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Plug: quadratic-time decoding of nested query/body parameters enables denial of service

High severity GitHub Reviewed Published Jun 23, 2026 in elixir-plug/plug • Updated Sep 23, 2026

Package

erlang plug (Erlang)

Affected versions

>= 1.15.0, < 1.15.5
>= 1.16.0, < 1.16.4
>= 1.17.0, < 1.17.2
>= 1.18.0, < 1.18.3
>= 1.19.0, < 1.19.3

Patched versions

1.15.5
1.16.4
1.17.2
1.18.3
1.19.3

Description

Summary

Plug's nested-parameter decoder (Plug.Conn.Query) parses URL-encoded keys in time quadratic in their bracket-nesting depth. Any unauthenticated remote attacker that can reach a Plug-based HTTP endpoint can pin a BEAM scheduler for minutes with a single small request.

Details

For a key like a[a][a]...=1, Plug.Conn.Query.split_keys/6 (in lib/plug/conn/query.ex) builds an accumulator of :binary.part prefixes (a, a[a], a[a][a], …) that grow ~3 bytes per level. Plug.Conn.Query.insert_keys/3 then does one Map.put per level keyed on that growing prefix, hashing the full byte range each time, and Plug.Conn.Query.finalize_pointer/2 repeats the prefix-keyed walk to materialize the structure. Total cost is O(N²) in nesting depth.

The same code path handles query strings, application/x-www-form-urlencoded bodies, and multipart field names via Plug.Conn.Query.decode/4 and decode_each/2. The default Plug.Parsers.URLENCODED cap is 1 MB (~333,000 nesting levels), but Plug.Parsers accepts urlencoded payloads up to its overall body limit (20 MB by default), so an attacker can scale the per-request work well beyond the urlencoded-specific cap. The decoder shows ~4× scaling per 2× input (16k levels ≈ 195 ms on a single scheduler).

PoC

  1. POST a[a][a]...[a]=1 as application/x-www-form-urlencoded to any endpoint of a Plug-based app. Even at the 1 MB urlencoded-parser default the payload carries ~333,000 nesting levels; with the broader Plug.Parsers body limit (20 MB default) a single request can carry millions.
  2. Launch one such request per scheduler concurrently. Each pins a scheduler for minutes; legitimate traffic stalls once all schedulers are busy.

Impact

A single low-bandwidth sender can render any internet-reachable Plug-based service (most Phoenix and standalone Elixir/Erlang web stacks) unresponsive. No credentials, specific endpoint, or prior knowledge of the application is required.

References

References

@josevalim josevalim published to elixir-plug/plug Jun 23, 2026
Published by the National Vulnerability Database Jun 23, 2026
Published to the GitHub Advisory Database Sep 23, 2026
Reviewed Sep 23, 2026
Last updated Sep 23, 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.
(60th percentile)

Weaknesses

Inefficient Algorithmic Complexity

An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached. Learn more on MITRE.

CVE ID

CVE-2026-54892

GHSA ID

GHSA-j43x-5hjq-rgxf

Source code

Credits

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