CVE-2026-25899
Published: 24 February 2026
Summary
CVE-2026-25899 is a high-severity Memory Allocation with Excessive Size Value (CWE-789) vulnerability in Gofiber Fiber. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 32.4th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified are NIST 800-53 SC-5 (Denial-of-service Protection) and SI-10 (Information Input Validation).
Deeper analysis
CVE-2026-25899 affects Fiber, an Express-inspired web framework written in Go, specifically versions on the v3 branch prior to 3.1.0. The vulnerability stems from the `fiber_flash` cookie, which triggers an unbounded memory allocation due to unvalidated msgpack deserialization. A crafted 10-character cookie value can force the server to attempt allocating up to 85GB of memory. Every endpoint in GoFiber v3 applications is vulnerable, regardless of whether the application uses flash messages.
Any unauthenticated attacker with network access can exploit this vulnerability by sending a malicious `fiber_flash` cookie in an HTTP request. This leads to memory exhaustion, enabling a denial-of-service attack that may crash the server or severely degrade performance. The CVSS v3.1 base score is 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), highlighting high availability impact, with associated CWEs-789 (Uncontrolled Memory Allocation) and CWE-770 (Allocation of Resources Without Limits or Throttling).
The Fiber project fixed this issue in version 3.1.0. Mitigation requires upgrading to v3.1.0 or later. Additional details are available in the release notes at https://github.com/gofiber/fiber/releases/tag/v3.1.0 and the security advisory at https://github.com/gofiber/fiber/security/advisories/GHSA-2mr3-m5q5-wgp6.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-8566
Vulnerability details
Fiber is an Express inspired web framework written in Go. In versions on the v3 branch prior to 3.1.0, the use of the `fiber_flash` cookie can force an unbounded allocation on any server. A crafted 10-character cookie value triggers an…
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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. Version 3.1.0 fixes the issue.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Unvalidated cookie deserialization in public-facing Go web framework enables direct memory exhaustion DoS via crafted HTTP request, matching application exploitation for endpoint availability impact.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Remediating the specific flaw by applying the v3.1.0 patch or later directly eliminates the unbounded memory allocation from unvalidated msgpack deserialization in the fiber_flash cookie.
Denial-of-service protection mechanisms such as resource limiting and rate limiting prevent memory exhaustion attacks triggered by crafted fiber_flash cookies.
Validating untrusted inputs like cookies before msgpack deserialization prevents malicious payloads from causing excessive memory allocation across all Fiber v3 endpoints.