CVE-2021-41773 Vulnerability

Published: 11/21/2025

What is CVE-2021-41773 vulnerability?

CVE-2021-41773 is a path traversal vulnerability in Apache HTTP Server 2.4.49 that allows attackers to map URLs to files located outside the expected document root. This critical flaw, when exploited, permits remote actors to view sensitive files or execute arbitrary code (in some cases) on vulnerable servers. Its primary risk lies in compromising data confidentiality, integrity, and availability, particularly for organizations using default configurations.

When was it discovered?

The vulnerability was disclosed on October 5, 2021. It was initially discovered by security researchers who submitted their findings to Apache's team. A follow-up patch for this vulnerability was quickly addressed, but some subsequent exploits continued against systems unpatched post-disclosure.

Affected products & versions

Product	Versions Affected	Fixed Versions / Patch Links
Apache HTTP Server	2.4.49	Apache 2.4.51

CVE-2021-41773 technical description

CVE-2021-41773 stems from insufficient input validation in Apache HTTP Server’s default configurations. Path traversal flaws allow unauthorized access to files outside the server’s public document structure, particularly when non-public directories lack proper protections.

Tactics, Techniques & Procedures (TTPs)

Attackers typically exploit the vulnerability by sending specially crafted GET requests that bypass normal file access restrictions, allowing traversal via encoded directory navigation patterns (“../../../”). Combined with other system weaknesses, this flaw can enable arbitrary file reads or remote code execution.

Indicators of compromise

Look out for unusual directory traversal requests in server access logs. Suspicious patterns, such as “../../../etc/passwd,” or repeated attempts to access sensitive configurations, often signal exploitation attempts. Ensure monitoring of unusual IPs, excessive failed requests, and unauthorized file activity.

Known proof-of-concepts & exploits

A public proof-of-concept (PoC) exploit has been released on platforms like GitHub, quickly incorporated into frameworks like Metasploit. Active exploitation campaigns targeting this vulnerability have been widely reported since its disclosure.

How to detect CVE-2021-41773 vulnerability

Detection can be performed using network-based intrusion detection systems (NIDS) for malicious traversal signatures. Analyze server access logs for exploitation patterns or deploy endpoint detection and response (EDR) solutions to identify unusual code execution or unauthorized file access. Sample SIEM queries should focus on HTTP requests containing encoded traversal strings.

Impact & risk of CVE-2021-41773 vulnerability

If unmitigated, this vulnerability poses critical business and technical risks. Exploits can expose sensitive data, configuration files, or enable system takeover via malicious scripts. For example, attackers gaining access to configuration files on a high-volume web server could lead to enterprise-wide outages or data breaches, damaging reputation and financial stability.

Mitigation & remediation strategies

To mitigate the CVE-2021-41773 vulnerability, upgrade affected Apache servers to 2.4.51 or later immediately. Apply proper access controls to all non-public directories and disable unnecessary modules to minimize risk exposure. To strengthen defenses, implement network segmentation and monitor critical server traffic for suspicious patterns.

CVE-2021-41773 Vulnerability FAQs

CVE-2021-41773 is a path traversal vulnerability in Apache HTTP Server 2.4.49 that allows unauthorized access to files outside the expected public directory. This is achieved by sending specially crafted requests that exploit improper input validation in the server.

CVE-2021-41773 doesn't "infect" systems like malware but relies on exploiting an insecure web server configuration to steal data or execute malicious actions. Attackers achieve this by submitting traversal requests to access sensitive files or run unauthorized commands.

While patches are available to mitigate CVE-2021-41773, it may remain exploitable if organizations run unpatched Apache versions. Ensuring regular updates and system audits is essential to neutralize lingering threats.

Organizations should patch Apache servers to the latest secure version and configure directory controls to prevent unauthorized access. Regular system monitoring and deployment of intrusion detection measures can further enhance protection.