CVE-2021-41773
A flaw was found in a change made to path normalization in Apache HTTP Server 2.
Executive Summary
CVE-2021-41773 is a critical severity vulnerability affecting appsec. It is classified as an undisclosed flaw. Ensure your systems and dependencies are patched immediately to mitigate exposure risks.
Precogs AI Insight
"This critical flaw stems from within A flaw, allowing a lack of rigorous type checking mechanisms. A threat actor could leverage this oversight to execute arbitrary code on the target system, potentially leading to full system compromise. Precogs AI Analysis Engine leverages inter-procedural taint tracking to harden the environment against lateral movement."
What is this vulnerability?
CVE-2021-41773 is categorized as a critical Path Traversal flaw. Based on our vulnerability intelligence, this issue occurs when the application fails to securely handle untrusted data boundaries.
A flaw was found in a change made to path normalization in Apache HTTP Server 2.4.49. An attacker could use a path traversal attack to map URLs to files ou.
This architectural defect enables adversaries to bypass intended security controls, directly manipulating the application's execution state or data layer. Immediate strategic intervention is required.
Risk Assessment
| Metric | Value |
|---|---|
| CVSS Base Score | 9.8 (CRITICAL) |
| Vector String | N/A |
| Published | March 21, 2026 |
| Last Modified | March 21, 2026 |
| Related CWEs | N/A |
Impact on Systems
✅ Sensitive File Disclosure: Unauthorized access to critical configuration files (/etc/passwd, .env files, private SSH keys).
✅ Application Source Leak: Attackers can download proprietary source code and hardcoded credentials.
✅ Remote Code Execution: By combining with log poisoning, attackers can write PHP/JSP shells into web-accessible directories.
How to fix this issue?
Implement the following strategic mitigations immediately to eliminate the attack surface.
1. Indirect References Avoid using direct file paths. Utilize indirect references (like database IDs) mapped to backend files.
2. Strict Path Resolution If direct paths are required, resolve the absolute path and rigorously verify the path starts with the expected base directory using native OS path resolving functions.
3. Chroot Jails Confine the application processes to highly restricted directory structures (chroot) with minimum readable boundaries.
Vulnerability Signature
// Vulnerable File Access
import os
def get_image(request):
filename = request.GET.get('file')
# VULNERABLE: No validation preventing moving upwards in the directory tree
filepath = os.path.join('/var/www/images/', filename)
return open(filepath, 'rb').read()
// EXPLOIT PAYLOAD: ?file=../../../../../../../../etc/passwd
References and Sources
Vulnerability Code Signature
Attack Data Flow
| Stage | Detail |
|---|---|
| Source | Untrusted User Input |
| Vector | Input flows through the application logic without sanitization |
| Sink | Execution or Rendering Sink |
| Impact | Application compromise, Logic Bypass, Data Exfiltration |
Vulnerable Code Pattern
# ❌ VULNERABLE: Unsanitized Input Flow
def process_request(request):
user_input = request.GET.get('data')
# Taint sink: processing untrusted data
execute_logic(user_input)
return {"status": "success"}
Secure Code Pattern
# ✅ SECURE: Input Validation & Sanitization
def process_request(request):
user_input = request.GET.get('data')
# Sanitized boundary check
if not is_valid_format(user_input):
raise ValueError("Invalid input format")
sanitized_data = sanitize(user_input)
execute_logic(sanitized_data)
return {"status": "success"}
How Precogs Detects This
Precogs AI Analysis Engine maps untrusted input directly to execution sinks to catch complex application security vulnerabilities.\n