Buffer Overflow via Parameter Expansion
Attack Pattern ID: 47 (Detailed Attack Pattern Completeness: Complete)Typical Severity: HighStatus: Draft
+ Description


In this attack, the target software is given input that the attacker knows will be modified and expanded in size during processing. This attack relies on the target software failing to anticipate that the expanded data may exceed some internal limit, thereby creating a buffer overflow.

Attack Execution Flow

  1. Consider parts of the program where user supplied data may be expanded by the program. Use a disassembler and other reverse engineering tools to guide the search.

  2. Find a place where a buffer overflow occurs due to the fact that the new expanded size of the string is not correctly accounted for by the program. This may happen perhaps when the string is copied to another buffer that is big enough to hold the original, but not the expanded string. This may create an opportunity for planting the payload and redirecting program execution to the shellcode.

  3. Write the buffer overflow exploit. To be exploitable, the "spill over" amount (e.g. the difference between the expanded string length and the original string length before it was expanded) needs to be sufficient to allow the overflow of the stack return pointer (in the case of a stack overflow), without causing a stack corruption that would crash the program before it gets to execute the shellcode. Heap overflow will be more difficult and will require the attacker to get more lucky, by perhaps getting a chance to overwrite some of the accounting information stored as part of using malloc().

+ Attack Prerequisites

The program expands one of the parameters passed to a function with input controlled by the user, but a later function making use of the expanded parameter erroneously considers the original, not the expanded size of the parameter.

The expanded parameter is used in the context where buffer overflow may becomes possible due to the incorrect understanding of the parameter size (i.e. thinking that it is smaller than it really is).

+ Typical Likelihood of Exploit

Likelihood: Medium

+ Methods of Attack
  • Injection
+ Examples-Instances


Attack Example: FTP glob()

The glob() function in FTP servers has been susceptible to attack as a result of incorrect resizing. This is an ftpd glob() Expansion LIST Heap Overflow Vulnerability. ftp daemon contains a heap-based buffer overflow condition. The overflow occurs when the LIST command is issued with an argument that expands into an oversized string after being processed by glob().

This buffer overflow occurs in memory that is dynamically allocated. It may be possible for attackers to exploit this vulnerability and execute arbitrary code on the affected host.

To exploit this, the attacker must be able to create directories on the target host.

The glob() function is used to expand short-hand notation into complete file names. By sending to the FTP server a request containing a tilde (~) and other wildcard characters in the pathname string, a remote attacker can overflow a buffer and execute arbitrary code on the FTP server to gain root privileges. Once the request is processed, the glob() function expands the user input, which could exceed the expected length. In order to exploit this vulnerability, the attacker must be able to create directories on the FTP server.

From G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.

Related Vulnerabilities



Buffer overflow in the glob implementation in libc in NetBSD-current before 20050914, and NetBSD 2.* and 3.* before 20061203, as used by the FTP daemon, allows remote authenticated users to execute arbitrary code via a long pathname that results from path expansion.

The limit computation of an internal buffer was done incorrectly. The size of the buffer in byte was used as element count, even though the elements of the buffer are 2 bytes long. Long expanded path names would therefore overflow the buffer.

Related Vulnerabilities


+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: High

Finding this particular buffer overflow may not be trivial. Also, stack and especially heap based buffer overflows require a lot of knowledge if the intended goal is aribtrary code execution. Not only that the attacker needs to write the shell code to accomplish his or her goals, but the attacker also needs to find a way to get the program execution to jump to the planted shellcode. There also needs to be sufficient room for the payload. So not every buffer overflow will be exploitable, even by a skilled attacker.

+ Resources Required

Access to the program source or binary. If the program is only available in binary then a disassembler and other reverse engineering tools will be helpful.

+ Solutions and Mitigations

Ensure that when parameter expansion happens in the code that the assumptions used to determine the resulting size of the parameter are accurate and that the new size of the parameter is visible to the whole system

+ Attack Motivation-Consequences
  • Privilege Escalation
  • Privilege Escalation
  • Denial of Service
  • Data Modification
+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')Targeted
119Failure to Constrain Operations within the Bounds of a Memory BufferTargeted
118Improper Access of Indexable Resource ('Range Error')Targeted
130Improper Handling of Length Parameter Inconsistency Targeted
131Incorrect Calculation of Buffer SizeTargeted
74Failure to Sanitize Data into a Different Plane ('Injection')Targeted
20Improper Input ValidationSecondary
680Integer Overflow to Buffer OverflowTargeted
697Insufficient ComparisonTargeted
+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
ChildOfAttack PatternAttack Pattern100Overflow Buffers 
Mechanism of Attack (primary)1000
+ 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.

+ Content History
G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.Cigital, Inc2007-03-01
Eugene LebanidzeCigital, Inc2007-02-26Fleshed 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 Name, Description and Related Attack Patterns
Sean BarnumCigital, Inc2007-04-13Modified pattern content according to review and feedback