A backdoor is a method, often secret, of bypassing normal authentication in a product, computer system, cryptosystem or algorithm etc. Backdoors are often used for securing unauthorized remote access to a computer, or obtaining access to plaintext in cryptographic systems.
- Object code backdoors
- Asymmetric backdoors
- Compiler backdoors
- List of known backdoors
A backdoor may take the form of a hidden part of a program, a separate program (e.g. Back Orifice may subvert the system through a rootkit), or may be a hardware feature. Although normally surreptitiously installed, in some cases backdoors are deliberate and widely known. These kinds of backdoors might have "legitimate" uses such as providing the manufacturer with a way to restore user passwords.
Default passwords can function as backdoors if they are not changed by the user. Some debugging features can also act as backdoors if they are not removed in the release version.
In 1993 the United States government attempted to deploy an encryption system, the Clipper chip, with an explicit backdoor for law enforcement and national security access. The chip was unsuccessful internationally and in business.
The threat of backdoors surfaced when multiuser and networked operating systems became widely adopted. Petersen and Turn discussed computer subversion in a paper published in the proceedings of the 1967 AFIPS Conference. They noted a class of active infiltration attacks that use "trapdoor" entry points into the system to bypass security facilities and permit direct access to data. The use of the word trapdoor here clearly coincides with more recent definitions of a backdoor. However, since the advent of public key cryptography the term trapdoor has acquired a different meaning (see trapdoor function), and thus the term "backdoor" is now preferred. More generally, such security breaches were discussed at length in a RAND Corporation task force report published under ARPA sponsorship by J.P. Anderson and D.J. Edwards in 1970.
A backdoor in a login system might take the form of a hard coded user and password combination which gives access to the system. An example of this sort of backdoor was used as a plot device in the 1983 film WarGames, in which the architect of the "WOPR" computer system had inserted a hardcoded password (his dead son's name) which gave the user access to the system, and to undocumented parts of the system (in particular, a video game-like simulation mode and direct interaction with the artificial intelligence).
Although the number of backdoors in systems using proprietary software (software whose source code is not publicly available) is not widely credited, they are nevertheless frequently exposed. Programmers have even succeeded in secretly installing large amounts of benign code as Easter eggs in programs, although such cases may involve official forbearance, if not actual permission.
Many computer worms, such as Sobig and Mydoom, install a backdoor on the affected computer (generally a PC on broadband running Microsoft Windows and Microsoft Outlook). Such backdoors appear to be installed so that spammers can send junk e-mail from the infected machines. Others, such as the Sony/BMG rootkit, placed secretly on millions of music CDs through late 2005, are intended as DRM measures—and, in that case, as data-gathering agents, since both surreptitious programs they installed routinely contacted central servers.
A sophisticated attempt to plant a backdoor in the Linux kernel, exposed in November 2003, added a small and subtle code change by subverting the revision control system. In this case, a two-line change appeared to check root access permissions of a caller to the sys_wait4 function, but because it used assignment
= instead of equality checking
==, it actually granted permissions to the system. This difference is easily overlooked, and could even be interpreted as an accidental typographical error, rather than an intentional attack.
In January 2014, a backdoor was discovered in certain Samsung Android products, like the Galaxy devices. The Samsung proprietary Android versions are fitted with a backdoor that provides remote access to the data stored on the device. In particular, the Samsung Android software that is in charge of handling the communications with the modem, using the Samsung IPC protocol, implements a class of requests known as remote file server (RFS) commands, that allows the backdoor operator to perform via modem remote I/O operations on the device hard disk or other storage. As the modem is running Samsung proprietary Android software, it is likely that it offers over-the-air remote control that could then be used to issue the RFS commands and thus to access the file system on the device.
Object code backdoors
Harder to detect backdoors involve modifying object code, rather than source code – object code is much harder to inspect, as it is designed to be machine-readable, not human-readable. These backdoors can be inserted either directly in the on-disk object code, or inserted at some point during compilation, assembly linking, or loading – in the latter case the backdoor never appears on disk, only in memory. Object code backdoors are difficult to detect by inspection of the object code, but are easily detected by simply checking for changes (differences), notably in length or in checksum, and in some cases can be detected or analyzed by disassembling the object code. Further, object code backdoors can be removed (assuming source code is available) by simply recompiling from source.
Thus for such backdoors to avoid detection, all extant copies of a binary must be subverted, and any validation checksums must also be compromised, and source must be unavailable, to prevent recompilation. Alternatively, these other tools (length checks, diff, checksumming, disassemblers) can themselves be compromised to conceal the backdoor, for example detecting that the subverted binary is being checksummed and returning the expected value, not the actual value. To conceal these further subversions, the tools must also conceal the changes in themselves – for example, a subverted checksummer must also detect if it is checksumming itself (or other subverted tools) and return false values. This leads to extensive changes in the system and tools being needed to conceal a single change.
Because object code can be regenerated by recompiling (reassembling, relinking) the original source code, making a persistent object code backdoor (without modifying source code) requires subverting the compiler itself – so that when it detects that it is compiling the program under attack it inserts the backdoor – or alternatively the assembler, linker, or loader. As this requires subverting the compiler, this in turn can be fixed by recompiling the compiler, removing the backdoor insertion code. This defense can in turn be subverted by putting a source meta-backdoor in the compiler, so that when it detects that it is compiling itself it then inserts this meta-backdoor generator, together with the original backdoor generator for the original program under attack. After this is done, the source meta-backdoor can be removed, and the compiler recompiled from original source with the compromised compiler executable: the backdoor has been bootstrapped. This attack dates to Karger & Schell (1974), and was popularized in Thompson's 1984 article, entitled "Reflections on Trusting Trust"; it is hence colloquially known as the "Trusting Trust" attack. See compiler backdoors, below, for details. Analogous attacks can target lower levels of the system, such as the operating system, and can be inserted during the system booting process; these are also mentioned in Karger & Schell (1974), and now exist in the form of boot sector viruses.
