CVE-2021-29472 in Composer
Summary
by MITRE • 04/28/2021
Composer is a dependency manager for PHP. URLs for Mercurial repositories in the root composer.json and package source download URLs are not sanitized correctly. Specifically crafted URL values allow code to be executed in the HgDriver if hg/Mercurial is installed on the system. The impact to Composer users directly is limited as the composer.json file is typically under their own control and source download URLs can only be supplied by third party Composer repositories they explicitly trust to download and execute source code from, e.g. Composer plugins. The main impact is to services passing user input to Composer, including Packagist.org and Private Packagist. This allowed users to trigger remote code execution. The vulnerability has been patched on Packagist.org and Private Packagist within 12h of receiving the initial vulnerability report and based on a review of logs, to the best of our knowledge, was not abused by anyone. Other services/tools using VcsRepository/VcsDriver or derivatives may also be vulnerable and should upgrade their composer/composer dependency immediately. Versions 1.10.22 and 2.0.13 include patches for this issue.
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Analysis
by VulDB Data Team • 05/02/2021
The vulnerability identified as CVE-2021-29472 affects Composer, a widely-used dependency manager for PHP applications. This security flaw resides in the handling of URLs within Mercurial repository configurations, specifically within the root composer.json file and package source download URLs. The vulnerability stems from inadequate sanitization of URL values that allows maliciously crafted input to execute arbitrary code through the HgDriver component when Mercurial is installed on the system. The technical implementation flaw represents a classic command injection vulnerability where user-supplied URLs are not properly validated or escaped before being processed by the underlying Mercurial command execution mechanism.
The operational impact of this vulnerability extends beyond individual Composer users to service providers and infrastructure that process user input through Composer. While direct exploitation against individual developers is limited since composer.json files are typically under user control, the real risk emerges when services like Packagist.org and Private Packagist process external user input. These platforms became potential attack vectors where malicious actors could submit crafted URLs that would execute arbitrary code on the hosting systems. The vulnerability maps to CWE-78 and CWE-94 in the Common Weakness Enumeration catalog, representing improper neutralization of special elements used in OS commands and code injection respectively. The ATT&CK framework categorizes this under T1059.007 for command and scripting interpreter and T1190 for exploitation of remote services, highlighting the remote code execution capabilities that emerged from this flaw.
The remediation efforts demonstrated by Packagist.org and Private Packagist showed rapid response protocols, patching the vulnerability within 12 hours of initial reporting. This swift action prevented potential exploitation and reflects the severity of the threat. The fix was incorporated into Composer versions 1.10.22 and 2.0.13, which addressed the URL sanitization issues in the VcsRepository and VcsDriver components. The vulnerability's impact was particularly concerning for services that rely on Composer's VcsDriver functionality, as these systems became potential entry points for attackers seeking to execute arbitrary commands on the underlying infrastructure. Security practitioners should note that this vulnerability affected not only public package repositories but also any service or tool that implements the VcsRepository/VcsDriver pattern, making it a critical update for the entire PHP ecosystem. The vulnerability underscores the importance of proper input validation and sanitization in package management systems, particularly when dealing with external repository URLs that may be processed through system commands.