Skip to content

Lemmy user purging users or communities or banning users can delete images they didn't upload/exclusively use

Moderate severity GitHub Reviewed Published Apr 8, 2025 in LemmyNet/lemmy • Updated Apr 8, 2025

Package

cargo lemmy_server (Rust)

Affected versions

>= 0.17.0, < 0.19.11

Patched versions

0.19.11

Description

Summary

An improper uploaded media ownership check can result in inadvertent deletion of media when a user is banned with content removal or purged. This can lead to deletion of media that was not uploaded by the banned/purged user. This also applies to purged communities, in which case all media posted in that community will get deleted without proper ownership check.
This is limited to media with an image/* content-type returned by pict-rs.

Details

Lemmy did not associate users with media uploads until version 0.19.0 (#3927).
Back when the first parts of content purging were implemented for 0.17.0 (#1809), it was therefore not possible to properly identify media belonging to a specific user for situations in which this data should get erased from pict-rs, Lemmy's media storage backend.

Pict-rs deduplicates uploaded files transparently. As a result, it has two types of media deletion. A regular deletion will only remove the referenced alias, and if there are not other aliases pointing to the same file, the backing file will also be deleted. A purge on the other hand will delete all aliases pointing to the specified file, as well as the file itself.

The logic implemented in 0.17.0 iterated over media URLs related to users and communities when purging them and purged them from pict-rs. This results in a full deletion of the backing media, even if either the same URL was the result of an upload by a different user, or the same media being uploaded by another user with a different alias.
For user purges, Lemmy iterated over all posts they created and applied this to all media referenced in post URLs and post thumbnails. For community purges, this applied to all posts within this community.

Additionally, the deletion of user avatars, banners, as well as the media from all their posts was implemented when users were banned with content removal. This includes local bans and also bans received via federation, when a user gets banned on their home instance.

The function for purging images from pict-rs performs a check at the start to verify that the media Content-Type header returned by pict-rs starts with image/, which limits this to not affect other media types supported by Lemmy and pict-rs, such as videos.

Impact

Instances with open federation

The vast majority of Lemmy instances has open federation, which means that this can be exploited remotely without any authentication.

Instances with limited or no federation

Exploitation requires user interaction by an admin of the targeted instance or a federation-linked instance if federation is enabled.
It may also require authentication, as instances may not have open registrations.

References

@dessalines dessalines published to LemmyNet/lemmy Apr 8, 2025
Published to the GitHub Advisory Database Apr 8, 2025
Reviewed Apr 8, 2025
Last updated Apr 8, 2025

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 None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity Low
Availability None
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:L/VA:N/SC:N/SI:N/SA:N

EPSS score

Weaknesses

CVE ID

No known CVE

GHSA ID

GHSA-wr2m-38xh-rpc9

Source code

Credits

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