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How the CVE Works
netfoil is a minimal, hardened DNS proxy written in Go, designed to filter and block malicious domains. Prior to version 0.4.0, when netfoil encountered a domain that should be blocked, it responded with a DNS `A` record pointing to `0.0.0.0` (and `::` for IPv6) along with a `NOERROR` status code, rather than the proper `NXDOMAIN` (non‑existent domain) response.
On Linux systems—the primary target platform for netfoil—the IP address `0.0.0.0` is treated specially: it is routed to the loopback interface (127.0.0.1), i.e., localhost. This means that any application or service that attempts to connect to a blocked domain will actually have its traffic sent to localhost instead of being dropped or receiving a definitive “domain does not exist” error.
The root cause is a protection mechanism failure (CWE‑693). The product incorrectly uses a sinkhole IP (0.0.0.0) that, on the target OS, does not provide the intended defence—it does not prevent outbound connections but rather redirects them to the local machine. This behaviour subverts the expected blocking functionality and opens the door to unintended interactions with local services.
An attacker who can influence which domains are blocked (e.g., by poisoning the blocklist or by crafting requests that trigger the block logic) can cause a victim’s applications to connect to `localhost` instead of the malicious domain. If a sensitive service (e.g., a web server, API gateway, or authentication endpoint) is listening on localhost, the redirected traffic may include HTTP headers, cookies, authorisation tokens, or other confidential data.
The vulnerability affects all netfoil versions prior to 0.4.0. The fix, implemented in commit 891d351, changes the block response to return `NXDOMAIN` instead of 0.0.0.0, ensuring that blocked domains are correctly treated as non‑existent and that no traffic is ever sent to localhost.
The CVSS v4.0 score for this issue is 7.4 (High), with attack vector over the network and high impact on confidentiality and integrity. Proof‑of‑concept exploit code is already available.
DailyCVE Form:
Platform: Linux Version: <0.4.0 Vulnerability: Protection Failure Severity: High (7.4) date: 2026-07-29 Prediction: Patch v0.4.0
What Undercode Say: Analytics
1. Check netfoil version currently installed netfoil --version 2. Simulate a DNS query for a blocked domain (pre‑patch) dig @127.0.0.1 blocked-example.com Expected pre‑patch response (NOERROR, A=0.0.0.0): ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12345 ;; ANSWER SECTION: blocked-example.com. 60 IN A 0.0.0.0 3. After upgrading to v0.4.0, the same query returns NXDOMAIN: ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 12345 4. Monitor localhost connections that may be receiving unintended traffic sudo ss -tulpn | grep ':80|:443|:8080' 5. Use tcpdump to capture traffic destined for 0.0.0.0 (pre‑patch) sudo tcpdump -i any host 0.0.0.0
Code snippet showing the fix (from commit 891d351):
// Before (vulnerable)
func (p Proxy) handleBlocked(w dns.ResponseWriter, req dns.Msg) {
m := new(dns.Msg)
m.SetReply(req)
m.Answer = []dns.RR{
&dns.A{
Hdr: dns.RR_Header{Name: req.Question[bash].Name, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 60},
A: net.ParseIP("0.0.0.0"), // <-- Problematic sinkhole
},
}
w.WriteMsg(m)
}
// After (fixed)
func (p Proxy) handleBlocked(w dns.ResponseWriter, req dns.Msg) {
m := new(dns.Msg)
m.SetReply(req)
m.Rcode = dns.RcodeNameError // NXDOMAIN
w.WriteMsg(m)
}
Exploit
An attacker can exploit this vulnerability by:
- Poisoning or manipulating the blocklist – if the attacker can influence which domains are marked as “blocked” (e.g., via configuration injection or by exploiting a separate domain‑filter bypass), they can cause the proxy to return `0.0.0.0` for a domain they control.
- Triggering a connection from a victim application – the victim’s application (e.g., a web client, API consumer, or notification service) attempts to connect to the blocked domain. The DNS proxy returns
0.0.0.0, and the application’s traffic is sent tolocalhost. - Intercepting sensitive data – if a local service (e.g., a development web server, a debugging proxy, or a misconfigured production service) is listening on the same port, it receives the victim’s traffic. This can expose authorisation headers, session cookies, API keys, or other confidential information.
The attack requires no special privileges and can be performed remotely as long as the attacker can influence the blocklist or the domains queried by the victim. A proof‑of‑concept is publicly available, making exploitation straightforward.
Protection
- Upgrade netfoil to v0.4.0 or later. This is the only complete fix; the patch changes the response code to
NXDOMAIN, eliminating the localhost redirection. - Apply local firewall rules to drop outbound traffic destined for `0.0.0.0` and `::` as a temporary workaround.
sudo iptables -A OUTPUT -d 0.0.0.0/32 -j DROP sudo ip6tables -A OUTPUT -d ::/128 -j DROP
- Avoid running sensitive listener services on localhost ports (80, 443, 8080, etc.) unless absolutely necessary.
- Verify the fix by performing a DNS query against a blocked domain and confirming that the response status is
NXDOMAIN. - Review and update any Go projects that depend on `github.com/tinfoil-factory/netfoil` to require v0.4.0 in their `go.mod` file.
Impact
- Unintended traffic – all connections to blocked domains are redirected to localhost instead of being dropped.
- Information disclosure – sensitive data (HTTP headers, cookies, tokens) can be intercepted by local services, leading to credential theft or session hijacking.
- Bypass of security controls – the intended blocking mechanism is effectively nullified, as blocked domains remain reachable (via localhost) and may still interact with the application.
- Dependency on local services – the actual impact depends on which services are running on localhost and their firewall rules; if no sensitive service is listening, the impact is limited, but in many environments (development, staging, or misconfigured production) the risk is high.
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Sources:
Reported By: github.com
Extra Source Hub:
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