CVE-2026-121577 in whatstk
Summary
by MITRE • 11/03/2026
Inefficient Regular Expression Complexity (CWE-1333) in generate_regex() (whatstk/whatsapp/parser.py) in whatstk from 0.2.0 before 0.9.0 allows a party that supplies a chat export to consume the CPU of the process that parses it for an unbounded time, because the function builds the header regular expression by replacing each %x code with its named capture group and leaves every other character of the header format in the pattern unescaped. A header whose date separator is a regular expression metacharacter therefore alters the pattern instead of matching itself: a separator of + quantifies the preceding capture group, giving (?P<day>\d{1,2})+(?P<month>\d{1,2})+(?P<year>\d{2,4}), and a run of digits that cannot complete the header then makes the engine try every way of partitioning that run. Header auto detection is enabled by default and derives the format from the text of the file, so the separator is taken from the export itself and no header format has to be passed by the caller. Separators of (, ) and \ instead raise re.PatternError, separators of *, | and ? raise TypeError, and separators of ^ and $ raise KeyError, none of which the library catches. This results in the parse of a file of a few hundred bytes consuming CPU without completing, or in an uncaught exception in the calling program.
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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability identified as CWE-1333 involves inefficient regular expression complexity within the whatstk Python library, specifically affecting versions from 0.2.0 up to but not including 0.9.0. This flaw resides in the generate_regex function located in the whatsapp/parser.py module and represents a classic case of Regular Expression Denial of Service, often referred to as ReDoS. The core technical issue stems from how the library constructs regular expressions for parsing WhatsApp chat export headers. When processing these exports, the system dynamically builds a header regular expression by replacing specific placeholder codes with named capture groups while leaving other characters in the header format unescaped. This design choice creates a critical security gap because it fails to properly escape metacharacters that may appear as separators within the data being parsed.
The operational impact of this vulnerability is severe, primarily manifesting as an unbounded consumption of CPU resources by the process parsing the chat export. The danger arises when the date separator used in the WhatsApp export file corresponds to a regular expression metacharacter with quantifying properties, such as the plus sign. In such scenarios, the resulting pattern becomes something akin to (?P<day>\d{1,2})+(?P<month>\d{1,2})+(?P<year>\d{2,4}). When the regular expression engine attempts to match a run of digits that cannot fully satisfy this complex quantified structure, it engages in catastrophic backtracking. The engine exhaustively tries every possible way to partition the input string among the capture groups, leading to exponential time complexity relative to the length of the input. Consequently, even a small file containing only a few hundred bytes can cause the CPU usage to spike indefinitely without completing the parse operation.
It is important to note that this vulnerability is particularly dangerous due to the default configuration of the library. Header auto-detection is enabled by default and derives the format directly from the text content of the imported file. This means an attacker does not need to provide a specific header format string; they simply need to supply a maliciously crafted chat export where the date separator acts as a regex metacharacter that triggers inefficient backtracking. The library attempts to handle other problematic separators like parentheses, asterisks, pipes, question marks, carets, and dollar signs by raising exceptions such as re.PatternError, TypeError, or KeyError. However, these errors are not caught within the calling code, which can lead to uncaught exceptions crashing the application rather than just causing a denial of service through resource exhaustion.
From an industry standards perspective, this vulnerability aligns with CWE-1333 for inefficient regular expression complexity and falls under the ATT&CK technique T1496, Resource Hijacking, specifically within the context of CPU consumption to disrupt availability. To mitigate this risk, developers using whatstk should immediately upgrade to version 0.9.0 or later where these regex construction flaws have been addressed. For applications that cannot yet upgrade, it is crucial to implement strict input validation and sanitization for any chat export files before passing them to the parser. Additionally, wrapping the parsing logic in try-except blocks can prevent uncaught exceptions from crashing the host application, although this does not fully resolve the CPU exhaustion issue caused by catastrophic backtracking. Ensuring that all special characters in dynamic regex components are properly escaped is a fundamental best practice for preventing such vulnerabilities across software systems handling user-supplied data.