CVE-2020-28935— Unbound 后置链接漏洞

Local symlink attack in Unbound and NSD

NLnet Labs Unbound, up to and including version 1.12.0, and NLnet Labs NSD, up to and including version 4.3.3, contain a local vulnerability that would allow for a local symlink attack. When writing the PID file, Unbound and NSD create the file if it is not there, or open an existing file for writing. In case the file was already present, they would follow symlinks if the file happened to be a symlink instead of a regular file. An additional chown of the file would then take place after it was written, making the user Unbound/NSD is supposed to run as the new owner of the file. If an attacker has local access to the user Unbound/NSD runs as, she could create a symlink in place of the PID file pointing to a file that she would like to erase. If then Unbound/NSD is killed and the PID file is not cleared, upon restarting with root privileges, Unbound/NSD will rewrite any file pointed at by the symlink. This is a local vulnerability that could create a Denial of Service of the system Unbound/NSD is running on. It requires an attacker having access to the limited permission user Unbound/NSD runs as and point through the symlink to a critical file on the system.

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在文件访问前对链接解析不恰当(链接跟随)

Unbound 后置链接漏洞

Unbound是荷兰NLnet Labs（Nlnet Labs）基金会的一款支持验证递归和缓存的DNS解析器。 NLnet Labs Unbound 1.12.0版本及之前版本和NLnet Labs NSD 4.3.3版本及之前版本存在安全漏洞，该漏洞允许一个本地符号链接攻击。在写入PID文件时，如果文件不存在，Unbound和NSD将创建该文件，或者打开现有文件进行写入。在文件已经存在的情况下，如果文件碰巧是符号链接而不是常规文件，则它们将遵循符号链接。

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