diff --git a/.github/scripts/audit_regressions.sh b/.github/scripts/audit_regressions.sh
new file mode 100755
index 00000000..55a236e4
--- /dev/null
+++ b/.github/scripts/audit_regressions.sh
@@ -0,0 +1,106 @@
+#!/usr/bin/env bash
+# Fail if a lockfile has a RustSec vulnerability that its base lockfile doesn't.
+#
+#   audit_regressions.sh [--config AUDIT_TOML] HEAD_LOCK [BASE_LOCK]
+#
+# A PR's Security Audit should fail on an advisory the PR brings in, and not on
+# one that was published against a dependency main already had: that one would
+# red every open PR the day it lands, whatever the PR changes (V2-1800). So
+# this audits both lockfiles against the same advisory database and fails only
+# on (advisory, crate) pairs that are in HEAD_LOCK and not in BASE_LOCK. Moving
+# an already-vulnerable crate to another vulnerable version is not new. The
+# vulnerabilities both share are listed as warnings, so they stay visible.
+#
+# With no BASE_LOCK every vulnerability in HEAD_LOCK counts as new.
+#
+# Only the ignores in AUDIT_TOML apply, to both audits; with no --config, none
+# do. cargo audit reads .cargo/audit.toml from its working directory, so it runs
+# from a scratch directory holding only AUDIT_TOML. CI passes the base branch's
+# copy: if a PR's own ignores applied, the PR could add an ignore for the
+# advisory it introduces and hide it from both audits. (cargo audit also reads
+# $CARGO_HOME/audit.toml, which belongs to the runner, not the PR.)
+set -euo pipefail
+
+config=
+if [ "${1:-}" = --config ]; then
+  config=$2
+  shift 2
+  [ -f "$config" ] || { echo "::error::no audit config at $config"; exit 1; }
+fi
+# Absolute, since cargo audit runs from the scratch directory.
+head_name=$1
+absolute() { case $1 in /*) echo "$1" ;; *) echo "$PWD/$1" ;; esac; }
+head_lock=$(absolute "$1")
+base_lock=${2:+$(absolute "$2")}
+work=$(mktemp -d)
+trap 'rm -rf "$work"' EXIT
+mkdir -p "$work/cwd/.cargo"
+if [ -n "$config" ]; then
+  cp "$config" "$work/cwd/.cargo/audit.toml"
+fi
+
+# audit LOCKFILE REPORT [cargo audit args]
+audit() {
+  # cargo audit exits non-zero when it finds a vulnerability, so its status
+  # can't tell a finding from a crash. A run that didn't produce a report fails
+  # the check on the jq test below instead.
+  (cd "$work/cwd" && cargo audit --json --file "$1" "${@:3}") > "$2" || true
+  if ! jq -e '.vulnerabilities.list | type == "array"' "$2" > /dev/null 2>&1; then
+    echo "::error::cargo audit produced no report for $1"
+    cat "$2"
+    exit 1
+  fi
+}
+
+# One line per vulnerability: "ID CRATE VERSION: title". The first two fields
+# are what "new" is judged on.
+vulnerabilities() {
+  jq -r '.vulnerabilities.list[] | "\(.advisory.id) \(.package.name) \(.package.version): \(.advisory.title)"' "$1" | sort -u
+}
+
+# One line per informational warning (unmaintained, unsound, yanked).
+warnings() {
+  jq -r '.warnings // {} | to_entries[] | .value[]
+         | "\(.kind) \(.advisory.id // "-") \(.package.name) \(.package.version)"' "$1" | sort -u
+}
+
+# The first audit fetches the advisory database; the second reuses that copy,
+# so both lockfiles are judged against the same advisories.
+audit "$head_lock" "$work/head.json"
+echo "ignores applied: $(jq -r '.settings.ignore | if length == 0 then "none" else join(", ") end' "$work/head.json")"
+vulnerabilities "$work/head.json" > "$work/head.txt"
+warnings "$work/head.json" > "$work/head-warnings.txt"
+if [ -n "$base_lock" ]; then
+  audit "$base_lock" "$work/base.json" --no-fetch
+  vulnerabilities "$work/base.json" > "$work/base.txt"
+  warnings "$work/base.json" > "$work/base-warnings.txt"
+else
+  : > "$work/base.txt"
+  : > "$work/base-warnings.txt"
+fi
+
+# Split HEAD's vulnerabilities by whether BASE has the same advisory for the
+# same crate. (FILENAME, not NR == FNR: that is also true on the second file
+# when the first is empty.)
+awk 'FILENAME == ARGV[1] { seen[$1 " " $2] = 1; next } !(($1 " " $2) in seen)' \
+  "$work/base.txt" "$work/head.txt" > "$work/new.txt"
+awk 'FILENAME == ARGV[1] { seen[$1 " " $2] = 1; next } ($1 " " $2) in seen' \
+  "$work/base.txt" "$work/head.txt" > "$work/existing.txt"
+# Informational warnings are never fatal; the new ones get an annotation.
