CVE-2021-32839 in sqlparse
Summary
by MITRE • 09/20/2021
sqlparse is a non-validating SQL parser module for Python. In sqlparse versions 0.4.0 and 0.4.1 there is a regular Expression Denial of Service in sqlparse vulnerability. The regular expression may cause exponential backtracking on strings containing many repetitions of '\r\n' in SQL comments. Only the formatting feature that removes comments from SQL statements is affected by this regular expression. As a workaround don't use the sqlformat.format function with keyword strip_comments=True or the --strip-comments command line flag when using the sqlformat command line tool. The issues has been fixed in sqlparse 0.4.2.
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Analysis
by VulDB Data Team • 11/04/2025
The vulnerability identified as CVE-2021-32839 affects the sqlparse Python module, a non-validating SQL parser that developers commonly use for parsing and formatting SQL statements. This module serves as a utility for SQL processing in various applications, particularly those requiring SQL statement manipulation, formatting, or analysis. The specific issue manifests within the regular expression patterns used by the module's formatting functionality, specifically when processing SQL comments. The vulnerability stems from the module's handling of strings containing multiple consecutive repetitions of the single quote character within SQL comments, creating a condition where regular expression engines can experience exponential backtracking. This behavior represents a classic example of a regular expression denial of service vulnerability, which falls under the CWE-400 category of uncontrolled resource consumption.
The technical flaw occurs in the regular expression engine's backtracking mechanism when processing SQL statements containing repeated single quote characters within comments. When the sqlformat.format function is invoked with the strip_comments=True parameter, or when using the --strip-comments command line flag, the vulnerable regular expression pattern triggers exponential backtracking behavior. This means that as the input string contains more repetitions of the problematic character sequence, the processing time increases exponentially rather than linearly, effectively consuming system resources and potentially causing application unresponsiveness. The vulnerability is particularly concerning because it affects the formatting feature that removes comments from SQL statements, which is a common operation in SQL processing workflows, making it a likely target for exploitation in applications that process user-provided SQL content.
The operational impact of this vulnerability extends beyond simple performance degradation to potential system availability issues. Applications that rely on sqlparse for SQL formatting and comment stripping may become unresponsive or crash when processing specially crafted SQL input containing repetitive single quote sequences. This affects not only direct usage of the module but also applications that depend on it as a library component, including database management tools, SQL analysis utilities, and automated SQL processing systems. The vulnerability can be exploited by attackers who craft malicious SQL input to cause resource exhaustion, leading to denial of service conditions that can disrupt legitimate operations. From an ATT&CK perspective, this vulnerability maps to the T1499.004 technique involving network denial of service, though the attack vector is more specific to resource exhaustion through regular expression processing rather than network-level attacks.
Mitigation strategies for CVE-2021-32839 involve immediate upgrading to sqlparse version 0.4.2 or later, which contains the patched regular expression patterns that prevent exponential backtracking. Organizations should also implement input validation measures to sanitize SQL input before processing, particularly when using the strip_comments functionality. The workaround suggested in the original advisory, which involves avoiding the use of strip_comments functionality, remains valid for systems unable to upgrade immediately. Security teams should monitor for applications that may be using vulnerable versions of sqlparse through dependency analysis tools and ensure proper patch management procedures are in place. The fix implemented in version 0.4.2 demonstrates proper regular expression design practices that prevent catastrophic backtracking while maintaining the module's core functionality. Additionally, organizations should consider implementing rate limiting and resource monitoring for SQL processing functions to detect and mitigate potential exploitation attempts. This vulnerability highlights the importance of regular expression security reviews and the need for robust input validation in parsing libraries that handle untrusted user input.