CVE-2025-49506: 
NixOS 脆弱性の分析と軽減

CVE-2025-49506 is a timing side-channel vulnerability in Apache Portable Runtime Utility (APR-util) affecting the apr_password_validate() function. The function performs non-constant-time comparisons of password hashes, potentially leaking hash or password content to an attacker via measurable response time differences. Affected versions are APR-util 1.2.0 through 1.6.3 (inclusive); version 1.6.4 resolves the issue. The vulnerability is particularly impactful on platforms lacking native crypt() support, such as Windows, BeOS, NetWare, and Android. It carries a CVSS v3.1 base score of 7.5 (High) (Apache Advisory).

The root cause is classified as CWE-208 (Observable Timing Discrepancy). The apr_password_validate() function in APR-util does not use a constant-time comparison algorithm when validating passwords or their hashes, meaning the time taken to return a result varies depending on how many characters match — a classic timing oracle. On platforms with native crypt() (e.g., most Linux distributions), the risk may be partially mitigated by the OS-level implementation, but on platforms without it (Windows, BeOS, NetWare, Android), the vulnerability is directly exploitable. An unauthenticated, network-accessible attacker can send repeated authentication attempts and statistically analyze response timing to deduce the correct password or hash value (Apache Advisory, Feedly).

Successful exploitation allows an unauthenticated remote attacker to gradually recover password or hash values through statistical timing analysis, resulting in a high confidentiality impact with no integrity or availability impact. This could lead to unauthorized access to systems or services that rely on APR-util for password validation, such as Apache HTTP Server with digest authentication or other APR-dependent applications. The scope is limited to the affected system, but credential compromise could enable lateral movement in broader environments (Apache Advisory, Feedly).

There is no public proof-of-concept exploit and no evidence of active in-the-wild exploitation as of the disclosure date (Feedly). The EPSS score is 0.0, reflecting very low current exploitation probability. The vulnerability is rated as automatable (no user interaction required) by NVD SSVC analysis, meaning a scripted timing attack is feasible in principle. CVE-2025-49506 is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

1. Reconnaissance: Identify services using APR-util for password validation (e.g., Apache HTTP Server with digest or basic authentication) on platforms without native crypt() support (Windows, Android, etc.) using network scanning tools like Nmap or Shodan.
2. Establish baseline timing: Send a large number of authentication requests with known-incorrect passwords to establish a statistical baseline for response times.
3. Iterative character guessing: Send authentication attempts varying one character at a time (e.g., brute-forcing the first character of the password), measuring response times for each attempt across many repetitions to reduce noise.
4. Statistical analysis: Use statistical methods (e.g., t-tests or median comparison) to identify which character value produces a measurably longer response time, indicating a partial match in the non-constant-time comparison.
5. Repeat for each character: Iterate the process for each subsequent character position until the full password or hash is recovered.
6. Authenticate: Use the recovered credentials to gain unauthorized access to the target service (Apache Advisory, Feedly).

• Network: High volume of repeated authentication attempts (failed logins) from a single or small set of IP addresses against APR-util-backed services; requests may appear as normal authentication traffic but with unusually high frequency.
• Logs: Authentication failure logs showing thousands of attempts with slight variations in credentials from the same source IP; access logs showing repeated requests to authentication endpoints over an extended period.
• Process/Behavior: No unusual process spawning expected, as this is a passive timing attack; however, anomalous authentication attempt rates in application or web server logs are the primary indicator.

The primary remediation is to upgrade APR-util to version 1.6.4 or later, which fixes the non-constant-time comparison in apr_password_validate() (Apache Advisory). If immediate patching is not feasible, implement rate limiting and account lockout policies on authentication endpoints to reduce the practical feasibility of timing attacks. Restricting network access to authentication endpoints and monitoring for anomalous authentication attempt volumes are additional defensive measures. Deployments on Windows, BeOS, NetWare, or Android should be prioritized for patching due to the absence of native crypt() support.

The vulnerability was disclosed via the Apache security mailing list and cross-posted to oss-security (oss-sec). There was minor social media activity on Bluesky shortly after disclosure. No significant vendor statements beyond the Apache advisory or notable researcher commentary have been identified at this time.