CVE-2024-23771: Darkhttpd Auth Bypass Vulnerability

CVE-2024-23771 Overview

CVE-2024-23771 is a timing side channel vulnerability affecting darkhttpd, a lightweight single-threaded HTTP server. The vulnerability exists in versions prior to 1.15 where the server uses the standard strcmp function to verify authentication credentials. Since strcmp is not constant-time, it returns as soon as it encounters a mismatched character, allowing remote attackers to measure response time differences and deduce valid authentication credentials through statistical analysis of multiple requests.

Critical Impact

Remote attackers can bypass authentication mechanisms without valid credentials by exploiting timing differences in password comparison, potentially gaining unauthorized access to protected resources.

Affected Products

• unix4lyfe darkhttpd versions prior to 1.15

Discovery Timeline

• 2024-01-22 - CVE-2024-23771 published to NVD
• 2025-05-30 - Last updated in NVD database

Technical Details for CVE-2024-23771

Vulnerability Analysis

This vulnerability is classified as CWE-203 (Observable Discrepancy), specifically a timing side channel attack. The root issue lies in how darkhttpd validates HTTP Basic Authentication credentials. The server compares the provided Authorization header value against the stored auth_key using the standard C library function strcmp. This function performs byte-by-byte comparison and returns immediately upon finding the first non-matching character.

Attackers can exploit this behavior by sending multiple authentication attempts while precisely measuring server response times. Shorter response times indicate the submitted password matched more characters before failing, while longer processing times for correct character prefixes allow attackers to systematically determine each character of the valid credential through statistical timing analysis.

Root Cause

The vulnerability stems from using strcmp for security-sensitive credential comparison. The strcmp function in the C standard library is designed for efficiency, not security. It compares two strings character by character and returns immediately when it finds a difference. This optimization creates a measurable timing difference depending on how many characters match before a mismatch is found.

For authentication purposes, a constant-time comparison function should be used that always takes the same amount of time regardless of where the first mismatch occurs, preventing attackers from gaining information through timing measurements.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can remotely send numerous HTTP requests with different password guesses to the darkhttpd server while measuring response latencies with microsecond precision. By performing statistical analysis on thousands of requests, the attacker can identify which password guesses take slightly longer (indicating more matching prefix characters) and progressively reconstruct the valid authentication credentials.

The attack is particularly effective against:

• Servers with stable network latency
• Servers under low load conditions
• Shorter authentication credentials

 static int want_chroot = 0, want_daemon = 0, want_accf = 0,
            want_keepalive = 1, want_server_id = 1;
 static char *server_hdr = NULL;
-static char *auth_key = NULL;
+static char *auth_key = NULL;       /* NULL or "Basic base64_of_password" */
 static char *custom_hdrs = NULL;
 static uint64_t num_requests = 0, total_in = 0, total_out = 0;
 static int accepting = 1;           /* set to 0 to stop accept()ing */

Source: GitHub Commit f477619

The patch introduces a new constant-time test_password_equal function to replace the vulnerable strcmp comparison, ensuring timing consistency regardless of input values.

Detection Methods for CVE-2024-23771

Indicators of Compromise

• Unusually high volume of failed authentication attempts from single IP addresses
• Rapid sequential HTTP requests with Authorization headers containing slight variations
• Network traffic patterns indicating timing measurement (requests sent at precise intervals)
• Statistical anomalies in authentication request patterns suggesting brute-force timing analysis

Detection Strategies

• Monitor for excessive 401 Unauthorized responses to the same client within short time windows
• Implement rate limiting and anomaly detection on authentication endpoints
• Deploy network intrusion detection systems configured to identify timing attack patterns
• Analyze server logs for authentication attempt frequency anomalies

Monitoring Recommendations

• Enable detailed logging of all authentication attempts including timestamps and source IPs
• Configure alerting thresholds for authentication failure rates exceeding normal baselines
• Monitor network latency patterns to detect precision timing measurements
• Implement centralized log aggregation for correlation of timing attack indicators across systems

How to Mitigate CVE-2024-23771

Immediate Actions Required

• Upgrade darkhttpd to version 1.15 or later which implements constant-time password comparison
• Implement network-level rate limiting on authentication endpoints as a defense-in-depth measure
• Place darkhttpd behind a reverse proxy with built-in timing attack mitigations
• Review access logs for signs of prior exploitation attempts

Patch Information

The vulnerability was fixed in darkhttpd version 1.15. The security patch commit replaces the vulnerable strcmp comparison with a new constant-time password_equal function that prevents timing-based information leakage. Users should upgrade to version 1.15 or later immediately.

For version comparison details, see the GitHub comparison between v1.14 and v1.15.

Workarounds

• Implement additional authentication layers such as client certificate validation or IP allowlisting
• Deploy a Web Application Firewall (WAF) configured to rate-limit and monitor authentication traffic
• Use a reverse proxy with constant-time authentication handling in front of darkhttpd
• Disable HTTP Basic Authentication and use alternative authentication mechanisms if possible

# Example: Rate limiting authentication attempts with iptables
# Limit new connections from same IP to 10 per minute
iptables -A INPUT -p tcp --dport 80 -m state --state NEW -m recent --set
iptables -A INPUT -p tcp --dport 80 -m state --state NEW -m recent --update --seconds 60 --hitcount 10 -j DROP