Iran seems to be developing an ICBM openly, but not openly. Instead, Iran launches what it says are efforts to put satellites into orbit and despite numerous failures to do that, the launches continue. Since an SLV (satellite launch vehicle) can also deliver a nuclear warhead over huge distances, the only real difference between an SLV and an ICBM is that the third stage of an ICBM has a warhead, not a satellite. For an ICBM to work you have to get the first two stages working reliably. In ICBM mode the flight control software is modified to go for distance, not orbital altitude.
For example, in February 2020 Iran failed, for the fourth time in a row, to launch a satellite using its three-stage Simorgh SLV. The previous failures were in early 2019, mid-2017 and early 2016. The first two launches were tests with no payload. The first test was declared a success anyway even though the SLV did not reach a high enough altitude to achieve an orbit. The second test experienced a failure in the second stage while the 2017 test saw the third stage fail. During the latest, the satellite failed to reach orbit. While that is a failure for an SLV it is good enough for an ICBM. The third stage of an SLV has a much more complicated job than the third stage of an ICBM. In effect, these four failed Simorgh SLV launches were a success for a Simorgh ICBM.
To make this deception work Iran has to make a major effort to sell the SLV story. When Iran announced a successful second test launch of the Simorgh SLV in mid-2017, the claim was quickly demonstrated as false. Within a week American intelligence officials revealed that while the Simorgh took off and disappeared from view of people on the ground without visible problems, as it approached the point, outside the atmosphere where, as an SLV, it would insert a satellite into orbit,.the Simorgh malfunctioned and fell back to earth without accomplishing anything. Moreover, American, South Korean and UN ballistic missile experts agree that the Simorgh appears to be based on the North Korean Unha SLV/ICBM design. Why do so many different sets of experts agree on what Simorgh actually is and its origins? That’s because Iran and North Korea have been trading weapons and weapons tech since the 1980s. After 2001 that relationship, never really publicized by either party, tried to disappear from public view. Simorgh/Unha is a good example of why disappearing has not worked.
Simorgh itself is an 87 ton two-stage liquid-fuel rocket that can put satellites of up to 350 kg (770 pounds) into an LEO (low earth orbit) of 500 kilometers. The first test was in early 2016 and it was not successful. Iran says it plans to use Simorgh to put surveillance satellites into orbit. That was supposed to happen earlier in 2017 but apparently another test launch was considered necessary and since the second test also failed more are apparently on the way. The Simorgh appears to be stuck at the same stage of development as the North Korean Unha it is based on.
In early 2009, using a modified Shahab 4 two-stage ballistic missile, now called the Safir 1 SLV, Iran put a crude satellite into low earth orbit. This was done to coincide with the 30th anniversary of their Islamic revolution. The satellite was described as a "communications satellite", but it was actually nothing more than a transponder, giving off a signal that could be tracked. What Iran has done is carry out the same kind of early satellite launches Russia and the United States did in the 1950s. Iran says it is building four more satellites, but given the level of technology they have access to, these will be low capability birds, launched into low orbit with short lifetimes.
Since 2008 Iran has used the Safir 1 eight times in efforts to put satellites in orbit. Since 2004 there had been reports of a longer range Shahab 4, based on Soviet-era SS-9 ICBM and North Korean No-Dong 2 missiles, under development. This ICBM was also large enough to put a satellite into orbit. Iran claimed Shahab 4 had a range of 3,000 kilometers. Former Soviet and Chinese rocket engineers are alleged to have helped the Iranians design this missile, and adapt it serve as a satellite launcher. This rocket became an SLV with the modification of the third stage to put a satellite into orbit.
Iran began working on the Shahab 4 in 1997 and used it to try and launch a satellite in 2008, but it failed to achieve orbit. In 2007 it tested the Shahab 4 as a satellite launcher, without attempting to actually orbit a satellite. The key to putting a satellite in orbit is the final (second or third) stage containing the satellite and small maneuvering rockets and a computer to align and inject the satellite at the right time, speed and altitude to achieve orbit. They were successful in 2009, 2011, 2012 and 2015. The two attempts since 2015, both in 2019, failed.
Iran has been launching satellites using its own SLVs since 2009 and having the Russians or Chinese do it for them twice in 2005 and 2008. Since then Iran has put four rather crude satellites into orbit using its own SLVs. Most of these satellites had short lifespans. But the SLV tests were a longer term investment in developing ICBM technology.
The first Iranian satellite launch efforts were similar to the Russian Sputnik launch of 1957, which was the first satellite ever put in orbit. The U.S. followed in 1958. Since then, eight other nations, including Iran, have done the same. Ukraine was the last to do so, in 1995. Israel launched its first satellite in 1988. France launched its first satellite in 1965, Japan and China in 1970, Britain in 1971, and India in 1980.
Early ICBMs used the same tech as existing SLVs. After the Cold War ended in 1991 there were thousands of unneeded ICBMs to be deposed of. The U.S. and Russia agreed to allow each other, after verification, to use many of those surplus ICBMs to launch. This was economically viable because satellite technology had made it profitable to launch smaller and lighter satellites that could do useful work and pay for themselves. SLVs are larger than ICBMs because SLVs lift much heavier third stages. ICBMs as SLVs can still reliably but a few tons of satellites into orbit and many have done just that. Iran noted that and is using that capability in reverse. It is developing SLVs that, if unable to reliably launch satellites, can be put into service delivering nuclear warheads.