Summary
Heimdall: Forwarded Header Injection via Unsanitized Host Header in Proxy Mode
When Heimdall operates in proxy mode, it constructs the Forwarded HTTP header after executing the matched rule pipeline by inserting the incoming request's Host header value directly into the header string without sanitizing commas or semicolons. This allows an attacker to inject additional parameters into the Forwarded header, potentially spoofing IP addresses for upstream services.
Vulnerable Code
File: proxy/request_context.go (line 201)
entry := "for=" + clientIP + ";host=" + in.Host + ";proto=" + proto
Go's net/http allows commas and semicolons in Host header values. No sanitization is applied before string concatenation.
PoC
# Inject a spoofed IP into the Forwarded header
curl -s -H "Host: evil.com,for=127.0.0.1" \
"http://TARGET:PORT/protected-resource"
This produces the following Forwarded header sent to the upstream service:
Forwarded: for=1.2.3.4;host=evil.com, for=127.0.0.1;proto=http
Upstream services that parse the Forwarded header according to RFC 7239 will see two entries. If the service trusts the last or any for= value, the attacker successfully spoofs 127.0.0.1 as the client IP.
# More targeted attack: spoof to bypass IP allowlist
curl -s -H "Host: legit.com;for=10.0.0.1;proto=https,for=192.168.1.1" \
"http://TARGET:PORT/admin-panel"
Impact
- IP spoofing: Upstream services behind Heimdall may trust the injected
for=value, believing the request originates from an internal/trusted IP - Access control bypass: Applications that restrict access based on IP address by themselves, without using the corresponding heimdall capabilities (e.g., admin panels, internal APIs), can be bypassed
- Affects all proxy-mode deployments where upstream services parse the
Forwardedheader.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-57209? CVE-2026-57209 is a high-severity improper input validation vulnerability in github.com/dadrus/heimdall (go), affecting versions <= 0.17.16. It is fixed in 0.17.17. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- Which versions of github.com/dadrus/heimdall are affected by CVE-2026-57209? github.com/dadrus/heimdall (go) versions <= 0.17.16 is affected.
- Is there a fix for CVE-2026-57209? Yes. CVE-2026-57209 is fixed in 0.17.17. Upgrade to this version or later.
- Is CVE-2026-57209 exploitable, and should I be worried? Whether CVE-2026-57209 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-57209 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-57209? Upgrade
github.com/dadrus/heimdallto 0.17.17 or later.