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Incomplete fix for CVE-2026-44838: `escape_regex_char/1` does not escape `-`, leaving room for an MQTT topic permission bypass

Moderate
michaelklishin published GHSA-q46v-hrvq-hp24 Jul 23, 2026

Package

rabbitmq-server (RabbitMQ)

Affected versions

>= 4.3.2, < 4.3.3
>= 4.2.0, < 4.2.9
>= 4.1.0, < 4.1.14
>= 4.0.0, < 4.0.23

Patched versions

4.3.3
4.2.9
4.1.14
4.0.23

Description

Affected product

RabbitMQ — internal authorization backend + rabbitmq_mqtt plugin topic permissions

Summary

The CVE-2026-44838 fix made expand_topic_permission/2 escape regex metacharacters in expanded topic-permission variables (escape_regex(V)), but escape_regex_char/1 escapes \ ^ $ . | ? * + ( ) [ ] { } and omits -. When a topic permission template places {client_id} inside a [...] character class (e.g. ^[{client_id}]-sensors$), an MQTT client using client_id = a-z expands the regex to ^[a-z]-sensors$, turning the class into a range and broadening topic authorization to b-sensors, m-sensors, etc. The repository's own OAuth2 scope-expansion code explicitly escapes - for exactly this reason.

Affected code

  • deps/rabbit/src/rabbit_auth_backend_internal.erl → escape_regex_char/1 (no $-).
  • deps/rabbitmq_mqtt/src/rabbit_mqtt_processor.erl → injects user-controlled client_id into the topic variable map; read and write checks share the same backend expansion.

Steps to reproduce

Using the provided reproduction materials, With set_topic_permissions ... '^[{client_id}]-sensors$':

  • mosquitto_pub -i 'a-z' -t 'b-sensors' → PUBACK (bypass);
  • mosquitto_pub -i 'x' -t 'b-sensors' → denied (control).

(Runtime-confirmed on official rabbitmq:4.3.2-management; a single benign placeholder message was published.)

Impact

A low-privileged authenticated MQTT user controlling its client_id can broaden topic authorization (read and write) when templates embed {client_id} in a [...] class. Narrower than the original raw-regex injection (requires that template shape), but RabbitMQ supports regex topic permissions and its OAuth2 tests already treat this range class as security-relevant.

Suggested remediation

Add $- to escape_regex_char/1, matching the OAuth2 implementation. Add a regression test (analogous to OAuth2 crafted_value_cannot_form_range_inside_char_class): expand_topic_permission(<<"^[{client_id}]-sensors$">>, #{<<"client_id">> => <<"a-z">>}) should yield <<"^[a\\-z]-sensors$">>, and b-sensors must be denied.

Credit

Reported as part of an incomplete-patch measurement study (responsible disclosure).

Severity

Moderate

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 Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality Low
Integrity High
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:L/UI:N/VC:N/VI:N/VA:N/SC:L/SI:H/SA:N

CVE ID

No known CVE

Weaknesses

No CWEs

Credits