String Format Overflow in syslog()
Attack Pattern ID: 67 (Detailed Attack Pattern Completeness: Complete)Typical Severity: Very HighStatus: Draft
+ Description


This attack targets the format string vulnerabilities in the syslog() function. An attacker would typically inject malicious input in the format string parameter of the syslog function. This is a common problem, and many public vulnerabilities and associated exploits have been posted.

Attack Execution Flow

  1. The attacker finds that he can inject data to the format string parameter of Syslog().

  2. The attacker craft a malicious input and inject it into the format string parameter. From now on, the attacker can exeute arbitrary code and do more damage.

+ Attack Prerequisites

The format string argument of the Syslog function can be tainted with user supplid data.

+ Typical Likelihood of Exploit

Likelihood: High

+ Methods of Attack
  • Injection
+ Examples-Instances


Format string vulnerability in TraceEvent function for ntop before 2.1 allows remote attackers to execute arbitrary code by causing format strings to be injected into calls to the syslog function, via (1) an HTTP GET request, (2) a user name in HTTP authentication, or (3) a password in HTTP authentication.

Related Vulnerabilities


+ Probing Techniques

If the source code of the application is available, an attacker can use static analysis tools to spot a syslog vulnerability (a simple grep may also work).

If the source code is not available, automated tools such as Fuzzer and advanced Web Scanner can be used. If the tool supplied data reaches the syslog's format string argument, the application under scrutiny may have unexpected behavior.

If the source code is not available, a more complexe technique involve the use of library and system call tracer combined with the use of binary auditing tool such as IDA Pro. Reverse Engineering technique can be used to find format string vulnerability in the syslog function call. For instance it is possible to get the address of the buffer that is later used as the format string when reading data

+ Solutions and Mitigations

The code should be reviewed for misuse of the Syslog function call. Manual or automated code review can be used. The reviewer needs to ensure that all format string functions are passed a static string which cannot be controlled by the user and that the proper number of arguments are always sent to that function as well. If at all possible, do not use the %n operator in format strings. The following code shows a correct usage of Syslog():

syslog(LOG_ERR, "%s", cmdBuf);

The following code shows a vulnerable usage of Syslog():

syslog(LOG_ERR, cmdBuf); // the buffer cmdBuff is taking user supplied data.
+ Attack Motivation-Consequences
  • Run Arbitrary Code
  • Denial of Service
  • Privilege Escalation
  • Data Modification
+ Injection Vector

Untrusted user supplied data is the injection vector.

+ Payload

The maliciously crafted data can read from the stack if passed to the Syslog function as a format String.

+ Activation Zone

The signature of the syslog function is as following : "void syslog(int priority, const char *format, ...);"

A vulnerable usage would be : "syslog(LOG_ERR, cmdBuf);" where cmdBuf is a user controlled data which leads to a format string vulnerability. The activation zone is the format String argument which accepts user supplied data.

+ Payload Activation Impact

The impacts of this attack can be execution of arbitrary code. Execution of arbitary code can lead to many problems such as corruption of data, unauthorized access, etc.

+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')Secondary
134Uncontrolled Format StringTargeted
74Failure to Sanitize Data into a Different Plane ('Injection')Secondary
20Improper Input ValidationSecondary
680Integer Overflow to Buffer OverflowTargeted
697Insufficient ComparisonTargeted
+ Related Vulnerabilities
Vulnerability IDRelationship Description

format string in bad call to syslog function


format string in bad call to syslog function


format string in bad call to syslog function

+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
ChildOfAttack PatternAttack Pattern100Overflow Buffers 
Mechanism of Attack (primary)1000
ChildOfCategoryCategory339WASC Threat Classification 2.0 - WASC-06 - Format String 
WASC Threat Classification 2.0333
ParentOfAttack PatternAttack Pattern69Target Programs with Elevated Privileges 
Mechanism of Attack1000
+ Relevant Security Requirements

Choose a language which is not subject to this flaw.

Do not use the Syslog() in your implementation.

Use manual or automated code review to spot potential format string vulnerability in functions such as Syslog(), Vsyslog(), snprintf(), etc.

+ Related Security Principles
  • Reluctance to Trust

+ Related Guidelines
  • Verify that input is of a limited size.

  • If the message is coming from an outside source, check for %s type parameters and ensure that bounds will not be overwritten.

  • Don't use text from an outside source as a format string.

+ Purposes
  • Penetration
  • Exploitation
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: High
+ Technical Context
Architectural Paradigms
+ References
G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.

CWE - Buffer Errors

Exploiting Format String Vulnerabilities, scut / team teso,

Halvar Flake, "Auditing binaries for security vulnerabilities",

Fortify Taxonomy of Vulnerabilities : - Fortify Software (

Syslog man page :

+ Content History
G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.Cigital, Inc2007-03-01
Eric DalciCigital, Inc2007-02-13Fleshed out content to CAPEC schema from the original descriptions in "Exploiting Software"
Sean BarnumCigital, Inc2007-03-05Review and revise
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in Related Attack Patterns
Sean BarnumCigital, Inc2007-04-13Modified pattern content according to review and feedback