EUVD-2026-42351
GHSA-gcfq-8gqf-4876
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
No credentials or special conditions needed beyond sending an HTTP header; integrity is Low because only IP-dependent control decisions at upstream are manipulated, not data directly.
Primary rating from Vendor (https://github.com/gofiber/fiber).
CVSS VectorVendor: https://github.com/gofiber/fiber
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Lifecycle Timeline
2DescriptionCVE.org
Summary
The BalancerForward proxy helper in GoFiber uses Header.Add() instead of Header.Set() when injecting the X-Real-IP header. This appends the real client IP as a second header value rather than replacing any attacker-supplied value. Upstream servers that read the first X-Real-IP header (nginx, Express, most HTTP servers) use the attacker's spoofed IP for logging, rate limiting, and access control.
Vulnerable Code
File: middleware/proxy/proxy.go, lines 270-285
func BalancerForward(servers []string, clients ...*fasthttp.Client) fiber.Handler {
r := &roundrobin{
current: 0,
pool: servers,
}
return func(c fiber.Ctx) error {
server := r.get()
if !strings.HasPrefix(server, "http") {
server = "http://" + server
}
c.Request().Header.Add("X-Real-IP", c.IP()) // line 282: Add, not Set
return Do(c, server+c.OriginalURL(), clients...)
}
}Data Flow
- Attacker sends request with
X-Real-IP: 10.0.0.1(spoofed internal IP) BalancerForwardhandler executes at line 282c.Request().Header.Add("X-Real-IP", c.IP())APPENDS the real IP as a second header- Upstream server receives:
X-Real-IP: 10.0.0.1ANDX-Real-IP: <real-attacker-ip> - Most HTTP servers (nginx, Node.js, Apache) read the FIRST value
- Upstream uses
10.0.0.1for all IP-dependent logic
Impact
- Rate limit bypass: IP-based rate limiting at the upstream uses the spoofed IP, allowing unlimited requests
- IP ACL bypass: Internal IP allowlists (e.g., admin panels restricted to
10.0.0.0/8) can be bypassed - Audit log poisoning: Security logs record the spoofed IP, making incident investigation unreliable
- Geolocation bypass: IP-based geofencing or region restrictions are circumvented
Fix
Replace Header.Add() with Header.Set() at line 282:
c.Request().Header.Set("X-Real-IP", c.IP())Header.Set() replaces any existing header value, ensuring only the real client IP is forwarded.
AnalysisAI
IP header spoofing in GoFiber's BalancerForward proxy middleware allows any remote unauthenticated attacker to inject a forged X-Real-IP header that upstream servers treat as authoritative. The middleware calls Header.Add() rather than Header.Set() when stamping the real client IP, causing the attacker-supplied value to remain as the first header instance - the one read by nginx, Express, Apache, and most HTTP servers for rate limiting, IP-based ACLs, and audit logging. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires that the target application uses GoFiber's BalancerForward middleware specifically; other GoFiber proxy helpers sharing the same codebase but not calling this function are not affected. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 3.1 score of 5.3 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N) accurately reflects low complexity and no authentication requirement but conservatively scores integrity impact as Low. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker sends an HTTP GET request to a GoFiber BalancerForward proxy with the header X-Real-IP: 10.0.0.1 set to a spoofed internal IP associated with a privileged allow-list. The BalancerForward middleware appends the attacker's real IP as a second X-Real-IP value, forwarding both to the upstream server. … |
| Remediation | The fix is a one-line change: replace Header.Add("X-Real-IP", c.IP()) with Header.Set("X-Real-IP", c.IP()) at line 282 of middleware/proxy/proxy.go. … Detailed patch versions, workarounds, and compensating controls in full report. |
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External POC / Exploit Code
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