Summary
Element Web and Element Desktop may accept guest credentials supplied through URL fragment parameters when loading a session. An attacker can trick a logged-in user into opening a crafted link, potentially forcing a guest session, logging the user out, and clearing locally stored data or unbacked end-to-end encryption keys.
Element Web and Element Desktop may accept guest credentials supplied through URL fragment parameters when loading a session. An attacker can trick a logged-in user into opening a crafted link, potentially forcing a guest session, logging the user out, and clearing locally stored data or unbacked end-to-end encryption keys.
Affected context
The issue occurs in Element Web and Element Desktop deployments that use the affected session-loading behavior and process guest credentials from URL fragments. Risk increases when a logged-in user opens an attacker-controlled link and locally stored encryption keys are not backed up or synchronized elsewhere.
The issue occurs in Element Web and Element Desktop deployments that use the affected session-loading behavior and process guest credentials from URL fragments. Risk increases when a logged-in user opens an attacker-controlled link and locally stored encryption keys are not backed up or synchronized elsewhere.
Exploit preconditions
- Attack surface: Web or desktop session-loading functionality
- Required access: No authenticated access to the target account is required
- Trigger point: A logged-in user opens a crafted link containing guest session parameters
- Why exploitation is possible: The application may trust URL fragment values as guest credentials and replace the active session without sufficient user intent or origin validation
- Attack surface: Web or desktop session-loading functionality
- Required access: No authenticated access to the target account is required
- Trigger point: A logged-in user opens a crafted link containing guest session parameters
- Why exploitation is possible: The application may trust URL fragment values as guest credentials and replace the active session without sufficient user intent or origin validation
Why this happens
The underlying issue is a cross-site request forgery pattern in session state management. The application accepts attacker-influenced URL fragment parameters and uses them to alter the user's active session, rather than treating the data as untrusted and requiring an explicit, validated transition.
This maps to CWE-352: Cross-Site Request Forgery, where an attacker causes a user's browser or client to perform an unintended state-changing action using the user's existing session context.
The underlying issue is a cross-site request forgery pattern in session state management. The application accepts attacker-influenced URL fragment parameters and uses them to alter the user's active session, rather than treating the data as untrusted and requiring an explicit, validated transition.
This maps to CWE-352: Cross-Site Request Forgery, where an attacker causes a user's browser or client to perform an unintended state-changing action using the user's existing session context.
Potential impact
- Forced logout from the legitimate user session
- Clearing of local application stores
- Loss of unbacked end-to-end encryption keys
- Temporary denial of access to locally available conversations or data
- User confusion, disruption, or social-engineering impact
- Forced logout from the legitimate user session
- Clearing of local application stores
- Loss of unbacked end-to-end encryption keys
- Temporary denial of access to locally available conversations or data
- User confusion, disruption, or social-engineering impact
General mitigation guidance
Mitigation generally focuses on preventing URL-controlled data from silently changing authenticated session state.
Common defensive patterns include:
- Do not accept guest credentials from untrusted URL fragments by default
- Require explicit user confirmation before switching or replacing an active session
- Validate the source, structure, and intended purpose of session-transition data
- Use narrowly scoped, single-use, and time-limited session handoff tokens where such flows are necessary
- Preserve or back up encryption keys before destructive session or storage operations
- Isolate guest-session storage from authenticated-user storage
- Apply least-privilege access to local stores and minimize destructive cleanup behavior
Mitigation generally focuses on preventing URL-controlled data from silently changing authenticated session state.
Common defensive patterns include:
- Do not accept guest credentials from untrusted URL fragments by default
- Require explicit user confirmation before switching or replacing an active session
- Validate the source, structure, and intended purpose of session-transition data
- Use narrowly scoped, single-use, and time-limited session handoff tokens where such flows are necessary
- Preserve or back up encryption keys before destructive session or storage operations
- Isolate guest-session storage from authenticated-user storage
- Apply least-privilege access to local stores and minimize destructive cleanup behavior
How teams detect issues like this
Organizations can assess this class of issue through:
- Static analysis for session changes driven directly by URL parameters
- Code review checks for authentication, logout, storage-clearing, or account-switching actions triggered without explicit user intent
- Browser testing with cross-origin links and attacker-controlled fragment values
- Policy enforcement requiring origin validation and confirmation for sensitive session transitions
- Automated security testing that verifies authenticated state and local encryption material remain intact when crafted links are opened
Organizations can assess this class of issue through:
- Static analysis for session changes driven directly by URL parameters
- Code review checks for authentication, logout, storage-clearing, or account-switching actions triggered without explicit user intent
- Browser testing with cross-origin links and attacker-controlled fragment values
- Policy enforcement requiring origin validation and confirmation for sensitive session transitions
- Automated security testing that verifies authenticated state and local encryption material remain intact when crafted links are opened
