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Canonical LXD Arbitrary File Read via Template Injection in Snapshot Patterns

High severity GitHub Reviewed Published Oct 2, 2025 in canonical/lxd • Updated Oct 2, 2025

Package

gomod github.com/lxc/lxd (Go)

Affected versions

>= 4.0, < 5.21.4
>= 6.0, < 6.5
>= 0.0.0-20200331193331-03aab09f5b5c, < 0.0.0-20250827065555-0494f5d47e41

Patched versions

5.21.4
6.5
0.0.0-20250827065555-0494f5d47e41

Description

Impact

In LXD's instance snapshot creation functionality, the Pongo2 template engine is used in the snapshots.pattern configuration for generating snapshot names. While code execution functionality has not been found in this template engine, it has file reading capabilities, creating a vulnerability that allows arbitrary file reading through template injection attacks.

Reproduction Steps

  1. Log in to LXD-UI with an account that has permissions to modify instance settings
  2. Set the following template injection payload in the instance snapshot pattern:
{% filter urlencode|slice:":100" %}{% include "/etc/passwd" %}{%endfilter %}

Note that the above template uses the Pongo2 template engine's include tag to read system files. It also uses urlencode and slice filters to bypass character count and type restrictions.

  1. Set scheduled snapshots to run every minute and wait for snapshot generation
  2. Wait about a minute and confirm that file contents can be obtained from the created snapshot name

Risk

The attack requires having configuration change permissions for LXD instances.
The attack allows reading arbitrary files accessible with LXD process permissions. This could lead to leakage of the following information:
-​ LXD host configuration files (/etc/passwd, /etc/shadow, etc.)
-​ LXD database files (containing information about all projects and instances)
-​ Configuration files and data of other instances
-​ Sensitive information on the host system

Countermeasures

Pongo2 provides mechanisms for sandboxing templates.

Template sandboxing (directory patterns, banned tags/filters)
( https://github.com/flosch/pongo2/tree/master?tab=readme-ov-file#features )

This functionality allows banning specific tags and filters by generating a custom TemplateSet.

At minimum, the following tags are considered to pose a risk of file leakage on the LXD host when used. Therefore, banning these can provide countermeasures against file reading attacks.
-​ include
-​ ssi
-​ extends
-​ import

The deny-list approach is prone to vulnerability recurrence due to missed countermeasures or new feature additions. Therefore, as the safest approach, we recommend using an allow-list format to permit only necessary functions.

However, as far as our investigation shows, pongo2 does not have functionality to retrieve a list of registered tags or filters, nor does it provide means to implement an allow-list approach. Therefore, it is necessary to either forcibly obtain the registration list through reflection and ban anything not on the allow-list, or ban everything from the current implemented list since the library has not been updated for about two years.

In LXD's implementation, template injection attacks can be prevented by modifying the RenderTemplate function in shared/util.go to use a restricted TemplateSet as shown above.

Patches

LXD Series Status
6 Fixed in LXD 6.5
5.21 Fixed in LXD 5.21.4
5.0 Ignored - Not critical
4.0 Ignored - EOL and not critical

References

Reported by GMO Flatt Security Inc.

References

@tomponline tomponline published to canonical/lxd Oct 2, 2025
Published by the National Vulnerability Database Oct 2, 2025
Published to the GitHub Advisory Database Oct 2, 2025
Reviewed Oct 2, 2025
Last updated Oct 2, 2025

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 Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity None
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:L/UI:N/VC:H/VI:N/VA:N/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.
(11th percentile)

Weaknesses

Improper Neutralization of Special Elements Used in a Template Engine

The product uses a template engine to insert or process externally-influenced input, but it does not neutralize or incorrectly neutralizes special elements or syntax that can be interpreted as template expressions or other code directives when processed by the engine. Learn more on MITRE.

CVE ID

CVE-2025-54287

GHSA ID

GHSA-w2hg-2v4p-vmh6

Source code

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