CVE-2021-30477

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-04-15 · updated 2026-06-17

Description

An issue was discovered in Zulip Server before 3.4. A bug in the implementation of replies to messages sent by outgoing webhooks to private streams meant that an outgoing webhook bot could be used to send messages to private streams that the user was not intended to be able to send messages to.

Affected products

zulip: before 3.4 — fixed in 3.4

How to fix

Upgrade to a fixed version: zulip 3.4. Check the vendor advisory for mitigations.

References

https://blog.zulip.com/2021/04/14/zulip-server-3-4/
https://blog.zulip.com/2021/04/14/zulip-server-3-4/

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