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Pdfium - Out-of-Bounds Read with Shading Pattern Backed by Pattern Colorspace Exploit
[Related to issue 1490 .
When parsing ShadingPatterns; according to the specification they shouldn't be permitted to have a pattern colorspace as their base colorspace, but this is not validated, leading to out-of-bounds reads when rendering using the malformed shading pattern.
bool CPDF_ShadingPattern::Load() {
// ... snip ...
CPDF_Object* pCSObj = pShadingDict->GetDirectObjectFor("ColorSpace");
if (!pCSObj)
return false;
// No validation here on the type of colorspace.
// ... snip ...
return true;
}
If we now look at the code called during rendering of this pattern, we call through DrawFreeGouraudShading (cpdf_renderstatus.cpp), which will call CPDF_MeshStream::ReadVertex for each vertex in the shading pattern, which will call CPDF_MeshStream::ReadColor.
std::tuple<float, float, float> CPDF_MeshStream::ReadColor() {
ASSERT(ShouldCheckBPC(m_type));
float color_value[kMaxComponents];
for (uint32_t i = 0; i < m_nComponents; ++i) {
color_value[i] = m_ColorMin[i] + m_BitStream->GetBits(m_nComponentBits) *
(m_ColorMax[i] - m_ColorMin[i]) /
m_ComponentMax;
}
// NB: color_value has only been initialised for the first m_nComponents elements
float r = 0.0;
float g = 0.0;
float b = 0.0;
if (m_funcs.empty()) {
m_pCS->GetRGB(color_value, &r, &g, &b); // <-- we're interested in this call here
return std::tuple<float, float, float>(r, g, b);
}
// ... snip ...
}
This call to GetRGB will be into the pattern cs
bool CPDF_PatternCS::GetRGB(float* pBuf, float* R, float* G, float* B) const {
if (m_pBaseCS) {
ASSERT(m_pBaseCS->GetFamily() != PDFCS_PATTERN);
PatternValue* pvalue = (PatternValue*)pBuf;
// pvalue->m_Comps is now pointing 5 dwords into an 8 dword sized buffer, and p_pBaseCS expects to be able to read 8 dwords from it.
if (m_pBaseCS->GetRGB(pvalue->m_Comps, R, G, B))
return true;
}
*R = 0.75f;
*G = 0.75f;
*B = 0.75f;
return false;
}
Originally reported without 90 day deadline as https://bugs.chromium.org/p/chromium/issues/detail?id=795251, since it wasn't clear that there was an easy way to use the oob-read to leak information in a way that was useful, deadline applied as of 15/12 after working out how to use this as an information leak for issue 1489 .
Proof of Concept:
https://github.com/offensive-security/exploit-database-bin-sploits/raw/master/bin-sploits/44083.zip
# 0day.today [2024-12-23] #