Missing JWT Signature Verification

Description

Splitting a JWT yourself or reading it with a decode-only API such as ReadJwtToken does not verify its signature. Trusting the resulting claims for authentication or authorization can allow attacker-forged values to be used.

Potential impact

Authentication bypass, privilege escalation, and user impersonation.

Remediation

Use JwtSecurityTokenHandler.ValidateToken or ASP.NET Core authentication middleware to validate the signature, issuer, audience, and expiry time.

Examples

Before

csharp

sealed class Jwt
{
    public string[] Decode(JwtValidationRequest request)
    {
        return request.Token.Split('.');
    }
}

sealed class JwtValidationRequest
{
    public string Token { get; set; } = "";
}

After

Provide the key from trusted configuration and set the issuer and audience to the actual service values. Treat validation failure as authentication failure.

csharp

using System;
using System.IdentityModel.Tokens.Jwt;
using System.Security.Claims;
using Microsoft.IdentityModel.Tokens;

ClaimsPrincipal ValidateJwt(string token, SecurityKey trustedSigningKey)
{
    var validationParameters = new TokenValidationParameters
    {
        RequireSignedTokens = true,
        ValidateIssuerSigningKey = true,
        IssuerSigningKey = trustedSigningKey,
        ValidAlgorithms = new[] { SecurityAlgorithms.RsaSha256 },
        ValidateIssuer = true,
        ValidIssuer = "https://issuer.example.com",
        ValidateAudience = true,
        ValidAudience = "api://orders",
        RequireExpirationTime = true,
        ValidateLifetime = true,
        ClockSkew = TimeSpan.FromMinutes(1)
    };

    return new JwtSecurityTokenHandler().ValidateToken(
        token, validationParameters, out _);
}

Explanation:

Before: Splitting a string is not validation. Trusting claims from an unvalidated token can admit forged or unsigned values.
After: Fix the trusted key and permitted algorithm, and validate the signature, issuer, audience, and expiry time. Adding a ValidateToken call while weakening its validation options is not safe.

References

CWE-347