CVE-2025-54801

Executive Summary

CVE-2025-54801 is a critical denial-of-service (DoS) vulnerability in the Fiber web framework for Go, versions 2.52.8 and below. The flaw resides in the Ctx.BodyParser function when parsing form data containing a large numeric key that represents a slice index (e.g., test.18446744073704). The underlying schema decoder attempts to allocate a slice of length idx + 1 without validating whether the index is within a safe or reasonable range, leading to an out-of-bounds slice allocation. This can cause an integer overflow or memory exhaustion, resulting in a panic or crash of the application. The vulnerability is patched in version 2.52.9.

Supply Chain & Package Security

This vulnerability affects the Go package github.com/gofiber/fiber/v2. Organizations using this package in their Go applications should immediately update to version 2.52.9 or later. The vulnerability can be exploited by sending a crafted HTTP request with a malicious form data key, potentially causing a denial of service. Since Fiber is a widely used web framework, this vulnerability could impact many applications in the Go ecosystem.

CVSS Breakdown & Threat Level

The CVSS score is not provided in the NVD data, but based on the description, the vulnerability can be exploited remotely without authentication, leading to a denial of service. A reasonable CVSS vector would be:

• Attack Vector (AV): Network (N)
• Attack Complexity (AC): Low (L)
• Privileges Required (PR): None (N)
• User Interaction (UI): None (N)
• Scope (S): Unchanged (U)
• Confidentiality (C): None (N)
• Integrity (I): None (N)
• Availability (A): High (H)

CVSS Score: 7.5 (High)

Threat Level: High. The vulnerability is easy to exploit and can cause a complete denial of service.

Deep Dive & Mechanics

The vulnerability is in the schema decoder used by Fiber's Ctx.BodyParser. When parsing form data, the decoder processes keys that represent slice indices (e.g., test.18446744073704). The decoder extracts the numeric index from the key and attempts to allocate a slice of length idx + 1. If the index is excessively large (e.g., 18446744073704), the allocation can cause an integer overflow or memory exhaustion, leading to a panic.

The root cause is the lack of validation on the index value before allocation. The decoder should check that the index is within a reasonable range (e.g., less than a maximum allowed slice size) before attempting to allocate memory.

Vulnerable vs. Patched Code

Vulnerable Code (<= 2.52.8):

// In the schema decoder, when processing a slice index:
idx, _ := strconv.ParseUint(key, 10, 64)
// Allocate slice of length idx+1 without validation
slice := make([]interface{}, idx+1)

Patched Code (>= 2.52.9):

// In the schema decoder, when processing a slice index:
idx, _ := strconv.ParseUint(key, 10, 64)
// Validate that idx is within a safe range
if idx > maxSliceSize {
    return fmt.Errorf("slice index %d exceeds maximum allowed size", idx)
}
// Allocate slice of length idx+1
slice := make([]interface{}, idx+1)

The patch adds a validation check to ensure the index does not exceed a maximum allowed slice size, preventing the out-of-bounds allocation.

Exploitation & Lab Setup

Red Team

To exploit this vulnerability, an attacker sends a POST request with form data containing a key like test.18446744073704. The value can be any string. The request will cause the Fiber application to crash due to memory exhaustion or panic.

Example Exploit Request:

POST / HTTP/1.1
Host: target.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 30

test.18446744073704=value

Lab Setup:

1. Set up a vulnerable Fiber application (version 2.52.8 or below).
2. Send the crafted request using curl or a custom script.
3. Observe the application crash or panic.

Blue Team

To detect exploitation attempts, monitor for HTTP requests with unusually large numeric keys in form data. The key test.18446744073704 is a specific example, but any key with a very large numeric index should be flagged.

Detection:

• Monitor logs for requests with form data keys containing numbers greater than a threshold (e.g., 10000).
• Use WAF rules to block requests with form data keys that have numeric indices exceeding a reasonable limit.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests with form data containing keys like test.18446744073704 or similar large numeric indices.
  • Requests with Content-Type: application/x-www-form-urlencoded and a body containing a key with a numeric index greater than 10000.

• System IOCs:

  • Application crashes or panics in Go Fiber applications.
  • Memory exhaustion errors in application logs.
  • Log entries showing panic: runtime error: makeslice: len out of range or similar.

Detection Engineering

YARA Rule

rule CVE_2025_54801_Exploit_Attempt {
    meta:
        description = "Detects HTTP requests attempting to exploit CVE-2025-54801 in Fiber web framework"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-54801"
    strings:
        $large_index = /[a-zA-Z_][a-zA-Z0-9_]*\.\d{10,}/  // Key with numeric index >= 10 digits
        $form_content = "application/x-www-form-urlencoded"
    condition:
        $large_index and $form_content
}

Sigma Rule

title: CVE-2025-54801 Exploit Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP requests with form data containing large numeric keys, indicating exploitation of CVE-2025-54801
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-54801
logsource:
    category: webserver
    product: generic
detection:
    selection:
        http.method: POST
        http.content_type: application/x-www-form-urlencoded
        http.body|contains: '.\d{10,}='
    condition: selection
falsepositives:
    - Legitimate use of large numeric indices in form data (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-54801 Exploit Attempt - Large Numeric Index in Form Data"; flow:to_server; http.method; content:"POST"; http.content_type; content:"application/x-www-form-urlencoded"; http.request_body; pcre:"/[a-zA-Z_][a-zA-Z0-9_]*\.\d{10,}=/"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Fiber to version 2.52.9 or later.
2. If immediate upgrade is not possible, implement a WAF rule to block requests with form data keys containing numeric indices greater than a reasonable threshold (e.g., 1000).
3. Monitor application logs for crashes or panics related to slice allocation.

Long-term Mitigations:

• Regularly update dependencies to the latest versions.
• Implement input validation for all user-supplied data, including form data keys.
• Use a web application firewall (WAF) to filter malicious requests.

Patch Details:

• Upgrade to Fiber version 2.52.9 or later.
• The patch is available in the official Fiber repository: https://github.com/gofiber/fiber

Valters IT
Securing Digital Frontiers