JWT首次验证失败二次成功，疑Microsoft.IdentityModel.Tokens存在问题

问题现象 #

C#应用中实现的JWT验证逻辑存在异常：首次调用验证方法时抛出以下异常，第二次调用却能验证成功，无法安全部署到生产环境：

Microsoft.IdentityModel.Tokens.SecurityTokenSignatureKeyNotFoundException: 'IDX10503: Signature validation failed. Token does not have a kid. Keys tried: 'System.Text.StringBuilder'.'

验证代码 #

public static TrustedCredential Validate(string token, string applicationId, string publicKey)
{
    TrustedCredential result = TryToValidate(token, applicationId, publicKey);

    if (ReferenceEquals(result, null))
    {
        result = TryToValidate(token, applicationId, publicKey);
    }

    return result;
}

private static TrustedCredential TryToValidate(string token, string applicationId, string publicKey)
{
    TokenValidationParameters validationParameters;
    JwtSecurityTokenHandler handler;
    ClaimsPrincipal claims;
    string sub, name, email, tenantId;
    
    RSA rsa = null;

    try
    {
        rsa = RSA.Create();

        rsa.FromXmlString(publicKey);

        validationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidIssuer = "codeconecta",
            ValidateAudience = true,
            ValidAudience = applicationId,
            ValidateIssuerSigningKey = true,
            IssuerSigningKey = new RsaSecurityKey(rsa),
            ValidateLifetime = true,
            ClockSkew = TimeSpan.Zero
        };

        handler = new JwtSecurityTokenHandler();
        claims = handler.ValidateToken(token, validationParameters, out SecurityToken validatedToken);

        sub = claims.FindFirst(MicrosoftRegisteredClaimNames.Sub).Value;
        email = claims.FindFirst(MicrosoftRegisteredClaimNames.Email).Value;
        tenantId = claims.FindFirst(MicrosoftRegisteredClaimNames.TenantId).Value;
        name = claims.FindFirst(JwtRegisteredClaimNames.Name).Value;

        return new TrustedCredential(tenantId, sub, email, name);
    }
    catch (SecurityTokenException)
    {
        return null;
    }
    finally
    {
        if (rsa != null)
        {
            rsa.Dispose();
            rsa = null;
        }
    }
}

JWT生成代码 #

SigningCredentials signingCredentials;
JwtSecurityToken token;

JwtHeader jwtHeader;
JwtPayload jwtPayload;
            
using (RSA rsa = RSA.Create())
{
    rsa.FromXmlString(privateKey);
    signingCredentials = new SigningCredentials(new RsaSecurityKey(rsa), SecurityAlgorithms.RsaSha256);

    jwtHeader = new JwtHeader(signingCredentials);

    jwtHeader["kid"] = "some_kid_value";

    jwtPayload = new JwtPayload (
        issuer: "codeconecta",
        audience: connectedApp.id,
        claims: claims,
        notBefore: DateTime.UtcNow,
        expires: DateTime.UtcNow.AddHours(1)
    );

    token = new JwtSecurityToken(jwtHeader, jwtPayload);

    return (new JwtSecurityTokenHandler()).WriteToken(token);
}

自定义Claim类代码 #

internal class MicrosoftRegisteredClaimNames
{
    internal const string Sub = "http://schemas.xmlsoap.org/ws/2005/05/identity/claims/nameidentifier";
    internal const string Email = "http://schemas.xmlsoap.org/ws/2005/05/identity/claims/emailaddress";
    internal const string TenantId = "http://schemas.microsoft.com/identity/claims/tenantid";
}

异常原因 #

核心问题在于Token的kid（密钥标识）与验证端的密钥KeyId不匹配：

1. 生成Token时，手动给JWT头部添加了kid: "some_kid_value"；
2. 验证端创建RsaSecurityKey时，没有设置对应的KeyId属性；
3. Microsoft.IdentityModel.Tokens组件在首次验证时，会严格匹配Token的kid与本地密钥的KeyId，因匹配失败抛出异常；
4. 二次验证时，组件内部可能存在缓存或 fallback 逻辑，跳过了kid匹配直接验证签名，因此验证成功。

解决方案 #

方案1：给验证用密钥设置匹配的KeyId（推荐） #

修改验证代码中RsaSecurityKey的创建逻辑，添加与生成Token时一致的KeyId：

validationParameters = new TokenValidationParameters
{
    // 其他参数保持不变
    IssuerSigningKey = new RsaSecurityKey(rsa) { KeyId = "some_kid_value" },
};

这样首次验证时组件就能匹配到对应的密钥，直接验证成功，无需二次重试。

方案2：规范Token生成时的kid设置方式 #

生成Token时，直接给RsaSecurityKey设置KeyId，无需手动修改JwtHeader，避免手动操作导致的不一致：

using (RSA rsa = RSA.Create())
{
    rsa.FromXmlString(privateKey);
    var rsaKey = new RsaSecurityKey(rsa);
    rsaKey.KeyId = "some_kid_value"; // 设置KeyId
    signingCredentials = new SigningCredentials(rsaKey, SecurityAlgorithms.RsaSha256);

    // 无需手动设置jwtHeader["kid"]，头部会自动从密钥的KeyId获取
    jwtHeader = new JwtHeader(signingCredentials);

    // 后续payload和token生成逻辑不变
}

方案3：关闭kid验证（不推荐，降低安全性） #

如果业务场景不需要通过kid区分密钥，可以在验证参数中设置忽略kid匹配：

validationParameters = new TokenValidationParameters
{
    // 其他参数保持不变
    KidValidator = (tokenKid, securityToken, validationParameters) => true
};

该方式会跳过kid校验，可能带来安全风险，仅在单一密钥场景下临时使用。

内容的提问来源于stack exchange，提问作者Bruno Leitão