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Fencer Intel
medium 6.1

CVE-2021-27889 json

Cross-site Scripting (XSS) vulnerability in MyBB before 1.8.26 via Nested Auto URL when parsing messages.

published 5y ago modified 4mo ago also EUVD-2021-14626

cvss
6.1
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
5.1%
EPSS 5.1% chance of exploitation in 30 days, percentile 92%
public exploits
1
Exploit-DB

Summary

Cross-site Scripting (XSS) vulnerability in MyBB before 1.8.26 via Nested Auto URL when parsing messages.

Patch / fix

  • mybb mybb 1.8.26

Is CVE-2021-27889 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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Timeline

  1. Published
  2. First public exploit (EDB-49696)

Exploit prediction

5.07% chance of exploitation in the next 30 days, higher than 92% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

  • EDB-49696 MyBB 1.8.25 - Chained Remote Command Execution webapps, 2021-03-22
  • packetstormsecurity.com http://packetstormsecurity.com/files/161908/MyBB-1.8.25-Remote-Command-Execution.html referenced PoC, tagged "exploit" by a source, unverified
  • blog.sonarsource.com https://blog.sonarsource.com/mybb-remote-code-execution-chain referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N

Classification

Weakness
  • CWE-79 Improper Neutralization of Input During Web Page Generation

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.