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CVE-2026-33020— libsixel: Integer Overflow in write_png_to_file() leads to Heap-based Buffer Overflow

Quick assessment

Affected
saitoha libsixel
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

libsixel是Hayaki Saito个人开发者的一个为DEC SIXEL图形和其他转换器程序提供编码/解码实现的软件包。 libsixel 1.8.7及之前版本存在输入验证错误漏洞，该漏洞源于sixel_frame_convert_to_rgb888函数存在整数溢出，可能导致堆缓冲区溢出，造成堆损坏、崩溃或任意代码执行。

CVSS 7.1 · High EPSS 0.21% · P11
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I. Basic Information for CVE-2026-33020

Vulnerability Information
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Vulnerability Title
libsixel: Integer Overflow in write_png_to_file() leads to Heap-based Buffer Overflow
Source: CVE Program / CVE List V5
Vulnerability Description
libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain an integer overflow which leads to a heap buffer overflow via sixel_frame_convert_to_rgb888() in frame.c, where allocation size and pointer offset computations for palettised images (PAL1, PAL2, PAL4) are performed using int arithmetic before casting to size_t. For images whose pixel count exceeds INT_MAX / 4, the overflow produces an undersized heap allocation for the conversion buffer and a negative pointer offset for the normalization sub-buffer, after which sixel_helper_normalize_pixelformat() writes the full image data starting from the invalid pointer, causing massive heap corruption confirmed by ASAN. An attacker providing a specially crafted large palettised PNG can corrupt the heap of the victim process, resulting in a reliable crash and potential arbitrary code execution. This issue has been fixed in version 1.8.7-r1.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
堆缓冲区溢出
Source: CVE Program / CVE List V5
Vulnerability Title
libsixel 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
libsixel是Hayaki Saito个人开发者的一个为DEC SIXEL图形和其他转换器程序提供编码/解码实现的软件包。 libsixel 1.8.7及之前版本存在输入验证错误漏洞，该漏洞源于sixel_frame_convert_to_rgb888函数存在整数溢出，可能导致堆缓冲区溢出，造成堆损坏、崩溃或任意代码执行。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
saitoha libsixel < 1.8.7-r1 -

II. Public POCs for CVE-2026-33020

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III. Intelligence Information for CVE-2026-33020

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Vendor Advisories for CVE-2026-33020 (1)

Same Patch Batch · saitoha · 2026-04-14 · 5 CVEs total

CVE-2026-33023 7.8 HIGH libsixel: Use-after-free in load_with_gdkpixbuf()
CVE-2026-33021 7.3 HIGH libsixel: Use-after-free in sixel_encoder_encode_bytes()
CVE-2026-33019 7.1 HIGH libsixel: Integer overflow leads to Out-of-bounds Read in img2sixel
CVE-2026-33018 7.0 HIGH libsixel: Use-After-Free in load_gif()

IV. Related Vulnerabilities

Same product: libsixel
Same vendor: saitoha
Same weakness: CWE-122

V. Comments for CVE-2026-33020

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