Publicly reachable TFTP servers can be abused as UDP reflectors: an attacker forges a request so it appears to come from a victim, and the server sends its reply to that victim. When the reply is larger than the request, the traffic is also amplified. Reducing exposure means disabling unneeded TFTP services and filtering spoofed or abusive UDP traffic; these protocol-level attacks are distinct from vulnerabilities in particular TFTP server products.
How TFTP reflection works
TFTP uses UDP, which does not establish a connection before data is sent. If an attacker can send a datagram with a forged source IP address, the attacker can make a request to a reachable TFTP server that appears to come from the intended victim. The server replies to the address in the request, directing its response traffic to the victim rather than the attacker.
When many servers are used this way, their replies combine into a distributed reflective denial-of-service attack, or DRDoS. CISA describes DRDoS as a DDoS pattern that relies on publicly accessible UDP servers and bandwidth amplification factors to overwhelm a victim with UDP traffic. The attacker can conceal the true source of the requests behind the spoofed address, while the victim receives responses from the servers.
Reflection and amplification are related, but different
- Reflection is about where traffic goes: the forged source address causes a server to send its reply to the victim.
- Amplification is about traffic size: a server’s response contains more bytes than the request that triggered it, increasing the traffic delivered to the victim.
CISA’s TA14-017A lists TFTP with a bandwidth amplification factor (BAF) of 60, based on research information credited to Christian Rossow. CISA defines BAF as the UDP payload bytes in a response compared with the UDP payload bytes in the request. The figure is a value in CISA’s research compilation—not a measurement of current attacks or a guaranteed ratio for every TFTP server, request, or deployment. The alert was first released on February 9, 2014, and last revised on December 18, 2019; its TFTP entry was added in December 2017.
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How to reduce TFTP reflector exposure
For operators, the most direct way to remove a TFTP reflector is to stop exposing a server that does not need to be reachable from the internet. Where TFTP is operationally necessary, restrict who can reach it and combine service-level controls with network controls that reduce spoofing and limit attack traffic.
- Disable or remove unneeded internet-facing TFTP. CISA recommends disabling or removing services that are not required. Review whether the server must be publicly reachable; if not, limit access to the networks and systems that need it.
- Apply ingress filtering to block spoofed packets. Network operators should prevent outbound traffic from leaving their networks with forged source addresses. CISA recommends ingress filtering as an anti-spoofing measure.
- Use suitable stateful UDP inspection and rate limits. CISA recommends stateful UDP inspection where appropriate and network-based rate limiting. These measures can help constrain abusive traffic, but should be designed around legitimate TFTP use.
- Prepare upstream mitigation. Maintain emergency contacts with upstream providers and coordinate response options, including remotely triggered blackholing where appropriate. Blackholing can protect other systems by discarding traffic to a targeted address, but makes that address unreachable while active.
Detecting and responding to a TFTP reflection attack
Reflection attacks can be difficult to identify because the traffic arrives from large, apparently legitimate servers rather than directly from the attacker. CISA recommends watching for unusually large UDP responses directed at one IP address and for unusual request or traffic patterns.
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- Look for traffic concentration and anomalies. Check whether UDP traffic is unusually high or whether large responses from multiple servers are converging on one destination.
- Separate mitigation from attribution. The apparent source addresses may be the reflecting servers, not the systems that forged the requests. Coordinate with your network and upstream providers rather than treating every responding server as the attacker.
- Apply proportionate controls. Use network rate limiting or appropriate UDP inspection, and coordinate upstream mitigation if the link or service is overwhelmed. Where operationally suitable, a provider may use remotely triggered blackholing.
- Address the source of exposure. If your own TFTP service is responding to spoofed requests, restrict or disable it as appropriate and ensure anti-spoofing controls are in place.
Do not confuse reflection with Cisco CVE-2015-0681
TFTP reflection is protocol abuse: it relies on UDP requests, spoofed source addresses, and reachable servers that send replies. Cisco CVE-2015-0681 was a separate vulnerability in the TFTP server feature of affected Cisco IOS and IOS XE releases. Cisco said multiple TFTP requests could allow an unauthenticated remote attacker to cause a device reload or hang when that feature was configured. It was not a universal flaw in the TFTP protocol.
Cisco’s advisory, first published July 22, 2015, said the TFTP server feature was not enabled by default. For affected deployments, administrators should verify current Cisco support and release guidance, check whether tftp-server is configured, and apply fixed software for the affected release. If the feature is needed, restrict access with TFTP access lists. Cisco cautions that spoofed UDP source addresses can undermine ACLs that trust source addresses, and recommends considering Unicast Reverse Path Forwarding (Unicast RPF) with TFTP access lists. Disable the server feature if it is not needed.
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Official guidance
- CISA alert TA14-017A: UDP-Based Amplification Attacks covers the reflection pattern, its TFTP BAF entry, detection, and mitigation recommendations.
- Cisco advisory for CVE-2015-0681 explains the affected TFTP server feature and Cisco’s product-specific mitigations.
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