A researcher discovered a severe vulnerability in the self-destruct message feature of Telegram, leaving deleted messages in the cache. While Telegram patched the flaw, it didn’t appropriately reward the researcher for the bug report.
Telegram Vulnerability In ‘Self-Destruct’ Feature
Elaborating the details in a blog post, security researcher Dmitrii discovered how the vulnerability in self-destruct feature allowed Telegram users to retrieve deleted data.
While inspecting the Telegram for Android client soon after the release of self-destruct update, the researcher found that the self-destruct feature would only work visually at the app level. In reality, the deleted messages would remain in the device folder /Storage/Emulated/0/Telegram/Telegram Image.
The researcher tested the vulnerability by setting the self-destruct timer for 24 hours. As observed, the sent-then-deleted picture remained in the device cache at both the sender’s and receiver’s end.
Dmitrii evaluated this feature on several devices running on Android 7 to 10 and would get the same results. The following video demonstrates the exploit.
As mentioned, this is a different but similar bug to the previously reported (and fixed) CVE-2019-16248 reported in 2019.
Researcher Denied The Bounty
Upon discovering the vulnerability, the researcher reached out to Telegram officials. However, he explained that the service didn’t respond adequately.
Specifically, Dmitrii reported the bug to Telegram in March 2021, which the firm even acknowledged. However, it didn’t fix the bug for several months despite recurrent updates for the Telegram client.
The researcher kept reminding of the vulnerability to telegram officials. Eventually, the service fixed the bug in a subsequent beta version released in August 2021 that the researcher confirmed.
Nonetheless, problems began when Telegram tried to restrict the researcher from disclosing the vulnerability at the time of rewarding the bug bounty, even after the fix. In response, the researcher sent some questions to Telegram regarding the agreement he was supposed to sign, but Dmitrii never got a response.
The researcher even noticed a lesser bounty offered to him (Euro 1000) for the bug than what the service offered previously for a similar flaw (Euro 2500).
Eventually, the researcher went ahead for full public disclosure for this vulnerability CVE-2021-41861.
Users should ensure using at least the Telegram app version 7.8 that includes the fix for it. (Though, Telegram hasn’t mentioned any such bug fix in the changelog.) Also, the researcher advised users not to rely on the self-destruct feature for media privacy. Instead, users should opt for manual deletion of messages that they don’t want the recipient to retain.
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