CVE-2021-20197 in Binutils
Summary
by MITRE • 03/27/2021
There is an open race window when writing output in the following utilities in GNU binutils version 2.35 and earlier:ar, objcopy, strip, ranlib. When these utilities are run as a privileged user (presumably as part of a script updating binaries across different users), an unprivileged user can trick these utilities into getting ownership of arbitrary files through a symlink.
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Analysis
by VulDB Data Team • 12/03/2025
The vulnerability identified as CVE-2021-20197 represents a critical race condition flaw affecting several core utilities within GNU binutils version 2.35 and earlier. This issue specifically impacts the ar, objcopy, strip, and ranlib commands that are fundamental components of the GNU binutils package used extensively in Unix-like operating systems for managing object files and archives. The vulnerability stems from an improper handling of file operations during the output writing process, creating an exploitable window where malicious actors can manipulate the system's file ownership semantics.
The technical flaw manifests when these utilities are executed with elevated privileges by privileged users, typically within automated scripts or build processes that update binaries across multiple user contexts. During the execution of these commands, a race condition exists between the time when the utility checks for file existence and when it actually writes to the target file. An unprivileged user can exploit this temporal gap by creating symbolic links that point to arbitrary target files, causing the privileged utility to write to unintended locations while inheriting the ownership of those files. This vulnerability is particularly dangerous because it allows attackers to escalate privileges and gain unauthorized control over system resources.
The operational impact of CVE-2021-20197 extends beyond simple privilege escalation, as it can enable attackers to manipulate system binaries, modify critical configuration files, or establish persistent access points within the target environment. The vulnerability affects systems where these utilities are commonly used in build environments, package management systems, or automated deployment scripts, making it a significant concern for enterprise environments and development pipelines. Attackers can leverage this weakness to subvert the integrity of system binaries and potentially compromise the entire system through file ownership manipulation. The vulnerability aligns with CWE-367, which specifically addresses Time-of-Check to Time-of-Use (TOCTOU) race conditions, and can be mapped to ATT&CK technique T1068, which covers the exploitation of privilege escalation vulnerabilities.
Mitigation strategies for CVE-2021-20197 primarily focus on updating to GNU binutils version 2.36 or later, where the race condition has been addressed through improved file handling mechanisms and proper atomic operations. System administrators should also implement restrictive file permissions and ensure that build environments are properly sandboxed to prevent unprivileged users from manipulating symbolic links in directories where privileged utilities are executed. Additionally, monitoring for suspicious symlink creation patterns and implementing proper file integrity checks can help detect exploitation attempts. Organizations should conduct comprehensive audits of their build and deployment processes to identify instances where these vulnerable utilities are executed with elevated privileges, particularly in shared or multi-user environments where the attack surface is most significant.