Summary
wolfSSH forwarding channels may be accepted without applying the configured authorization policy or verifying that the requested forwarding was previously registered. A malicious SSH peer could abuse this behavior to create unauthorized forwarding channels and potentially cause excessive memory allocation or denial of service when forwarding support is enabled.
wolfSSH forwarding channels may be accepted without applying the configured authorization policy or verifying that the requested forwarding was previously registered. A malicious SSH peer could abuse this behavior to create unauthorized forwarding channels and potentially cause excessive memory allocation or denial of service when forwarding support is enabled.
Affected context
The issue occurs in wolfSSH deployments built with forwarding support enabled. It affects channel-open processing when forwarded-tcpip channels are admitted without the same authorization checks applied to other forwarding channel types, and when clients do not verify that requested forwarding matches an earlier tcpip-forward registration.
The issue occurs in wolfSSH deployments built with forwarding support enabled. It affects channel-open processing when forwarded-tcpip channels are admitted without the same authorization checks applied to other forwarding channel types, and when clients do not verify that requested forwarding matches an earlier tcpip-forward registration.
Exploit preconditions
- Attack surface: SSH channel-opening and TCP forwarding functionality
- Required access: Ability to connect as, or act as, an SSH peer accepted by the endpoint
- Trigger point: Processing a
forwarded-tcpipchannel-open request - Configuration condition: Forwarding support must be enabled
- Why exploitation is possible: The channel type can bypass the forwarding policy callback, may not be limited in quantity, and client-side requests may not be checked against registered forwarding destinations
- Attack surface: SSH channel-opening and TCP forwarding functionality
- Required access: Ability to connect as, or act as, an SSH peer accepted by the endpoint
- Trigger point: Processing a
forwarded-tcpipchannel-open request - Configuration condition: Forwarding support must be enabled
- Why exploitation is possible: The channel type can bypass the forwarding policy callback, may not be limited in quantity, and client-side requests may not be checked against registered forwarding destinations
Why this happens
The underlying bug is an authorization-control gap in protocol state handling. The channel-open implementation applies the forwarding policy callback to direct-tcpip channels but does not consistently enforce equivalent authorization for forwarded-tcpip channels. In addition, the client does not bind incoming forwarding channels to the addresses and ports authorized by a prior forwarding request, mapping to CWE-862: Missing Authorization and CWE-863: Incorrect Authorization.
The underlying bug is an authorization-control gap in protocol state handling. The channel-open implementation applies the forwarding policy callback to direct-tcpip channels but does not consistently enforce equivalent authorization for forwarded-tcpip channels. In addition, the client does not bind incoming forwarding channels to the addresses and ports authorized by a prior forwarding request, mapping to CWE-862: Missing Authorization and CWE-863: Incorrect Authorization.
Potential impact
- An attacker may establish forwarding channels that the application did not authorize
- An endpoint could allocate excessive per-channel buffers and experience resource exhaustion
- A service may suffer denial of service through unbounded or insufficiently constrained channel creation
- Network access may be extended to destinations or ports outside the intended forwarding policy
- Security monitoring and network segmentation assumptions could be bypassed
- An attacker may establish forwarding channels that the application did not authorize
- An endpoint could allocate excessive per-channel buffers and experience resource exhaustion
- A service may suffer denial of service through unbounded or insufficiently constrained channel creation
- Network access may be extended to destinations or ports outside the intended forwarding policy
- Security monitoring and network segmentation assumptions could be bypassed
General mitigation guidance
Mitigation generally focuses on enforcing authorization consistently across every forwarding channel type and binding protocol actions to explicit application policy.
Common defensive patterns include:
- Applying the forwarding policy callback to all relevant channel-open requests
- Verifying incoming forwarding channels against the exact address and port registrations maintained by the client
- Enforcing bounded limits on concurrent channels, buffers, and forwarding resources
- Disabling forwarding when an application does not require it
- Running SSH services with least-privilege identities and narrowly scoped network access
- Treating peer-supplied forwarding parameters as untrusted until authorization and state checks succeed
Mitigation generally focuses on enforcing authorization consistently across every forwarding channel type and binding protocol actions to explicit application policy.
Common defensive patterns include:
- Applying the forwarding policy callback to all relevant channel-open requests
- Verifying incoming forwarding channels against the exact address and port registrations maintained by the client
- Enforcing bounded limits on concurrent channels, buffers, and forwarding resources
- Disabling forwarding when an application does not require it
- Running SSH services with least-privilege identities and narrowly scoped network access
- Treating peer-supplied forwarding parameters as untrusted until authorization and state checks succeed
How teams detect issues like this
Organizations can identify this class of issue through:
- Static analysis and code review of channel-dispatch paths for authorization checks applied inconsistently by channel type
- Review of state validation connecting channel-open requests to previously registered forwarding requests
- Tests that submit unauthorized
forwarded-tcpiprequests and verify rejection - Policy checks for deployments that enable forwarding without explicit resource limits
- Runtime monitoring for unexpected forwarding destinations, unusual channel counts, rapid channel creation, or abnormal buffer growth
- Automated security review tools that flag missing authorization and unchecked resource allocation paths
Organizations can identify this class of issue through:
- Static analysis and code review of channel-dispatch paths for authorization checks applied inconsistently by channel type
- Review of state validation connecting channel-open requests to previously registered forwarding requests
- Tests that submit unauthorized
forwarded-tcpiprequests and verify rejection - Policy checks for deployments that enable forwarding without explicit resource limits
- Runtime monitoring for unexpected forwarding destinations, unusual channel counts, rapid channel creation, or abnormal buffer growth
- Automated security review tools that flag missing authorization and unchecked resource allocation paths
