Pillow EUVD-2026-104660

Exploitation requires that the target application call Pillow's Image.quantize(palette=p) with an attacker-controlled palette image whose palette has zero entries (p.palette.palette == b""), causing ImagingPaletteCacheUpdate() to leave its local cache array uninitialized and the topalette() conversion path to emit those bytes as output pixel indices in the returned PNG/GIF. Limiting factors: this is not Pillow's default or implicit behavior - the application must expose the quantize-with-palette code path to untrusted input, and the attacker must control the palette image specifically. Impact is confined to disclosure of uninitialized stack memory (addresses, stack-canary data) with no code execution, corruption, or crash. Affects Pillow 9.1.0 through versions before 13.0.0.

This is a medium-severity information-disclosure flaw (CVSS 4.0 base 6.9) rated CWE-457 (use of uninitialized variable). The CVSS 4.0 vector AV:N/AC:L/AT:N/PR:N/UI:N indicates network-reachable, low-complexity, unauthenticated triggering with no user interaction, but impact is confined to confidentiality at Low (VC:L; VI:N/VA:N) - a pure read of uninitialized stack memory with no corruption, out-of-bounds write, or crash. No EPSS score was provided, so exploitation probability cannot be quantified; the CVE is NOT in CISA KEV and no public exploit code was supplied, so exploitation status is 'no public exploit identified at time of analysis'. The source-code evidence (fix commit 13943c8) is a patch, not a weaponized PoC. The practical risk is tempered by a real precondition the raw vector obscures: the AV:N/PR:N scoring assumes the attacker can reach the vulnerable call path, but exploitation only occurs when an application invokes Image.quantize() with an attacker-controlled palette image that has zero entries - not Pillow's default behavior. Net assessment: a genuine but lower-tier priority - valuable as an ASLR/stack-canary-defeating leak primitive to chain with a separate memory-corruption bug rather than an independently critical issue. Patch to 13.0.0 where the quantize path is a realistic attack surface.

A web application lets users upload an image and recolor it against a user-supplied palette via Pillow's Image.quantize(palette=...). An attacker uploads a palette image whose palette is empty (zero entries); Pillow's conversion path emits uninitialized stack bytes as pixel indices, and the attacker downloads the resulting PNG/GIF and reconstructs leaked stack contents such as pointers and stack-canary values. No public exploit code is known, but the low attack complexity (AC:L) makes reliable triggering straightforward once the palette path is reachable.