CVE-2005-3420: usercp_register.php in phpBB 2.0.17 allows remote attackers to modify regular expressions and execute PHP code

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HIGH 7.5DOWNGRADE

CVE-2005-3420 is a high-severity flaw (CVSS 7.5) affecting phpbb. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

usercp_register.php in phpBB 2.0.17 allows remote attackers to modify regular expressions and execute PHP code via the signature_bbcode_uid parameter, as demonstrated by injecting an "e" modifier into a preg_replace statement.

Why this priority

High severity

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

Risk facts

CVSS score	7.5 (High)
EPSS exploit chance	2.4% (top 16%)
CISA exploited list	No
Published	2005-11-01
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: phpbb group

Last updated 2026-06-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2005-11-01 21:02 UTCPublished CVE record published
2026-06-16 22:16 UTCUpdated Record last modified

phpbb group phpbb DOWNGRADE

Affected: 2.0.0 · 2.0.1 · 2.0.2 · 2.0.3 · 2.0.4 · 2.0.5 · 2.0.6 · 2.0.6c

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.0.0

1. Check installed version

apt-cache policy phpbb || rpm -q phpbb || phpbb --version

2. No fixed release: DOWNGRADE to a release before 2.0.0 (Debian / Ubuntu)

apt-cache madison phpbb sudo apt install phpbb=<VERSION_BEFORE_2.0.0> sudo apt-mark hold phpbb

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

dnf --showduplicates list phpbb sudo dnf downgrade phpbb-<VERSION_BEFORE_2.0.0> sudo dnf versionlock add phpbb

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.

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-2005-3420?

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

Is CVE-2005-3420 being exploited?

CVE-2005-3420 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-2005-3420 affect?

CVE-2005-3420 affects phpbb. Severity is high (CVSS 7.5). usercp_register.php in phpBB 2.0.17 allows remote attackers to modify regular expressions and execute PHP code via the signature_bbcode_uid parameter, as demonstrated by injecting an "e" modifier into a preg_replace statement.

How urgent is CVE-2005-3420?

Priority 47/100. High severity

Software pages

PHP vulnerabilities

Related CVEs

CVE-2026-29199 HIGH 8.1 UPGRADE phpBB before 3.3.16 is vulnerable to Host Header Injection that can lead to password rest link poisoning
CVE-2025-70811 MEDIUM 4.3 DOWNGRADE Cross Site Request Forgery vulnerability in Phpbb phbb3 v.3.3.15 allows a local attacker to execute arbitrary
CVE-2025-70810 HIGH 8.8 DOWNGRADE Cross Site Request Forgery vulnerability in Phpbb phbb3 v.3.3.15 allows a local attacker to execute arbitrary
CVE-2023-5917 LOW 2.4 UPGRADE A vulnerability, which was classified as problematic, has been found in phpBB up to 3.3.10
CVE-2020-8226 MEDIUM 5.8 UPGRADE A vulnerability exists in phpBB <v3.2.10 and <v3.3.1 which allowed remote image dimensions check to be used to

References

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