Telegram rlottie 6.1.1_1946 VGradientCache::generateGradientColorTable Heap Buffer Overflow


Telegram rlottie 6.1.1_1946 is affected by a Heap Buffer Overflow in the VGradientCache::generateGradientColorTable function: a remote attacker might be able to overwrite heap memory out-of-bounds on a victim device. Note: we’ll walk through the android app sources, but the issue applies to iOS and macOS Telegram apps too.

Product description (from vendor)

“Telegram is a cloud-based mobile and desktop messaging app with a focus on security and speed.”. For more information visit



Root cause analysis

Telegram uses a custom fork of rlottie to render animated stickers. The bug is an heap-based buffer overflow in VGradientCache::generateGradientColorTable (starting at ): an out-of-bounds write access is caused by inaccurate boundary checks.

The while loop starting on line does not limit pos size, which with a great enough input can become larger than size (which is the colorTable array size), leading to writing out-of-bounds 4 bytes.

Specifically, while fpos and incr are static, curr->first comes directly from the animated sticker. colorTable is an uint32_t array of size 1024, hence it is possible to overwrite an arbitrary amount memory after it by carefully using a specific float number as curr->first in the animated sticker file.

The written values are controlled via the sticker file too, but not 100% arbitrary because of ARGB codes constraints readable in premulARGB() and getColorReplacement()

Proof of concept

A blogpost will be published soon on our blog with a PoC walkthrough and further details.


A remote attacker might be able to overwrite Telegram’s heap memory out-of-bounds on a victim device.


Upgrade to Telegram 6.2.0 (1984) or later.

Disclosure timeline

  • 4/06/2020:
    • Telegram releases version 6.2.0 (1984) with a patch


`polict` of Shielder

This advisory was first published on


16 February 2021