While CVE-2021-41773 was initially documented as Path traversal and File disclosure vulnerability additional research concluded that the vulnerability can be further exploited to conduct remote code execution when mod_cgi module is enabled on the Apache HTTP server, this allows an attacker to leverage the path traversal vulnerability and call any binary on the system using HTTP POST requests. Please note that the default configuration of Apache HTTP server has the entire filesystem directory directive configured as Require all denied and hence is not vulnerable. Gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin List:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin Uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin News:x:9:9:news:/var/spool/news:/usr/sbin/nologin Mail:x:8:8:mail:/var/mail:/usr/sbin/nologin Lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin Man:x:6:12:man:/var/cache/man:/usr/sbin/nologin Games:x:5:60:games:/usr/games:/usr/sbin/nologin Vulnerable Configuration:ĭaemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin %2e/ and bypasses the check.Īlong with Path traversal check bypass, for an Apache HTTP server to be vulnerable, the HTTP Server configuration should either contain the directory directive for entire server’s filesystem as Require all granted or the directory directive should be completely missing from the configuration file. Hence when URL encoding the second dot as %2e, the logic fails to recognize %2e as dot thereby not decoding it, this converts the characters. To prevent path traversal attacks, the normalization function which is responsible to resolve URL-encoded values from the requested URI, resolved Unicode values one at a time. The path traversal vulnerability was introduced due to the new code change added for path normalization i.e., for URL paths to remove unwanted or dangerous parts from the pathname, but it was inadequate to detect different techniques of encoding the path traversal characters “dot-dot-slash (./)”
About CVE-2021-41773Īccording to CVE-2021-41773, Apache HTTP Server 2.4.49 is vulnerable to Path Traversal and Remote Code execution attacks. Once successfully detected, users can remediate the vulnerabilities by upgrading to Apache HTTP Sever 2.4.51 or greater. With both the CVEs being actively exploited, Qualys Web Application Scanning has released QID 150372, 150373, 150374 which sends specially crafted HTTP request to the target server to determine if it is exploitable. As the vulnerabilities are configuration dependent, checking the version of Apache web server is not enough to identify vulnerable servers.
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