields or returns.

4. Root cause

parse_entries/2 has no guard that detects zero-byte progress after a failed parse_entry call and no fallback to advance past the offending byte.

PoC

  1. Start an HTTP server that responds with the header Alt-Svc: ! (any single non-token byte suffices).
  2. Issue any HTTP GET request via hackney to that server: 
    hackney:request(get, "http://attacker.example/", [], <<>>, [])
  3. Observe that the call never returns; the Erlang scheduler hosting the connection process is pinned at 100% CPU indefinitely.

Alternatively, call the parser directly: hackney_altsvc:parse(<<"!">>) — the process hangs immediately.

Impact

Denial of service via unbounded CPU consumption. Any application using hackney 2.0.0-beta.1 through 4.0.0 that connects to attacker-controlled HTTP endpoints is affected. No authentication is required; a single response header byte is sufficient to hang the connection process. Fixed in hackney 4.0.1. CVSS v4.0 score: 8.7 (HIGH).

Resources

References

@benoitc benoitc published to benoitc/hackney May 25, 2026
Published by the National Vulnerability Database May 25, 2026
Published to the GitHub Advisory Database Jun 26, 2026
Reviewed Jun 26, 2026
Last updated Jun 26, 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 None
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:N/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

Loop with Unreachable Exit Condition ('Infinite Loop')

The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop. Learn more on MITRE.

CVE ID

CVE-2026-47066

GHSA ID

GHSA-6cp8-v795-jr2j

Source code

Credits

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