CVE-2026-97701: Zulip: Missing content-length limit on POST /messages/render allows denial-of-service via oversized requests
Zulip is an open-source team collaboration tool. In Zulip Server versions through 12.1, the authenticated POST /messages/render endpoint renders the content parameter without enforcing the normal message length limit used by message send and edit operations. An authenticated user can submit Markdown up to the server body-size limit, causing disproportionate backend CPU consumption, and repeated or concurrent requests can degrade Markdown rendering performance or backend availability for other users. This issue is fixed in version 12.2.
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Zulip Serverto a version that resolves this vulnerability.Fixed in 12.2
Event History
Frequently Asked Questions
Who can exploit this issue?
Any authenticated Zulip user can send oversized content to the POST /messages/render endpoint. No user interaction is required, and the affected endpoint is reachable over the network.
Are default message length protections sufficient?
No. The endpoint does not enforce the normal message length limit applied to message send and edit operations. Request size is instead bounded by the server body-size limit.
What is the impact of exploitation?
Large Markdown payloads can cause disproportionate backend CPU use during rendering. Repeated or concurrent requests can degrade rendering performance or reduce backend availability for other users.
Which versions are affected and what fixes the issue?
Zulip Server versions through 12.1 are affected. The issue is fixed in version 12.2.