cbcvebase.
CVE-2021-41773
published 2021-10-07

CVE-2021-41773: It was found that the fix for CVE-2021-41773 in Apache HTTP Server 2.4.50 was insufficient. An attacker could use a path traversal attack to map URLs to files…

PriorityP198critical9.8CVSS 3.1
AVNACLPRNUINSUCHIHAH
KEVITWEXPLOITRansomwareInitial access
CISA Known Exploited Vulnerabilitydue 2021-11-17
Exploited in the wild
EPSS
99.99%
100.0th percentile
It was found that the fix for CVE-2021-41773 in Apache HTTP Server 2.4.50 was insufficient. An attacker could use a path traversal attack to map URLs to files outside the directories configured by Alias-like directives. If files outside of these directories are not protected by the usual default configuration "require all denied", these requests can succeed. If CGI scripts are also enabled for these aliased pathes, this could allow for remote code execution. This issue only affects Apache 2.4.49 and Apache 2.4.50 and not earlier versions.

Affected

16 ranges
VendorProductVersion rangeFixed in
apachehttp_server
apachehttp_server
apachehttpd
apache_software_foundationapache_http_server
apache_software_foundationapache_http_server
debianapache2< apache2 2.4.51-1 (bookworm)apache2 2.4.51-1 (bookworm)
debianapache2< apache2 2.4.50-1 (bookworm)apache2 2.4.50-1 (bookworm)
fedoraprojectfedora
fedoraprojectfedora
msrccm1_httpd_2.4.51-1_on_cbl_mariner_1.0
oracleinstantis_enterprisetrack
oracleinstantis_enterprisetrack
oracleinstantis_enterprisetrack
oraclejd_edwards_enterpriseone_tools< 9.2.6.09.2.6.0
oraclesecure_backup< 18.1.0.1.018.1.0.1.0
paloaltopan-os

Detection & IOCsextracted from sources · hover to see the quote

path/cgi-bin/.%2e/.%2e/.%2e/.%2e/etc/passwd
path/cgi-bin/.%2e/.%2e/.%2e/.%2e/bin/sh
path/cgi-bin/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/etc/passwd
path/cgi-bin/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/bin/sh
commandecho;id
bytes
.%2e/ (dot-percent-2e-slash path traversal encoding)
bytes
%%32%65 (double URL-encoded dot for CVE-2021-42013 bypass)
  • Detect path traversal attempts using the .%2e/ encoding pattern in HTTP request URIs targeting /cgi-bin/ paths, indicative of CVE-2021-41773 exploitation on Apache 2.4.49.
  • Detect double URL-encoded path traversal using %%32%65 in HTTP request URIs, indicative of CVE-2021-42013 exploitation on Apache 2.4.50 (bypass of the insufficient first patch).
  • Alert on HTTP POST requests to /cgi-bin/ paths containing traversal sequences (.%2e or %%32%65) targeting /bin/sh, indicating RCE attempts via mod_cgi.
  • Monitor for outgoing GET requests via cURL to file hosting sites (GitHub, pastebin, etc.) accessing .php files, as this is a post-exploitation indicator associated with Androxgh0st operators leveraging CVE-2021-41773.
  • Flag Apache HTTP Server response headers advertising version 2.4.49 or 2.4.50 (e.g., 'Server: Apache/2.4.49 (Unix)') as these are the exclusively vulnerable versions.
  • Androxgh0st botnet actively scans for CVE-2021-41773 alongside CVE-2017-9841 and CVE-2018-15133; correlate scanning activity across these three CVEs as a combined campaign indicator.
  • ·CVE-2021-41773 affects ONLY Apache HTTP Server version 2.4.49. The patch in 2.4.50 was insufficient and introduced CVE-2021-42013; the fully fixed version is 2.4.51. Version checking alone is insufficient to confirm exploitability — configuration must also be assessed.
  • ·In a default install of Apache 2.4.49, only the document root and cgi-bin directories are accessible; exploitation is blocked unless the directory directive is misconfigured.

CVSS provenance

nvdv3.19.8CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
nvdv2.04.3MEDIUMAV:N/AC:M/Au:N/C:P/I:N/A:N
osv9.8CRITICAL
vulncheck9.8CRITICAL
cisa9.8CRITICAL
vendor_apache9.8CRITICAL
vendor_debian9.8CRITICAL
vendor_redhat9.8CRITICAL
vendor_cisco7.5HIGH
vendor_msrc7.5HIGH
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