Session Fixation in Buffalo with Dynamodb

Session fixation occurs when an application assigns a user a session identifier before authentication and fails to regenerate that identifier after login. In a Buffalo application using DynamoDB as the session store, this can happen if session IDs are created on the initial HTTP request and stored in DynamoDB without renewal after successful authentication. Because Buffalo does not inherently manage session lifecycle beyond what the developer configures, using DynamoDB as the backend requires explicit handling of session rotation to avoid fixation.

When sessions are stored in DynamoDB, the session record typically includes fields such as session ID, user ID, expiration timestamp, and potentially a CSRF token. If an attacker sets a known session ID on a victim’s browser (for example, via a link with a crafted query parameter or a malicious cookie) and the application does not issue a new session ID after authentication, the attacker can later use the same session ID to hijack the authenticated session. This is particularly risky when session identifiers are predictable or when the application relies on cookies without the SameSite and Secure attributes.

Buffalo’s default session handling can be extended to use DynamoDB by implementing a custom session store. If the custom store reuses the incoming session ID for the DynamoDB item without regenerating it post-login, the store effectively preserves the pre-authentication session binding. DynamoDB’s eventual consistency model does not directly cause fixation, but improper handling of conditional writes or missing uniqueness checks on session IDs can exacerbate the issue by allowing collisions or reuse that facilitate predictable session tokens.

Real-world attack patterns include an attacker crafting a URL with a session cookie parameter, tricking a user into visiting it, and then logging in as that user. If the application does not rotate the session, the attacker’s session remains valid after the victim authenticates. Additionally, if the DynamoDB item is updated in place without invalidating the old session record, there may be multiple active identifiers mapping to the same user, complicating auditability and increasing exposure.

To mitigate this specific combination, developers must ensure that any session store integration explicitly regenerates session identifiers after authentication and binds them securely to the user. This includes enforcing fresh UUIDs for session IDs, validating ownership on each request, and ensuring that DynamoDB conditional updates prevent stale or conflicting session entries. Tools such as middleBrick can be used to scan Buffalo applications that use DynamoDB session stores, checking for missing session regeneration and related configuration issues as part of the unauthenticated attack surface analysis.

Remediation focuses on ensuring that every authenticated session has a newly generated, cryptographically random session identifier stored in DynamoDB, with no linkage to pre-authentication identifiers. The following example demonstrates a Buffalo handler that uses the AWS SDK for Go to manage sessions safely.

First, define a session structure that includes a unique ID and metadata:

type Session struct {
    SessionID string    `json:"session_id"`
    UserID    string    `json:"user_id"`
    ExpiresAt int64     `json:"expires_at"`
    CSRF      string    `json:"csrf_token"`
}

When a user logs in, generate a new session ID and write it to DynamoDB with a condition that ensures no existing active session with the same ID is reused. Use uuid.NewString() to create unpredictable identifiers and set an appropriate TTL:

import (
    "github.com/gobuffalo/buffalo"
    "github.com/aws/aws-sdk-go/service/dynamodb"
    "github.com/aws/aws-sdk-go/aws"
    "github.com/google/uuid"
)

func LoginHandler(c buffalo.Context) error {
    // Validate credentials, omitted for brevity
    userID := "user-123"

    newSession := Session{
        SessionID: uuid.NewString(),
        UserID:    userID,
        ExpiresAt: time.Now().Add(24 * time.Hour).Unix(),
        CSRF:      uuid.NewString(),
    }

    input := &dynamodb.PutItemInput{
        TableName: aws.String("sessions"),
        Item: map[string]types.AttributeValue{
            "session_id": &types.AttributeValueMemberS{Value: newSession.SessionID},
            "user_id":    &types.AttributeValueMemberS{Value: newSession.UserID},
            "expires_at": &types.AttributeValueMemberN{Value: strconv.FormatInt(newSession.ExpiresAt, 10)},
            "csrf_token": &types.AttributeValueMemberS{Value: newSession.CSRF},
        },
        ConditionExpression: aws.String("attribute_not_exists(session_id)"),
    }

    _, err := dynamoClient.PutItem(input)
    if err != nil {
        return c.Render(500, r.HTML("errors/internal.html", buffalo.M{"error": err}))
    }

    // Set secure cookie
    c.Response().AddCookie(&http.Cookie{
        Name:     "session_id",
        Value:    newSession.SessionID,
        HttpOnly: true,
        Secure:   true,
        SameSite: http.SameSiteStrictMode,
        Path:     "/",
        MaxAge:   86400,
    })

    return c.Redirect(303, "/dashboard")
}

On each subsequent request, validate the session by querying DynamoDB using the cookie value and ensure the record exists and is not expired. When logging out or changing privileges, delete the DynamoDB item and issue a new session ID to prevent reuse:

func ValidateSession(c buffalo.Context) bool {
    cookie, err := c.Request().Cookie("session_id")
    if err != nil {
        return false
    }

    result, err := dynamoClient.GetItem(&dynamodb.GetItemInput{
        TableName: aws.String("sessions"),
        Key: map[string]types.AttributeValue{
            "session_id": &types.AttributeValueMemberS{Value: cookie.Value},
        },
    })
    if err != nil || result.Item == nil {
        return false
    }

    expiresAt, _ := strconv.ParseInt(*result.Item["expires_at"].(*types.AttributeValueMemberN).Value, 10, 64)
    if time.Now().Unix() > expiresAt {
        return false
    }

    return true
}

By always generating a new session ID upon authentication and binding it exclusively in DynamoDB, the application eliminates fixation risks. Developers should also configure the Buffalo app to use secure, HTTP-only cookies and enforce HTTPS to complete the secure session handling strategy. middleBrick scans can verify that session regeneration is implemented and that DynamoDB items are created with conditions that prevent predictable or reused identifiers.