+comm -13 "$work/base-warnings.txt" "$work/head-warnings.txt" > "$work/new-warnings.txt"
+
+while IFS= read -r line; do
+  echo "::warning::already in the base lockfile, not introduced here: $line"
+done < "$work/existing.txt"
+while IFS= read -r line; do
+  echo "::warning::new informational advisory: $line"
+done < "$work/new-warnings.txt"
+
+echo "vulnerabilities: $(wc -l < "$work/head.txt") in $head_name," \
+  "$(wc -l < "$work/existing.txt") already in the base, $(wc -l < "$work/new.txt") new"
+if [ -s "$work/new.txt" ]; then
+  while IFS= read -r line; do
+    echo "::error::introduced by this change: $line"
+  done < "$work/new.txt"
+  exit 1
+fi
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 58bcb0b4..d4156fdd 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,6 +25,7 @@ env:
 #                  tests without default features
 #   filesystems    the storage tests on ext4, XFS and btrfs
 #   build-release  the shipped Linux binary, built exactly as release.yml does
+#   security       RustSec vulnerabilities the PR adds to Cargo.lock
 jobs:
   quick-checks:
     name: Quick checks
@@ -258,27 +259,91 @@ jobs:
           fi
           echo "no failpoint in the release build"
 
-  # Kept outside CI Gate until V2-1800 settles how advisories are handled: an
-  # advisory comes from the RustSec database, not from the PR, so today this
-  # reds every PR (RUSTSEC-2026-0387, V2-1743).
+  # Fails on a RustSec vulnerability that the PR brings into Cargo.lock (V2-1800).
+  #
+  # A plain `cargo audit` fails on every advisory in the lockfile. A new advisory
+  # comes from the RustSec database, not from a PR, so the day one is published
+  # against a dependency main already has, every open PR goes red. That made
+  # this job red on 43 of 50 PRs (RUSTSEC-2026-0387, V2-1743), and the red
+  # stopped meaning anything. So the job audits the base branch's lockfile and
+  # the PR's against the same database, and fails only on what the PR adds.
+  # Advisories already on main are printed as warnings. A PR that doesn't
+  # change Cargo.lock passes without auditing, since it can't add anything.
+  #
+  # Finding advisories newly published against main is the daily org-wide sweep
+  # (V2-1767), not this job.
   security:
     name: Security Audit
     runs-on: ubuntu-latest
     timeout-minutes: 20
     steps:
       - uses: actions/checkout@v4
+        with:
+          # The PR's merge commit and its first parent, the base branch tip.
+          fetch-depth: 2
+      - name: Does this PR change the lockfile or the audit?
+        id: inputs
+        shell: bash
+        run: |
+          set -euo pipefail
+          # Fail safe: anything other than a merge commit with its base parent
+          # present audits the whole lockfile, with nothing counted as existing.
+          if ! git rev-parse --verify -q 'HEAD^2' > /dev/null \
+            || ! git rev-parse --verify -q 'HEAD^1' > /dev/null; then
+            echo "not a pull request merge commit; auditing everything"
+            echo "audit=true" >> "$GITHUB_OUTPUT"
+            echo "base=false" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          changed=$(git diff --name-only 'HEAD^1' HEAD -- \
+            Cargo.lock .cargo/audit.toml .github/scripts/audit_regressions.sh .github/workflows/ci.yml)
+          if [ -n "$changed" ]; then
+            echo "inputs changed:"
+            echo "$changed"
+            echo "audit=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "::notice::No change to Cargo.lock, .cargo/audit.toml or the audit itself; nothing new to audit"
+            echo "audit=false" >> "$GITHUB_OUTPUT"
+          fi
+          echo "base=true" >> "$GITHUB_OUTPUT"
       - uses: dtolnay/rust-toolchain@stable
+        if: steps.inputs.outputs.audit == 'true'
       - name: Install cargo-audit
-        run: cargo install cargo-audit
-      - name: Run security audit
-        run: cargo audit
+        if: steps.inputs.outputs.audit == 'true'
+        run: cargo install cargo-audit --locked --version 0.22.2
+      - name: Audit for advisories this PR introduces
+        if: steps.inputs.outputs.audit == 'true'
+        shell: bash
+        run: |
+          set -euo pipefail
+          if [ "${{ steps.inputs.outputs.base }}" = true ]; then
+            git show 'HEAD^1:Cargo.lock' > "$RUNNER_TEMP/base.lock"
+            # The base branch's ignores, not the PR's: an ignore the PR adds
+            # would hide the advisory it is meant to catch from both audits.
+            # A change to them takes effect once it is on the base branch.
+            if git cat-file -e 'HEAD^1:.cargo/audit.toml' 2> /dev/null; then
+              git show 'HEAD^1:.cargo/audit.toml' > "$RUNNER_TEMP/base-audit.toml"
+            else
+              : > "$RUNNER_TEMP/base-audit.toml"
+            fi
+            if ! git diff --quiet 'HEAD^1' HEAD -- .cargo/audit.toml; then
+              echo "::warning::This PR changes .cargo/audit.toml. The audit applies the base branch's ignores; the PR's apply once it merges."
+              git diff 'HEAD^1' HEAD -- .cargo/audit.toml
+            fi
+            bash .github/scripts/audit_regressions.sh --config "$RUNNER_TEMP/base-audit.toml" \
+              Cargo.lock "$RUNNER_TEMP/base.lock"
+          elif [ -f .cargo/audit.toml ]; then
+            bash .github/scripts/audit_regressions.sh --config .cargo/audit.toml Cargo.lock
+          else
+            bash .github/scripts/audit_regressions.sh Cargo.lock
+          fi
 
   # The single check that says whether a PR is green. It fails unless every job
   # it needs succeeded; a skipped or cancelled job counts as a failure.
   gate:
     name: CI Gate
     if: always()
-    needs: [quick-checks, msrv, notices, test, filesystems, build-release]
+    needs: [quick-checks, msrv, notices, test, filesystems, build-release, security]
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps: