Fix/string table oom on malformed apk#6
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MiroslavDrbal merged 2 commits intomasterfrom Apr 10, 2026
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Two allocation vulnerabilities in string table parsing allow a crafted
resources.arsc to cause multi-GiB heap allocations:
parseStringTable — res.data = make([]byte, r.N)
r.N is derived from the chunk's totalLen field, a raw uint32 that a
malformed APK can set to ~4 GiB. make() commits the memory immediately;
io.ReadFull then fails (if the file is smaller), but only after the 4 GiB
block is live on the heap. The existing stringCnt >= 2M guard does not
bound the data section size. Guard with a 50 MiB limit before the make.
parseString16 — buf = make([]uint16, strCharacters)
The extended 31-bit length encoding (high bit of the first uint16 set)
allows strCharacters up to 0x7FFF_FFFF (4 GiB as []uint16) per call.
r is bytes.NewReader(t.data[offset:]), so binary.Read fails immediately
when the actual data is smaller -- but only after the allocation lands.
Fix with two guards in order:
1. Validate strCharacters against br.Len(): a correctly-formed string
cannot claim more chars than remaining_bytes/2. Catches both crafted
fields and any future decoding misalignment.
2. Secondary cap at 64 KiB: no legitimate resource string exceeds this.
Also fix &buf -> buf in binary.Read calls (parseString16, parseString8):
Passing *[]uint16 / *[]uint8 instead of the slice value itself bypasses
binary.Read's intDataSize fast-path switch (which handles []uint16 /
[]uint8 but not pointer-to-slice). The fallback reflection path allocates
an extra []byte temp buffer of the same size and calls reflect.New per
element -- visible as ~3 GiB flat in encoding/binary.Read and ~378 MB
in reflect.New in heap profiles of affected servers. Passing the slice
directly uses the fast path, is functionally identical, and is zero-
reflection.
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Add hierarchical size validation to the binary resource parser. Every size field parsed from the file is now validated against the bytes actually available from the parent before it can drive an allocation or loop. common.go: - parseChunkHeader enforces headerLen >= chunkHeaderSize and totalLen >= headerLen, rejecting structurally invalid chunks. binxml.go: - ParseXml: replaced uint32 totalLen subtraction with int64 dataLen to eliminate underflow that could create ~4 GiB loop bounds. Loop position tracking changed to int64. Inner chunks validated against remaining parent data. Magic 2*4 replaced with chunkHeaderSize constant. - parseResourceIds: capped at 1M entries (~4 MB). resources.go: - ParseResourceTable: same int64 arithmetic and inner chunk bounds check as ParseXml. - parseTypeSpec: signature tightened to *io.LimitedReader, entryCount validated against r.N before the append loop. stringtable.go: - parseStringTable: 4*stringCnt validated against r.N before allocating the string offsets buffer. - parseStringTableWithChunk: uint32 subtraction moved to int64 space.
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