---
id: CVE-2026-121577
title: >-
  Unescaped header separators in whatstk build a regular expression that
  backtracks exponentially
summary: >-
  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 u…
severity: medium
cvss: 6.7
cvssVector: 'CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N'
cvssSource: cna
cwe:
  - CWE-1333
vendor: lucasrodes
product: whatstk
affected:
  - whatstk >= 0.2.0 < 0.9.0
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-11-06T19:49:38.845810Z'
published: '2026-11-03'
updated: '2026-11-06'
sourceUpdated: '2026-11-06T19:49:49.291Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-121577'
references:
  - url: 'https://github.com/lucasrodes/whatstk/pull/181'
  - url: 'https://github.com/lucasrodes/whatstk/commit/01d66d8'
  - url: 'https://github.com/lucasrodes/whatstk/releases/tag/v0.9.0'
  - url: >-
      https://secur0.com/en/cna/cve-list/cve-2026-121577-whatstk-header-regex-redos
tags:
  - cve.org
epss: 0.00129
epssPercentile: 0.02515
ingestedAt: '2026-11-05T20:33:06.423Z'
---

## Overview

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.

## Affected

- `whatstk >= 0.2.0 < 0.9.0`

## Remediation

Upgrade to whatstk 0.9.0 or later.