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Medium, CVSS 5.3

Fiber vulnerable to XSS in AutoFormat Content Negotiation

CVE-2026-42554GHSA-qjv7-627w-8qjv

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (github.com/gofiber/fiber/v3 3.2.0; github.com/gofiber/fiber/v2 2.52.13), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
5.3 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N · score source
EPSS
  • CVE-2026-42554: 0.21% estimated exploitation activity in the next 30 days; percentile 11.58 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary Description A Cross-Site Scripting (CWE-79) vulnerability in Go Fiber allows a remote attacker to inject arbitrary HTML/JavaScript by supplying Accept: text/html on any request whose handler passes attacker-influenced data to the AutoFormat() feature. This affects github.com/gofiber/fiber/v3 (DefaultRes.AutoFormat) through version 3.1.0 and github.com/gofiber/fiber/v2 (Ctx.Format) through version 2.52.12. The developer opts into content negotiation by calling AutoFormat(), but does not opt into raw HTML emission for a particular request; Fiber chooses that branch from attacker-controlled Accept. Five of the six branches of the same method already escape. JSON, XML, MsgPack, and CBOR all route through encoders that neutralize markup; the txt branch emits text/plain and cannot execute. The html branch is the sole outlier in a method whose name (AutoFormat) and symmetrical structure actively telegraph "safe, format-agnostic reply." Details The issue resides in res.go within…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-qjv7-627w-8qjv

Agent brief

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# Patch Next advisory brief: CVE-2026-42554

Advisory: Fiber vulnerable to XSS in AutoFormat Content Negotiation
IDs: CVE-2026-42554, GHSA-qjv7-627w-8qjv
Record: https://patchnext.com/advisories/cve-2026-42554 (revision 1, source checked 26 Oct 2026, 15:20 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/gofiber/fiber/v3 3.2.0; github.com/gofiber/fiber/v2 2.52.13. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/gofiber/fiber/v3: affected <= 3.1.0; fixed in 3.2.0
- Go github.com/gofiber/fiber/v2: affected <= 2.52.12; fixed in 2.52.13

## Suggested inspection (read-only)
- Go github.com/gofiber/fiber/v3: check the installed version with your package manager.
- Go github.com/gofiber/fiber/v2: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/gofiber/fiber/v3, github.com/gofiber/fiber/v2.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-qjv7-627w-8qjv (GitHub-reviewed advisory GHSA-qjv7-627w-8qjv lists affected ranges: Go github.com/gofiber/fiber/v3 >=0 =0 <=2.52.12)

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