CVE-2026-56657

Gitea SSH Key Parser Denial of Service

Gitea SSH Key Parser Denial of Service

Gitea's SSH key ingestion endpoint accepts keys in RFC 4716 (SSH2) format and normalises them before storage. The normalisation function contains an O(N²) string concatenation loop with no input size limit, meaning a single malicious key submission can force the server to perform an amount of work that grows quadratically with the size of the input. Any authenticated user can exploit this to exhaust the server's CPU and memory, taking the instance offline.

Root Cause

An attacker sends a POST /api/v1/user/keys request with a Bearer token and a JSON body whose key field contains a malicious RFC 4716 (SSH2) public key. The key consists of a valid SSH2 header followed by a very large number of short content lines — for example, 400,000 lines of 100 characters each (~38 MB total).

The request reaches CreateUserPublicKey with no prior size check:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/routers/api/v1/user/key.go#L201-L212

This calls CheckPublicKeyString which immediately calls parseKeyString. Inside parseKeyString, the SSH2 branch splits the input on newlines and accumulates the key body one line at a time using keyContent += line:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/models/asymkey/ssh_key_parse.go#L60-L79

Because Go strings are immutable, each += at line 77 allocates a new backing array and copies the entire accumulated string into it. For N lines the total bytes copied is N*(N+1)/2, making the operation O(N²) in both time and allocations. The validity of the key is only checked after the loop completes, so the entire quadratic work is performed regardless of whether the input is a real SSH key.

This is only possible because neither the web form field nor the API struct carries a size constraint:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/services/forms/user_form.go#L308-L317

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/structs/repo_key.go#L33-L49

PoC

To reproduce, clone gitea and checkout commit 9155a81b9daf1d46b2380aa91271e623ac947c1e. Then create the following files from the gitea root directory:

poc/Dockerfile

FROM golang:1.26-alpine AS builder

RUN apk add --no-cache git build-base

WORKDIR /gitea

# Download deps in a separate layer so rebuilds are fast after source changes.
COPY go.mod go.sum ./
RUN go mod download

# Copy full source (needed for fixtures, config templates, and compilation).
COPY . .

# Compile the integration test binary.
# modernc sqlite (pure Go, no CGO needed) is the default driver.
RUN CGO_ENABLED=0 go test -c \
      -o /integration.test \
      gitea.dev/tests/integration

# ── runtime image ────────────────────────────────────────────────────────────
FROM alpine:3.22

# git is required at runtime: the test framework initialises git repos.
RUN apk add --no-cache git

COPY --from=builder /integration.test /integration.test
# Keep the full source at /gitea so runtime.Caller(0) path resolution works
# and fixtures / config templates are accessible.
COPY --from=builder /gitea /gitea

RUN adduser -D -u 1000 poc && chown -R poc:poc /gitea

WORKDIR /gitea

USER poc

ENTRYPOINT ["/integration.test", \
            "-test.run", "TestDoSSSHKeyParserOOM", \
            "-test.v", \
            "-test.timeout", "600s"]

tests/integration/poc_dos_test.go

package integration

import (
	"fmt"
	"runtime"
	"runtime/debug"
	"strings"
	"sync"
	"sync/atomic"
	"testing"
	"time"

	auth_model "gitea.dev/models/auth"
	api "gitea.dev/modules/structs"
	"gitea.dev/tests"
)

func TestDoSSSHKeyParserOOM(t *testing.T) {
	defer tests.PrepareTestEnv(t)()

	// Raise the GC trigger so intermediate strings accumulate faster,
	// matching realistic server behaviour under sustained…

Gitea SSH Key Parser Denial of Service

Gitea SSH Key Parser Denial of Service

Gitea's SSH key ingestion endpoint accepts keys in RFC 4716 (SSH2) format and normalises them before storage. The normalisation function contains an O(N²) string concatenation loop with no input size limit, meaning a single malicious key submission can force the server to perform an amount of work that grows quadratically with the size of the input. Any authenticated user can exploit this to exhaust the server's CPU and memory, taking the instance offline.

Root Cause

An attacker sends a POST /api/v1/user/keys request with a Bearer token and a JSON body whose key field contains a malicious RFC 4716 (SSH2) public key. The key consists of a valid SSH2 header followed by a very large number of short content lines — for example, 400,000 lines of 100 characters each (~38 MB total).

The request reaches CreateUserPublicKey with no prior size check:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/routers/api/v1/user/key.go#L201-L212

This calls CheckPublicKeyString which immediately calls parseKeyString. Inside parseKeyString, the SSH2 branch splits the input on newlines and accumulates the key body one line at a time using keyContent += line:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/models/asymkey/ssh_key_parse.go#L60-L79

Because Go strings are immutable, each += at line 77 allocates a new backing array and copies the entire accumulated string into it. For N lines the total bytes copied is N*(N+1)/2, making the operation O(N²) in both time and allocations. The validity of the key is only checked after the loop completes, so the entire quadratic work is performed regardless of whether the input is a real SSH key.

This is only possible because neither the web form field nor the API struct carries a size constraint:

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/services/forms/user_form.go#L308-L317

https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/structs/repo_key.go#L33-L49

PoC

To reproduce, clone gitea and checkout commit 9155a81b9daf1d46b2380aa91271e623ac947c1e. Then create the following files from the gitea root directory:

poc/Dockerfile

FROM golang:1.26-alpine AS builder

RUN apk add --no-cache git build-base

WORKDIR /gitea

# Download deps in a separate layer so rebuilds are fast after source changes.
COPY go.mod go.sum ./
RUN go mod download

# Copy full source (needed for fixtures, config templates, and compilation).
COPY . .

# Compile the integration test binary.
# modernc sqlite (pure Go, no CGO needed) is the default driver.
RUN CGO_ENABLED=0 go test -c \
      -o /integration.test \
      gitea.dev/tests/integration

# ── runtime image ────────────────────────────────────────────────────────────
FROM alpine:3.22

# git is required at runtime: the test framework initialises git repos.
RUN apk add --no-cache git

COPY --from=builder /integration.test /integration.test
# Keep the full source at /gitea so runtime.Caller(0) path resolution works
# and fixtures / config templates are accessible.
COPY --from=builder /gitea /gitea

RUN adduser -D -u 1000 poc && chown -R poc:poc /gitea

WORKDIR /gitea

USER poc

ENTRYPOINT ["/integration.test", \
            "-test.run", "TestDoSSSHKeyParserOOM", \
            "-test.v", \
            "-test.timeout", "600s"]

tests/integration/poc_dos_test.go

package integration

import (
	"fmt"
	"runtime"
	"runtime/debug"
	"strings"
	"sync"
	"sync/atomic"
	"testing"
	"time"

	auth_model "gitea.dev/models/auth"
	api "gitea.dev/modules/structs"
	"gitea.dev/tests"
)

func TestDoSSSHKeyParserOOM(t *testing.T) {
	defer tests.PrepareTestEnv(t)()

	// Raise the GC trigger so intermediate strings accumulate faster,
	// matching realistic server behaviour under sustained…

Gitea SSH Key Parser Denial of Service in gitea.dev

This update for govulncheck-vulndb fixes the following issues:

• Update to version 0.0.20260723T184607 2026-07-23T18:46:07Z (jsc#PED-11136).

Go CVE Numbering Authority IDs added or updated with aliases:

• GO-2026-4730 CVE-2026-33022 GHSA-cv4x-93xx-wgfj
• GO-2026-4761 CVE-2026-33211 GHSA-j5q5-j9gm-2w5c
• GO-2026-5102 CVE-2026-45021 GHSA-3vcp-chfh-f6r2
• GO-2026-5272 CVE-2026-40938 GHSA-94jr-7pqp-xhcq
• GO-2026-5486 CVE-2026-40924 GHSA-m2cx-gpqf-qf74
• GO-2026-5542 CVE-2026-40090 GHSA-pj97-4p9w-gx3q
• GO-2026-5630 CVE-2026-25542 GHSA-rmx9-2pp3-xhcr
• GO-2026-5643 CVE-2026-40923 GHSA-rx35-6rhx-7858
• GO-2026-5675 CVE-2026-41492 GHSA-vvf7-6rmr-m29q
• GO-2026-5711 CVE-2026-40161 GHSA-wjxp-xrpv-xpff
• GO-2026-5746 CVE-2026-41567 GHSA-x86f-5xw2-fm2r
• GO-2026-5749 CVE-2026-41328 GHSA-x92x-px7w-4gx4
• GO-2026-5837 CVE-2026-44840 GHSA-q2m9-6jp9-c6mc
• GO-2026-5929 CVE-2026-44342 GHSA-26v7-h57m-gh9m
• GO-2026-5930 CVE-2026-33655 GHSA-6qcr-qxgr-m7fv
• GO-2026-5933 GHSA-3g4q-2f67-2gvh
• GO-2026-5934 GHSA-59qp-cfj3-rp64
• GO-2026-5935 GHSA-7856-g3gv-9wq8
• GO-2026-5936 CVE-2026-53552 GHSA-26rh-24rg-j3vv
• GO-2026-5937 CVE-2026-53508 GHSA-2jcc-mxv7-p3f9
• GO-2026-5938 CVE-2026-53553 GHSA-4g5x-hcwm-82jw
• GO-2026-5939 CVE-2026-10517 GHSA-698x-9w2p-7vvp
• GO-2026-5940 CVE-2026-53572 GHSA-6w3m-4hhp-775q
• GO-2026-5941 CVE-2026-53487 GHSA-gvhc-wv3v-7pf8
• GO-2026-5942 CVE-2026-46600
• GO-2026-5944 CVE-2026-50197 GHSA-659f-rgp5-w4wf
• GO-2026-5945 CVE-2026-6815 GHSA-rmxx-v9rj-vpvg
• GO-2026-5946 CVE-2026-52831 GHSA-v5px-423j-pf7p
• GO-2026-5947 CVE-2026-53649 GHSA-xqhv-chqm-fhcc
• GO-2026-5948 CVE-2026-50554 GHSA-588f-fvcv-xhvf
• GO-2026-5949 CVE-2026-50553 GHSA-rqrh-8wpv-x7hh
• GO-2026-5950 CVE-2026-53602 GHSA-339v-266x-79xr
• GO-2026-5951 CVE-2026-10219 GHSA-6jm8-4fhr-5w64
• GO-2026-5952 CVE-2026-49834 GHSA-9vcr-p3rj-q5q6
• GO-2026-5953 CVE-2026-9094 GHSA-c9w5-qp6m-m395
• GO-2026-5956 CVE-2026-55252 GHSA-h5g6-xmh4-hc37
• GO-2026-5957 CVE-2026-54158 GHSA-5xfx-xj4h-5p7r
• GO-2026-5958 CVE-2026-54068 GHSA-gcm7-57gf-953c
• GO-2026-5959 CVE-2026-54072 GHSA-h29v-hj44-q8cv
• GO-2026-5961 CVE-2026-54069 GHSA-hvr9-72v2-fff3
• GO-2026-5962 CVE-2026-54088 GHSA-m93h-4hw7-5qcm
• GO-2026-5963 CVE-2026-54067 GHSA-mvjr-vv3c-w4qv
• GO-2026-5964 CVE-2026-54066 GHSA-p4m3-mgmm-c664
• GO-2026-5965 CVE-2026-54070 GHSA-w7cg-whh7-xp28
• GO-2026-5966 CVE-2026-54089 GHSA-xqp3-jq6g-x3qm
• GO-2026-5967 CVE-2026-50551 GHSA-56mp-4f3v-fgj2
• GO-2026-5968 CVE-2026-54174 GHSA-fpg8-7664-jc5q
• GO-2026-5969 GHSA-g936-7jqj-mwv8
• GO-2026-5970 CVE-2026-56852
• GO-2026-5973 CVE-2026-54250 GHSA-jxr7-mqhw-9p98
• GO-2026-5974 CVE-2026-44300 GHSA-wmj8-9953-vff5
• GO-2026-5975 CVE-2026-50018 GHSA-42j2-w334-qxw7
• GO-2026-5976 CVE-2026-50141 GHSA-g7mm-9vx7-jm7h
• GO-2026-5977 CVE-2026-50013 GHSA-qrh4-p6v4-mrfg
• GO-2026-5978 CVE-2026-50125 GHSA-qw5r-ppcg-f8rj
• GO-2026-5979 CVE-2026-50006 GHSA-xrcf-6jh3-ggvx
• GO-2026-5980 CVE-2026-50158 GHSA-2c7f-fxww-6w6c
• GO-2026-5981 GHSA-mqxv-9rm6-w8qc
• GO-2026-5982 GHSA-pqg7-v6wh-3pfp
• GO-2026-5984 CVE-2026-53604 GHSA-2p2f-px33-4vv5
• GO-2026-5985 GHSA-7rx3-5wx3-5v76
• GO-2026-5986 CVE-2026-61699 GHSA-cm26-5974-52h8
• GO-2026-5987 CVE-2026-55513 GHSA-g4x6-jcvr-9m3g
• GO-2026-5988 CVE-2026-54628 GHSA-hwrq-8wxh-q4xv
• GO-2026-5989 CVE-2026-55512 GHSA-m3cx-mwpg-32jg
• GO-2026-5990 CVE-2026-54629 GHSA-mf78-3rpf-r784
• GO-2026-5991 CVE-2026-53603 GHSA-q4vm-pq3q-8wgq
• GO-2026-5992 CVE-2026-61549 GHSA-qf34-295c-26v8
• GO-2026-5996 CVE-2026-54450 GHSA-pph6-vfjv-vpjw
• GO-2026-5997 CVE-2026-54452 GHSA-xgch-x3mx-cm3c
• GO-2026-5998 CVE-2026-54495 GHSA-398h-7f66-3h4p
• GO-2026-5999 CVE-2026-10814 GHSA-jh6h-v6mp-h22v
• GO-2026-6002 CVE-2026-58196 GHSA-pr64-jmmf-jp54
• GO-2026-6003 CVE-2026-53714 GHSA-22xc-xg2r-9j7v
• GO-2026-6004 CVE-2026-52833 GHSA-3v79-m2cg-89ww
• GO-2026-6005 CVE-2026-53715 GHSA-8fv2-88gg-hm7q
• GO-2026-6006 CVE-2026-53716 GHSA-cxpq-8v7q-cg56 *…