Fiber: X-Real-IP Spoofing via Header.Add() in BalancerForward

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Fiber is an Express inspired web framework written in Go. Prior to 3.3.0 and 2.52.14, the BalancerForward proxy helper in middleware/proxy/proxy.go uses Header.Add() instead of Header.Set() when injecting X-Real-IP, allowing an attacker-supplied first X-Real-IP value to be forwarded to upstream servers for logging, rate limiting, and access control. This issue is fixed in version 3.3.0 and 2.52.14.

Source published

8 Jul 2026 19:26:26 UTC

Source modified

9 Jul 2026 13:20:08 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Fiber is an Express inspired web framework written in Go. Prior to 3.3.0 and 2.52.14, the BalancerForward proxy helper in middleware/proxy/proxy.go uses Header.Add() instead of Header.Set() when injecting X-Real-IP, allowing an attacker-supplied first X-Real-IP value to be forwarded to upstream servers for logging, rate limiting, and access control. This issue is fixed in version 3.3.0 and 2.52.14.

Source published

Jul 8, 2026, 7:26:26 PM (timezone not stated)

Source modified

Jul 9, 2026, 1:20:08 PM (timezone not stated)

Observed here

7 Nov 2026 13:22:49 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Fiber is an Express inspired web framework written in Go. Prior to 3.3.0 and 2.52.14, the BalancerForward proxy helper in middleware/proxy/proxy.go uses Header.Add() instead of Header.Set() when injecting X-Real-IP, allowing an attacker-supplied first X-Real-IP value to be forwarded to upstream servers for logging, rate limiting, and access control. This issue is fixed in version 3.3.0 and 2.52.14.

Source published

8 Jul 2026 20:16:50 UTC

Source modified

25 Oct 2026 17:10:00 UTC

Observed here

27 Oct 2026 11:12:23 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

## 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 ```go 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 1. Attacker sends request with `X-Real-IP: 10.0.0.1` (spoofed internal IP) 2. `BalancerForward` handler executes at line 282 3. `c.Request().Header.Add("X-Real-IP", c.IP())` APPENDS the real IP as a second header 4. Upstream server receives: `X-Real-IP: 10.0.0.1` AND `X-Real-IP: <real-attacker-ip>` 5. Most HTTP servers (nginx, Node.js, Apache) read the FIRST value 6. Upstream uses `10.0.0.1` for 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: ```go c.Request().Header.Set("X-Real-IP", c.IP()) ``` `Header.Set()` replaces any existing header value, ensuring only the real client IP is forwarded.

Source published

2 Jul 2026 13:50:45 UTC

Source modified

10 Sep 2026 03:50:52 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Publisher rating · MODERATE

GoFiber Vulnerable to X-Real-IP Spoofing via Header.Add() in BalancerForward in github.com/gofiber/fiber

Source published

24 Jul 2026 18:35:55 UTC

Source modified

24 Jul 2026 19:00:26 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.