CVE-2026-25899: Github.com/gofiber/fiber/v3 Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2mr3-m5q5-wgp6",
  "modified" : "2026-02-27T20:37:07Z",
  "published" : "2026-02-24T20:57:25Z",
  "aliases" : [ "CVE-2026-25899" ],
  "summary" : "Fiber is Vulnerable to Denial of Service via Flash Cookie Unbounded Allocation",
  "details" : "### Summary\nThe use of the `fiber_flash` cookie can force an unbounded allocation on any server. A crafted 10-character cookie value triggers an attempt to allocate up to 85GB of memory via unvalidated msgpack deserialization. No authentication is required. Every GoFiber v3 endpoint is affected regardless of whether the application uses flash messages.\n\n### Details\nRegardless of configuration, the flash cookie is checked:\n\n```go\nfunc (app *App) requestHandler(rctx *fasthttp.RequestCtx) {\n\t// Acquire context from the pool\n\tctx := app.AcquireCtx(rctx)\n\tdefer app.ReleaseCtx(ctx)\n\n\t\t// Optional: Check flash messages\n\t\trawHeaders := d.Request().Header.RawHeaders()\n\t\tif len(rawHeaders) > 0 && bytes.Contains(rawHeaders, flashCookieNameBytes) {\n\t\t\td.Redirect().parseAndClearFlashMessages()\n\t\t}\n\t\t_, err = app.next(d)\n\t} else {\n\t\t// Check if the HTTP method is valid\n\t\tif ctx.getMethodInt() == -1 {\n\t\t\t_ = ctx.SendStatus(StatusNotImplemented) //nolint:errcheck // Always return nil\n\t\t\treturn\n\t\t}\n\n\t\t// Optional: Check flash messages\n\t\trawHeaders := ctx.Request().Header.RawHeaders()\n\t\tif len(rawHeaders) > 0 && bytes.Contains(rawHeaders, flashCookieNameBytes) {\n\t\t\tctx.Redirect().parseAndClearFlashMessages()\n\t\t}\n}\n```\n\nThe cookie value is hex-decoded and passed directly to msgpack deserialization with no size or content validation:\n\nhttps://github.com/gofiber/fiber/blob/f8f34f642fb3682c341ede7816e7cf861aa7df89/redirect.go#L371\n\n```go\n// parseAndClearFlashMessages is a method to get flash messages before they are getting removed\nfunc (r *Redirect) parseAndClearFlashMessages() {\n\t// parse flash messages\n\tcookieValue, err := hex.DecodeString(r.c.Cookies(FlashCookieName))\n\tif err != nil {\n\t\treturn\n\t}\n\n\t_, err = r.c.flashMessages.UnmarshalMsg(cookieValue)\n\tif err != nil {\n\t\treturn\n\t}\n\n\tr.c.Cookie(&Cookie{\n\t\tName:   FlashCookieName,\n\t\tValue:  \"\",\n\t\tPath:   \"/\",\n\t\tMaxAge: -1,\n\t})\n}\n```\n\nThe auto-generated `tinylib/msgp` deserialization reads a `uint32` array header from the attacker-controlled byte stream and passes it directly to `make()` with no bounds check:\n\nhttps://github.com/gofiber/fiber/blob/f8f34f642fb3682c341ede7816e7cf861aa7df89/redirect_msgp.go#L242\n\n```go\n// UnmarshalMsg implements msgp.Unmarshaler\nfunc (z *redirectionMsgs) UnmarshalMsg(bts []byte) (o []byte, err error) {\n\tvar zb0002 uint32\n\tzb0002, bts, err = msgp.ReadArrayHeaderBytes(bts)\n\tif err != nil {\n\t\terr = msgp.WrapError(err)\n\t\treturn o, err\n\t}\n\tif cap((*z)) >= int(zb0002) {\n\t\t(*z) = (*z)[:zb0002]\n\t} else {\n\t\t(*z) = make(redirectionMsgs, zb0002)\n\t}\n\tfor zb0001 := range *z {\n\t\tbts, err = (*z)[zb0001].UnmarshalMsg(bts)\n\t\tif err != nil {\n\t\t\terr = msgp.WrapError(err, zb0001)\n\t\t\treturn o, err\n\t\t}\n\t}\n\to = bts\n\treturn o, err\n}\n```\n\nwhere\n `zb0002, bts, err = msgp.ReadArrayHeaderBytes(bts)` translates the attacker-controlled value into the element count and `make(redirectionMsgs, zb0002)` performs the unbounded allocation\n\nSo we can craft a gofiber cookie that will force a huge allocation: \n`curl -H \"Cookie: fiber_flash=dd7fffffff\" http://localhost:5000/hello`\n\nThe cookie val is a hex-encoded msgpack array32 header:\n- `dd` = msgpack array32 marker\n- `7fffffff` = 2 147 483 647 elements\n\n### Impact\nUnauthenticated remote Denial of Service (CWE-789). Anyone running a gofiber v3.0.0 or v3 server is affected. The flash cookie parsing is hardcoded.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/gofiber/fiber/v3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.1.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.0.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/gofiber/fiber/security/advisories/GHSA-2mr3-m5q5-wgp6"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-25899"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/gofiber/fiber"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/gofiber/fiber/releases/tag/v3.1.0"
  }, {
    "type" : "WEB",
    "url" : "https://pkg.go.dev/vuln/GO-2026-4534"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770", "CWE-789" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-24T20:57:25Z",
    "nvd_published_at" : "2026-02-24T22:16:31Z"
  }
}