Name Symlink Attack
Summary An attacker positions a symbolic link in such a manner that the targeted user or application accesses the link's endpoint, assuming that it is accessing a file with the link's name. The endpoint file may be either output or input. If the file is output, the result is that the endpoint is modified, instead of a file at the intended location. Modifications to the endpoint file may include appending, overwriting, corrupting, changing permissions, or other modifications. In some variants of this attack the attacker may be able to control the change to a file while in other cases they cannot. The former is especially damaging since the attacker may be able to grant themselves increased privileges or insert false information, but the latter can also be damaging as it can expose sensitive information or corrupt or destroy vital system or application files. Alternatively, the endpoint file may serve as input to the targeted application. This can be used to feed malformed input into the target or to cause the target to process different information, possibly allowing the attacker to control the actions of the target or to cause the target to expose information to the attacker. Moreover, the actions taken on the endpoint file are undertaken with the permissions of the targeted user or application, which may exceed the permissions that the attacker would normally have.
Prerequisites The targeted application must perform the desired activities on a file without checking whether the file is a symbolic link or not. The attacker must be able to predict the name of the file the target application is modifying and be able to create a new symbolic link where that file would appear.
Solutions Design: Check for the existence of files to be created, if in existence verify they are neither symlinks nor hard links before opening them. Implementation: Use randomly generated file names for temporary files. Give the files restrictive permissions.
Related Weaknesses
CWE ID Description
CWE-59 Improper Link Resolution Before File Access ('Link Following')
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