CVE-2021-23354 in printf Package
Summary
by MITRE • 03/13/2021
The package printf before 0.6.1 are vulnerable to Regular Expression Denial of Service (ReDoS) via the regex string /\%(?:\(([\w_.]+)\)|([1-9]\d*)\$)?([0 +\-\]*)(\*|\d+)?(\.)?(\*|\d+)?[hlL]?([\%bscdeEfFgGioOuxX])/g in lib/printf.js. The vulnerable regular expression has cubic worst-case time complexity.
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Analysis
by VulDB Data Team • 04/01/2021
The vulnerability identified as CVE-2021-23354 affects the printf package version 0.6.1 and earlier, presenting a significant security risk through Regular Expression Denial of Service. This issue stems from the package's implementation of a regular expression pattern designed to parse format specifiers in printf-style string formatting operations. The specific regex pattern /\%(?:\(([\w_.]+)\)|([1-9]\d)\$)?([0 +\-\])(\|\d+)?(\.)?(\|\d+)?[hlL]?([\%bscdeEfFgGioOuxX])/g demonstrates a critical flaw in its design that allows attackers to exploit computational complexity issues. The regex engine processes this pattern with cubic worst-case time complexity, meaning that input strings designed to trigger specific patterns can cause exponential execution time growth, effectively creating a denial of service condition.
This vulnerability operates through the manipulation of format string arguments passed to printf functions, where an attacker can craft malicious input that causes the regular expression engine to perform excessive backtracking operations. When the vulnerable package processes such inputs, the regex engine enters into a state where it must explore numerous possible matches due to the nested quantifiers and alternation constructs within the pattern. The pattern structure includes optional groups with quantifiers that create multiple paths for the regex engine to evaluate, particularly when encountering input strings that match the beginning of the pattern but fail to match the complete expression. This backtracking behavior leads to the exponential time complexity that characterizes ReDoS attacks, where processing time increases dramatically with input length.
The operational impact of this vulnerability extends beyond simple service disruption to potentially affect applications that rely on printf-style formatting for user input processing or logging operations. Systems utilizing the affected package may experience complete service unavailability when processing maliciously crafted format strings, as the computational overhead can cause timeouts or resource exhaustion. The vulnerability affects any application or system where the printf package is used for string formatting operations, particularly those that process untrusted input through printf functions. The cubic time complexity means that even relatively small malicious inputs can cause significant computational overhead, making this attack vector particularly effective for resource exhaustion attacks.
Security mitigations for CVE-2021-23354 involve updating to version 0.6.1 or later of the printf package where the vulnerable regular expression has been replaced with a more efficient implementation. Organizations should also implement input validation and sanitization measures to prevent malicious format strings from reaching the vulnerable parsing logic. The remediation process requires identifying all systems using the affected package and ensuring proper version updates are deployed across the environment. Additionally, implementing rate limiting and input length restrictions on format string processing can provide defense-in-depth measures against exploitation attempts. This vulnerability aligns with CWE-400, which specifically addresses the vulnerability of regular expressions to denial of service attacks through excessive backtracking, and represents a clear example of how improper regex design can create security weaknesses that directly impact system availability. The ATT&CK framework categorizes this under T1499.004, which covers Network Denial of Service, as the vulnerability enables attackers to consume system resources through crafted input processing.