Grub2 1.98/1.99/2.00/2.01/2.02 Rescue Shell auth.c grub_username_get access control

CVSS Meta Temp Score
CVSS is a standardized scoring system to determine possibilities of attacks. The Temp Score considers temporal factors like disclosure, exploit and countermeasures. The unique Meta Score calculates the average score of different sources to provide a normalized scoring system.
Current Exploit Price (≈)
Our analysts are monitoring exploit markets and are in contact with vulnerability brokers. The range indicates the observed or calculated exploit price to be seen on exploit markets. A good indicator to understand the monetary effort required for and the popularity of an attack.
CTI Interest Score
Our Cyber Threat Intelligence team is monitoring different web sites, mailing lists, exploit markets and social media networks. The CTI Interest Score identifies the interest of attackers and the security community for this specific vulnerability in real-time. A high score indicates an elevated risk to be targeted for this vulnerability.

A vulnerability was found in Grub2 1.98/1.99/2.00/2.01/2.02. It has been declared as problematic. This vulnerability affects the function grub_username_get of the file auth.c of the component Rescue Shell. The manipulation with an unknown input leads to a privilege escalation vulnerability. The CWE definition for the vulnerability is CWE-264. As an impact it is known to affect confidentiality, integrity, and availability.

The issue has been introduced in 12/01/2009. The weakness was presented 12/14/2015 by Hector Marco and Ismael Ripoll as not defined posting (Blog). The advisory is shared for download at This vulnerability was named CVE-2015-8370 since 11/26/2015. The attack needs to be approached locally. No form of authentication is required for a successful exploitation. Technical details and also a public exploit are known. The MITRE ATT&CK project declares the attack technique as T1068. Responsible for the vulnerability is the following code:

if (key == '\b')
The advisory points out:
The fault (bug) is in the code of Grub since version 1.98 (December, 2009). The commit which introduced the fault was b391bdb2f2c5ccf29da66cecdbfb7566656a704d, affecting the grub_password_get() function. There are two functions which suffer the same integer underflow fault. The grub_username_get() and grub_password_get() located in grub-core/normal/auth.c and lib/crypto.c respectively. Both functions are equal except for a call to printf() in the grub_username_get(). The PoC described here is based only on exploiting the grub_username_get() to obtain a Grub rescue shell.

A public exploit has been developed by Hector Marco and been published immediately after the advisory. It is declared as proof-of-concept. It is possible to download the exploit at The vulnerability was handled as a non-public zero-day exploit for at least 2204 days. During that time the estimated underground price was around $5k-$25k. The vulnerability scanner Nessus provides a plugin with the ID 87422 (CentOS 7 : grub2 (CESA-2015:2623)), which helps to determine the existence of the flaw in a target environment. It is assigned to the family CentOS Local Security Checks and running in the context local. The commercial vulnerability scanner Qualys is able to test this issue with plugin 124566 (Solaris 11.3 Support Repository Update (SRU) 4.5.0 Missing). The code used by the exploit is:

memset destination address = buf + cur_len
The advisory illustrates:
To quickly check if your system is vulnerable, when the Grub ask you the username, press the Backspace 28 times. If your machine reboots or you get a rescue shell then your Grub is affected.

Applying a patch is able to eliminate this problem. The bugfix is ready for download at A possible mitigation has been published 22 hours after the disclosure of the vulnerability. The vulnerability will be addressed with the following lines of code:

if (key == '\b' && cur_len)

The vulnerability is also documented in the databases at Vulnerability Center (SBV-55312) and Tenable (87422). Additional details are provided at See 80389, 80403, 80401 and 80398 for similar entries.



CPE 2.3info

CPE 2.2info



VulDB Meta Base Score: 5.9
VulDB Meta Temp Score: 5.4

VulDB Base Score: 5.9
VulDB Temp Score: 5.4
VulDB Vector: 🔍
VulDB Reliability: 🔍



VulDB Base Score: 🔍
VulDB Temp Score: 🔍
VulDB Reliability: 🔍

NVD Base Score: 🔍


Class: Privilege escalation
CWE: CWE-264
ATT&CK: T1068

Local: Yes
Remote: No

Availability: 🔍
Access: Public
Status: Proof-of-Concept
Author: Hector Marco
Download: 🔍

EPSS Score: 🔍
EPSS Percentile: 🔍

Price Prediction: 🔍
Current Price Estimation: 🔍


Nessus ID: 87422
Nessus Name: CentOS 7 : grub2 (CESA-2015:2623)
Nessus File: 🔍
Nessus Risk: 🔍
Nessus Family: 🔍
Nessus Context: 🔍

OpenVAS ID: 703421
OpenVAS Name: Debian Security Advisory DSA 3421-1 (grub2 - security update)
OpenVAS File: 🔍
OpenVAS Family: 🔍

Qualys ID: 🔍
Qualys Name: 🔍

Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍


Recommended: Patch
Status: 🔍

Reaction Time: 🔍
0-Day Time: 🔍
Exposure Time: 🔍
Exploit Delay Time: 🔍



12/01/2009 🔍
11/26/2015 +2186 days 🔍
12/13/2015 +17 days 🔍
12/14/2015 +0 days 🔍
12/14/2015 +0 days 🔍
12/14/2015 +0 days 🔍
12/14/2015 +0 days 🔍
12/15/2015 +1 days 🔍
12/16/2015 +0 days 🔍
12/16/2015 +0 days 🔍
12/17/2015 +0 days 🔍
07/01/2022 +2388 days 🔍


Advisory: RHSA-2015:2623
Researcher: Hector Marco/Ismael Ripoll
Status: Not defined
Confirmation: 🔍

CVE: CVE-2015-8370 (🔍)

SecurityTracker: 1034422
Vulnerability Center: 55312 - Grub2 1.98 - 2.02 Local Security Bypass Vulnerability in grub_password_get(), High
SecurityFocus: 79358 - GNU GRUB2 CVE-2015-8370 Multiple Local Authentication Bypass Vulnerabilities

scip Labs:
Misc.: 🔍
See also: 🔍


Created: 12/17/2015 09:28 AM
Updated: 07/01/2022 09:49 AM
Changes: (1) advisory_identifier
Complete: 🔍


No comments yet. Languages: en.

Please log in to comment.

Want to stay up to date on a daily basis?

Enable the mail alert feature now!