CVE-2021-3337: The Hide-Thread-Content plugin through 2021-01-27 for MyBB allows remote attackers to bypass intended content-

This week

HIGH 7.5REMOTEDOWNGRADE

CVE-2021-3337 is a high-severity flaw (CVSS 7.5) affecting hide thread content. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The Hide-Thread-Content plugin through 2021-01-27 for MyBB allows remote attackers to bypass intended content-reading restrictions by clicking on reply or quote in the postbit.

Why this priority

High severity
EPSS top 4%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	11.6% (top 4%)
CISA exploited list	No
Weakness	CWE-863
Published	2021-01-28
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: hide thread content project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-01-28 20:15 UTCPublished CVE record published
2026-06-17 04:04 UTCUpdated Record last modified

hide thread content project hide thread content DOWNGRADE

Affected: 1.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.0

1. Check installed version

apt-cache policy hide-thread-content || rpm -q hide-thread-content || hide-thread-content --version

2. No fixed release: DOWNGRADE to a release before 1.0 (Debian / Ubuntu)

apt-cache madison hide-thread-content sudo apt install hide-thread-content=<VERSION_BEFORE_1.0> sudo apt-mark hold hide-thread-content

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list hide-thread-content sudo dnf downgrade hide-thread-content-<VERSION_BEFORE_1.0> sudo dnf versionlock add hide-thread-content

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Exploit-DB: MyBB Hide Thread Content Plugin 1.0 - Information Disclosure

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-3337?

hide thread content: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-3337 being exploited?

CVE-2021-3337 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-3337 affect?

CVE-2021-3337 affects hide thread content. Severity is high (CVSS 7.5). The Hide-Thread-Content plugin through 2021-01-27 for MyBB allows remote attackers to bypass intended content-reading restrictions by clicking on reply or quote in the postbit.

How urgent is CVE-2021-3337?

Priority 60/100. High severity; EPSS top 4%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

References

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