Source: cve@mitre.org
Off-by-one buffer overflow in Basic Authentication in Acme Labs thttpd 1.95 through 2.20 allows remote attackers to cause a denial of service and possibly execute arbitrary code.
Acme Labs thttpd versions 1.95 through 2.20 are vulnerable to a critical buffer overflow in the Basic Authentication process, allowing remote attackers to trigger a denial-of-service (DoS) condition and potentially achieve remote code execution (RCE). Successful exploitation could lead to complete system compromise. This vulnerability poses a significant risk to any system running the affected versions of thttpd.
Step 1: Malformed Request: The attacker crafts an HTTP request with a specially crafted Authorization header. This header contains a username or password that is longer than the allocated buffer size, designed to overflow it.
Step 2: Authentication Attempt: The thttpd server receives the malicious HTTP request and attempts to authenticate the user based on the provided credentials.
Step 3: Buffer Overflow: During the authentication process, the server's code copies the attacker-supplied username or password into a fixed-size buffer. Due to the lack of proper bounds checking and the off-by-one error, the copy operation overflows the buffer, overwriting adjacent memory locations.
Step 4: Denial of Service/Code Execution: The overwritten memory corrupts critical data structures or control flow. This can lead to a crash, resulting in a denial-of-service (DoS). Alternatively, with carefully crafted input, the attacker can overwrite function pointers or other critical code segments, potentially leading to remote code execution (RCE).
The vulnerability lies within the Basic Authentication handling of thttpd. Specifically, an off-by-one error occurs when processing the user credentials provided in the Authorization header. The code fails to properly validate the length of the user credentials before copying them into a fixed-size buffer. This leads to a buffer overflow, where an attacker can provide a crafted Authorization header with an overly long username or password, overwriting adjacent memory. This overwrite can corrupt critical data structures, leading to a crash (DoS) or, with careful crafting, the execution of arbitrary code (RCE). The root cause is a lack of bounds checking on the input data before it's copied into the buffer, combined with an incorrect calculation of the buffer size or an off-by-one error in the copy operation.
Due to the age of the vulnerability, it's difficult to attribute it to specific APT groups with high confidence. However, any threat actor seeking to compromise web servers could utilize this exploit. CISA KEV status is unlikely due to the age and limited adoption of the affected software. However, if this were a more modern vulnerability, it would likely be a target for ransomware gangs and nation-state actors alike.
Network Intrusion Detection Systems (NIDS) can be configured to detect malicious HTTP requests with excessively long Authorization headers, specifically targeting the Basic Authentication scheme.
Web Application Firewalls (WAFs) can be deployed to filter and block requests with overly long credentials in the Authorization header.
Security Information and Event Management (SIEM) systems can be configured to analyze web server logs for suspicious activity, such as frequent failed authentication attempts with unusually long usernames or passwords.
Monitor server logs for crashes or unexpected behavior related to the web server process, particularly those associated with authentication modules.
Forensic analysis of memory dumps following a crash can reveal evidence of buffer overflows and overwritten memory regions.
Upgrade to a patched version of thttpd (if available). Since the software is old, this might not be possible, and a different web server should be considered.
If upgrading is not feasible, consider removing thttpd from the environment entirely and replacing it with a more secure web server solution (e.g., Apache, Nginx).
Implement a Web Application Firewall (WAF) to filter malicious requests, including those with overly long credentials in the Authorization header.
Regularly scan the system for vulnerabilities using vulnerability scanners.
Implement strong authentication practices, including the use of strong passwords and multi-factor authentication (MFA) where possible.
Review and harden the server's configuration, disabling unnecessary features and services.
Monitor server logs for suspicious activity and security events.