A traditional backdoor is a symmetric backdoor: anyone that finds the backdoor can in turn use it. The notion of an asymmetric backdoor was introduced by Adam Young and Moti Yung in the Proceedings of Advances in Cryptology: Crypto '96. An asymmetric backdoor can only be used by the attacker who plants it, even if the full implementation of the backdoor becomes public (e.g., via publishing, being discovered and disclosed by reverse engineering, etc.). Also, it is computationally intractable to detect the presence of an asymmetric backdoor under black-box queries. This class of attacks have been termed kleptography; they can be carried out in software, hardware (for example, smartcards), or a combination of the two. The theory of asymmetric backdoors is part of a larger field now called cryptovirology. Notably, NSA inserted a kleptographic backdoor into the Dual_EC_DRBG standard.
There exists an experimental asymmetric backdoor in RSA key generation. This OpenSSL RSA backdoor was designed by Young and Yung, utilizes a twisted pair of elliptic curves, and has been made available.
A sophisticated form of black box backdoor is a compiler backdoor, where not only is a compiler subverted (to insert a backdoor in some other program, such as a login program), but it is further modified to detect when it is compiling itself and then inserts both the backdoor insertion code (targeting the other program) and the code modifying self-compilation, like the mechanism how retroviruses infect their host. This can be done by modifying the source code, and the resulting compromised compiler (object code) can compile the original (unmodified) source code and insert itself: the exploit has been boot-strapped.
This attack was originally presented in Karger & Schell (1974, p. 52, section 3.4.5: "Trap Door Insertion"), which was a United States Air Force security analysis of Multics, where they described such an attack on a PL/I compiler, and call it a "compiler trap door"; they also mention a variant where the system initialization code is modified to insert a backdoor during booting, as this is complex and poorly understood, and call it an "initialization trapdoor"; this is now known as a boot sector virus.
This attack was then actually implemented and popularized by Ken Thompson, in his Turing Award acceptance speech in 1983 (published 1984), "Reflections on Trusting Trust", which points out that trust is relative, and the only software one can truly trust is code where every step of the bootstrapping has been inspected. This backdoor mechanism is based on the fact that people only review source (human-written) code, and not compiled machine code (object code). A program called a compiler is used to create the second from the first, and the compiler is usually trusted to do an honest job.
Thompson's paper describes a modified version of the Unix C compiler that would:
Because the compiler itself was a compiled program, users would be extremely unlikely to notice the machine code instructions that performed these tasks. (Because of the second task, the compiler's source code would appear "clean".) What's worse, in Thompson's proof of concept implementation, the subverted compiler also subverted the analysis program (the disassembler), so that anyone who examined the binaries in the usual way would not actually see the real code that was running, but something else instead.
An updated analysis of the original exploit is given in Karger & Schell (2002, Section 3.2.4: Compiler trap doors), and a historical overview and survey of the literature is given in Wheeler (2009, Section 2: Background and related work).
Thompson's version was, officially, never released into the wild. It is believed, however, that a version was distributed to BBN and at least one use of the backdoor was recorded. There are scattered anecdotal reports of such backdoors in subsequent years.
This attack was recently (August 2009) discovered by Sophos labs: The W32/Induc-A virus infected the program compiler for Delphi, a Windows programming language. The virus introduced its own code to the compilation of new Delphi programs, allowing it to infect and propagate to many systems, without the knowledge of the software programmer. An attack that propagates by building its own Trojan horse can be especially hard to discover. It is believed that the Induc-A virus had been propagating for at least a year before it was discovered.
Once a system has been compromised with a backdoor or Trojan horse, such as the Trusting Trust compiler, it is very hard for the "rightful" user to regain control of the system – typically one should rebuild a clean system and transfer data (but not executables!) over. However, several practical weaknesses in the Trusting Trust scheme have been suggested. For example, a sufficiently motivated user could painstakingly review the machine code of the untrusted compiler before using it. As mentioned above, there are ways to hide the Trojan horse, such as subverting the disassembler; but there are ways to counter that defense, too, such as writing your own disassembler from scratch.
A generic method to counter trusting trust attacks is called Diverse Double-Compiling (DDC). The method requires a different compiler and the source code of the compiler-under-test. That source, compiled with both compilers, results in two different stage-1 compilers, which however should have the same behavior. Thus the same source compiled with both stage-1 compilers must then result in two identical stage-2 compilers. A formal proof is given that the latter comparison guarantees that the purported source code and executable of the compiler-under-test correspond, under some assumptions. This method was applied by its author to verify that the C compiler of the GCC suite (v. 3.0.4) contained no trojan, using icc (v. 11.0) as the different compiler.
In practice such verifications are not done by end users, except in extreme circumstances of intrusion detection and analysis, due to the rarity of such sophisticated attacks, and because programs are typically distributed in binary form. Removing backdoors (including compiler backdoors) is typically done by simply rebuilding a clean system. However, the sophisticated verifications are of interest to operating system vendors, to ensure that they are not distributing a compromised system, and in high-security settings, where such attacks are a realistic concern.