CWE-134: Use of Externally-Controlled Format String
The product uses a function that accepts a format string as an argument, but the format string originates from an external source.
How it's found
Use of Externally-Controlled Format String is typically found by tracing untrusted input from where it enters the system to the point where it is used without the check or neutralization this weakness describes, combining manual code review with dynamic testing.
Consequences
- Read Memory: Format string problems allow for information disclosure which can severely simplify exploitation of the program.
- Modify Memory, Execute Unauthorized Code or Commands: Format string problems can result in the execution of arbitrary code, buffer overflows, denial of service, or incorrect data representation.
Mitigations
- Requirements: Choose a language that is not subject to this flaw.
- Implementation: 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, use functions that do not support the %n operator in format strings. [REF-116] [REF-117]
- Build and Compilation: Run compilers and linkers with high warning levels, since they may detect incorrect usage.
Where this fits in a TurboPentest engagement
This weakness is not covered by the automated black-box pentest. IntegSec pentesters cover it in a manual engagement.
Frequently asked questions
What is CWE-134?
The product uses a function that accepts a format string as an argument, but the format string originates from an external source.
How do you find Use of Externally-Controlled Format String?
Use of Externally-Controlled Format String is typically found by tracing untrusted input from where it enters the system to the point where it is used without the check or neutralization this weakness describes, combining manual code review with dynamic testing.
What is the impact of CWE-134?
Read Memory: Format string problems allow for information disclosure which can severely simplify exploitation of the program. Modify Memory, Execute Unauthorized Code or Commands: Format string problems can result in the execution of arbitrary code, buffer overflows, denial of service, or incorrect data representation.
Does TurboPentest test for Use of Externally-Controlled Format String?
This weakness is not covered by the automated black-box pentest. IntegSec pentesters cover it in a manual engagement.
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Written and reviewed by
Michel Chamberland - Founder & CEO, IntegSec
CISSP, OSCP, OSCE, CEH, GIAC, CCSK · 20+ years in offensive security
Michel has spent 20+ years on offensive security teams including IBM X-Force Red and Trustwave SpiderLabs, leading penetration tests, red team engagements, and breach response for Fortune 500 customers. He is the founder of IntegSec and the architect of TurboPentest.
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