phpBB 3.2.5 Denial Of Service

Credit: Colin Snover
Risk: Medium
Local: No
Remote: Yes

CVSS Base Score: 5/10
Impact Subscore: 2.9/10
Exploitability Subscore: 10/10
Exploit range: Remote
Attack complexity: Low
Authentication: No required
Confidentiality impact: None
Integrity impact: None
Availability impact: Partial

Vulnerability information ========================= Title: phpBB Native Fulltext Search denial of service CVE ID: CVE-2019-9826 CVSSv3 score: 8.6 (AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H) Vulnerability description ========================= Improper input validation in the Native Fulltext Search component of phpBB 3.2.5 and earlier allows an unauthenticated remote user to trigger a denial of service attack via the keywords URL parameter of search.php. Successful exploitation generates a slow SQL query which causes the database engine used by phpBB to consume all available CPU resources. Depending upon the database engine, users will also be completely unable to create or modify posts due to locks on the search index tables. The slowness of the query depends on the size of the search_wordlist and search_wordmatch tables. Because the denial of service is caused by a long-running database query, for a typical phpBB installation running on MySQL/Linux, only <# CPUs> requests need to be made by an attacker in order to consume all CPU resources available to the database engine. The slow query will continue to run after the attacker disconnects because PHP does not detect connection aborts until it tries to send data back to the client. Vulnerable packages =================== phpBB 3.2.5 and earlier when configured to use the Native Fulltext search component. (This is the default configuration.) Solutions and workarounds ========================= The vendor has released phpBB 3.2.6, which improves input validation in the Native Fulltext Search component. Mitigations are available for earlier versions of phpBB: 1. Set “Search backend” to an engine other than “phpBB Native Fulltext” 2. Set the “Can search board” group permission to “No” for all untrusted user groups 3. Set “Enable search facilities” to “No” Proof of concept ================ Due to the triviality of the attack, proof of concept code is withheld for the moment in order to allow users some time to test and install the vendor patch. Report timeline =============== The vendor was given a initial disclosure deadline of 2019-04-22. A one week grace period was granted so that a fixed release could be available at the time of disclosure. 2019-02-18: Initial disclosure to vendor with PoC and candidate patch 2019-02-19: Vendor acknowledges receipt of report 2019-03-12: Update requested 2019-03-13: Vendor verifies vulnerability 2019-03-15: Vendor assigns CVE ID 2019-03-19: Follow-up, no response 2019-04-15: Second follow-up 2019-04-18: Vendor requests extension to disclosure date 2019-04-22: One week extension granted 2019-04-29: Vendor patch released 2019-04-29: Public disclosure

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