Fiber is Vulnerable to Denial of Service via Flash Cookie Unbounded Allocation

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-25899: 0.45% estimated exploitation activity in the next 30 days; percentile 39.17 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The use of the fiberflash 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. Details Regardless of configuration, the flash cookie is checked: The cookie value is hex-decoded and passed directly to msgpack deserialization with no size or content validation: https://github.com/gofiber/fiber/blob/f8f34f642fb3682c341ede7816e7cf861aa7df89/redirect.go#L371 The 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: https://github.com/gofiber/fiber/blob/f8f34f642fb3682c341ede7816e7cf861aa7df89/redirectmsgp.go#L242 where zb0002, bts, err = msgp.ReadArrayHeaderBytes(bts) translates the attacker-controlled value into…

Published by the source

24 Feb 2026, 20:57 UTC

Last modified by the source

27 Feb 2026, 20:37 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-2mr3-m5q5-wgp6

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# Patch Next advisory brief: CVE-2026-25899

Advisory: Fiber is Vulnerable to Denial of Service via Flash Cookie Unbounded Allocation
IDs: CVE-2026-25899, GHSA-2mr3-m5q5-wgp6
Record: https://patchnext.com/advisories/cve-2026-25899 (revision 1, source checked 26 Oct 2026, 15:14 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
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 3.1.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/gofiber/fiber/v3: affected <= 3.0.0; fixed in 3.1.0

## Suggested inspection (read-only)
- Go github.com/gofiber/fiber/v3: 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.
- 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-2mr3-m5q5-wgp6 (GitHub-reviewed advisory GHSA-2mr3-m5q5-wgp6 lists affected ranges: Go github.com/gofiber/fiber/v3 >=0 <=3.0.0)

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