No reverse path verification on an untrusted interface
At a glance
Base severity
Area
Platform
Evidence level
Benchmark reference
Reads the export, not the device
ConfigProof reads a configuration export. This control is evaluated against the objects in that file, and it reports what the file says. It does not connect to the device and sees no traffic, so a finding here describes the configuration, not what the firewall is doing right now.
Why it matters
Without unicast reverse path forwarding the ASA accepts packets whose source address could not have arrived on that interface, which lets spoofed traffic in.
How to fix it
Web interface
The steps below are written for the web interface. A finding raised on your own export carries the same steps with your object names filled in.
- In ASDM: Configuration > Firewall > Advanced > Anti-Spoofing, select the interface and Enable. Apply and save.
- On the CLI: ip verify reverse-path interface <nameif>, and write memory.
Command line
Cisco ASA CLI
configure terminalip verify reverse-path interface <nameif>endwrite memory
An example. Values in angle brackets are yours to supply; a finding on your own export fills in the names it read. Typed in configuration mode; write memory saves it, with no separate commit. Read every line first.
Read it back
show running-config ip verify reverse-pathValues to fill in1
- <nameif>
- The interface name (nameif)
Before you type it2
- Written from the ASA 9.x command reference and not run on a device: read each line before you paste it.
- In multiple context mode, type it in the context it applies to (changeto context <name>).
How to confirm the fix
- Run show running-config ip verify reverse-path and confirm the interface is listed.
Limits
- Asymmetric routing makes reverse path verification drop legitimate traffic; check the routing first.
Compliance references
CIS Cisco ASA 9.x Firewall Benchmark v1.1.0
Covers the whole recommendation
Where a control aligns with a recommendation of the CIS Cisco ASA 9.x Firewall Benchmark v1.1.0, that recommendation is cited as a reference. Only that version was checked. A reference is not a certification.
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