CloudSEK Detects Threat Actors Exploiting OTP APIs for Large-Scale SMS Bombing


CloudSEK researchers have found that threat actors have developed automated software programs that exploit OTP (One-Time Password) verification APIs to flood mobile devices with excessive OTP SMS messages.

These rogue scripts, when unleashed, hold the potential to cause targeted outages of telecommunication services, leading to financial and reputational damage for affected brands. The situation raises alarm about the potential for ” Multi-factor authentication (MFA) fatigue” or “exhaustion” attacks in scenarios of account takeover.

Rising Threat Landscape

CloudSEK’s contextual AI digital risk platform, XVigil, has uncovered multiple GitHub repositories containing references to global companies and their APIs. These APIs allow unlimited OTP SMS messages to be sent to any number, lacking rate limiting or captcha protection. This vulnerability has led to the abuse of these APIs by automated tools, resulting in increased API costs, legal repercussions, and reputational damage to affected brands.

Attack Chain Analysis

The attack follows a distinct chain of events:

· Collecting Target Phone Numbers: Threat actors input target phone numbers manually or import lists of numbers from files, with motivation varying from pranks to dedicated attacks.

· Continuous Operation: The software sends messages relentlessly until a preset limit is reached or manually stopped, inundating the target’s device with messages and calls.

· Impact on the Target: The continuous influx of messages and calls overwhelms the target device, potentially causing slowdowns, freezes, crashes, and significant disruption to normal device usage. This overload could also lead to “MFA fatigue” or “exhaustion” attacks, hampering the target’s ability to respond to genuine OTP requests.

“This attack could be used as a veil to hide illegitimate login attempts made by the threat actors to gain access to the users’ device. This also implies that while the attack is going on the user may miss out on critical notifications. Further, due to the constant request of OTPs a service might block your account and you might not be able to access it,” said Mudit Bansal, Cyber Threat Researcher, CloudSEK.

Legal Repercussions

Bombarding phones with SMS messages, even after activating DND (Do Not Disturb) services, constitutes harassment and nuisance under IPC Section 268, and further qualifies as theft, cheating, and dishonest inducement of property delivery under IPC Sections 378 & 420.

Accessibility and Financial Impact

CloudSEK’s findings also underline the accessibility and financial aspects of these malicious services:

· Numerous online tools enable anyone to launch such campaigns effortlessly.

· The tools are available for free, as the primary cost burden falls on the brands owning the SMS-sending APIs.

· A single OTP SMS could cost a brand up to 20 paisa.

Impacted Companies and Exposed APIs

The affected companies, categorised by region, are as follows:

India: 44 exposed APIs

Indonesia: 1 exposed API

Russia: 81 exposed APIs


Note – These graphs are based on APIs found and collated from the source code of multiple SMS bombing tools, some of which are archived now. Cloudsek researchers refrained from testing these APIs for their current vulnerability status.

CloudSEK remains committed to raising awareness about these threats and working towards bolstering cybersecurity measures to protect organisations and individuals from such attacks. As this threat landscape continues to evolve, proactive cybersecurity measures are critical to safeguarding against the potentially devastating consequences of automated SMS bombing campaigns.


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