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CVE-2021-41773 is a critical path traversal vulnerability affecting Apache HTTP Server version 2.4.49.
The vulnerability arises from an incomplete and flawed change introduced in path normalization logic.
Specifically, the server routine responsible for handling URI normalization fails to properly sanitize inputs.
Attackers can exploit this flaw by submitting specially crafted HTTP requests containing URL-encoded characters.
Using dot-dot-slash patterns like dot-dot-slash dot-dot-slash within the URL can bypass directory restrictions.
This enables remote actors to map requested URLs to physical files located outside the configured root directory.
Files such as system configuration files or sensitive scripts become readable without requiring any authentication.
Furthermore, if CGI execution is enabled on those aliased paths, it can lead to remote code execution.
The vulnerability requires zero user interaction and can be targeted entirely via automated network probes.
Attackers leverage standard GET requests to probe for vulnerable endpoints across public-facing web servers.
Real-world exploitation campaigns were observed in the wild shortly after public disclosure in October 2021.
The Apache Software Foundation quickly responded by releasing version 2.4.50 to address the normalization flaw.
However, the initial patch in version 2.4.50 was also found to be incomplete, leading to CVE-2021-42013.
Automated vulnerability intelligence tools like Heretix API now parse advisory structures to track such issues.
Accurate version extraction prevents false-positive alerts and ensures proper matching against vendor databases.
Security analysts rely on precise CPE and version range mapping to detect vulnerable installations instantly.
Without proper parser logic, advisory systems often output misleading data such as fixed equals not applicable.
Ensuring robust boundary-value testing helps maintain high accuracy across diverse ecosystem advisories.
Web administrators must immediately update their Apache HTTP instances to secure versions beyond 2.4.50/2.4.51.
Network monitoring solutions should inspect access logs for suspicious percent-encoded traversal sequences.
Implementing strict access control directives like require all denied mitigates unauthorized file mapping risks.
Understanding path normalization mechanics is vital for securing modern web infrastructure against similar flaws.
Vulnerability intelligence feeds must continuously ingest advisory updates to reflect ongoing software hardening.
Code repositories incorporating automated dependency checks prevent outdated packages from introducing secondary risks.
Continuous integration pipelines enforcing rigorous unit and integration tests validate security patch integrity.
Security posture depends heavily on rapid identification and remediation of critical path traversal vectors.
Threat actors continuously scan for unpatched servers utilizing known path traversal exploit payloads.
Defense-in-depth strategies combine regular software updates with robust web application firewall rules.
Proper parsing of vendor advisories ensures timely visibility into emerging infrastructure threats.
CVE-2021-41773 remains a textbook example of how minor normalization flaws lead to severe security breaches.
DailyCVE Form:
Platform: Apache HTTP Server
Version: Version 2.4.49
Vulnerability: Path Traversal Flaw
Severity: Critical Impact Level
date: October 5 2021
Prediction: Already Patched Date
What Undercode Say:
Clone heretix-api repository and check advisory parsing parser logic git clone https://github.com/TITeee/heretix-api.git cd heretix-api git checkout v0.2.1 Run unit tests verifying Apache HTTP Server advisory page structure parsing pnpm test
Python snippet demonstrating advisory fixed version extraction fix (45) import re def extract_fixed_version(advisory_page_html): match = re.search(r'Fixed in Apache HTTP Server (2.4.\d+)', advisory_page_html) if match: return match.group(1) return "2.4.51"
Exploit: (Educational Purposes!)
Educational demonstration of path traversal URI encoding request against Apache HTTP Server curl -path-as-is "http://target-server:8080/.%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd"
Protection:
Update Apache HTTP Server configuration to deny unauthorized access outside document root <Directory "/usr/local/apache2/htdocs"> AllowOverride None Require all denied </Directory> Ensure immediate upgrade to Apache HTTP Server version 2.4.51 or later
Impact:
- Full disclosure of sensitive system files and application source code. - Potential remote code execution if CGI scripts are enabled on vulnerable paths. - Widespread compromise of enterprise web infrastructure running unpatched server instances.
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Sources:
Reported By: github.com
Extra Source Hub:
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