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Fencer Intel
high 7.2

CVE-2019-12831 json

In MyBB before 1.8.21, an attacker can abuse a default behavior of MySQL on many systems (that leads to truncation of strings that are too…

published 7y ago modified 4mo ago also EUVD-2019-4411

cvss
7.2
version 3.0, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.5%
EPSS 1.5% chance of exploitation in 30 days, percentile 73%
public exploits
0
none known

Summary

In MyBB before 1.8.21, an attacker can abuse a default behavior of MySQL on many systems (that leads to truncation of strings that are too long for a database column) to create a PHP shell in the cache directory of a targeted forum via a crafted XML import, as demonstrated by truncation of aaaaaaaaaaaaaaaaaaaaaaaaaa.php.css to aaaaaaaaaaaaaaaaaaaaaaaaaa.php with a 30-character limit, aka theme import stylesheet name RCE.

Patch / fix

  • mybb mybb 1.8.21

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Exploit prediction

1.47% chance of exploitation in the next 30 days, higher than 73% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

  • blog.ripstech.com https://blog.ripstech.com/2019/mybb-stored-xss-to-rce/ referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.0 7.2 CVSS:3.0/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 6.5 AV:N/AC:L/Au:S/C:P/I:P/A:P

Classification

Weakness
  • CWE-20 Improper Input Validation

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.